Digital Camera Patent Abstract
An optical probe apparatus and method for determining a position
of an image sensor within a digital camera relative to a reference
surface. The apparatus includes an optical probe assembly removably
mountable to the digital camera. A non-coherent light interferometer
in communication with the optical probe assembly is utilized to
determine a depth from a reference surface to the image sensor and
optical probe assembly. Digital Camera Patent Claims
What is claimed is:
1. In a digital camera, a method for determining a position of
an image sensor relative to a reference surface on the digital camera,
the image sensor comprising an imager plane and an optically transparent
plate, the imager plane spaced from the plate to define an interstice,
the plate being disposed intermediate the reference surface and
the imager plane, comprising the steps of:
mounting a removable optical probe assembly having an optical probe
and a pellicle to said digital camera, said pellicle being disposed
intermediate said reference surface and said optically transparent
plate;
locking said optical probe assembly to said digital camera in a
predetermined orientation such that said pellicle is disposed at
a first depth relative to said reference surface;
utilizing non-coherent light interferometry to determine (i) a
second depth from said reference surface to a front surface of said
optically transparent plate, (ii) an optical thickness of said optically
transparent plate, and (iii) a third depth from a back surface of
said optically transparent plate to said imager plane; and
calculating the position of the imager plane relative to the reference
surface.
2. The method according to claim 1 wherein the steps of mounting,
locking, utilizing, and calculating is accomplished within a 10
second measurement cycle.
3. The method according to claim 2 wherein a measurement reproducibility
is within at least 2.5 microns.
4. The method according to claim 1 wherein the step of utilizing
non-coherent light interferometry includes the steps of:
providing a non-coherent light source emitting a non-coherent light
signal incident on said optical probe;
transmitting a portion of said non-coherent light signal through
said pellicle;
applying a portion of said non-coherent light signal to said image
sensor, a portion of said non-coherent light signal being reflected
by (i) said pellicle, (ii) said optically transparent plate front
surface, (iii) said optically transparent plate back surface, and
(iv) said imager plane;
collecting said reflected signal;
dividing said reflected signal into a first and second light signals;
applying said first light signal to a stationary reference device,
a portion of said first light signal being reflected by said stationary
reference device to form a reference signal;
collecting said reference signal;
applying said second light signal to a variable optical path delay
element to form a delay signal, said variable optical path delay
element varying the optical path length of said delay signal;
collecting said delay signal;
displacing said variable optical path delay element with a predetermined
displacement profile as a function of time over a distance sufficient
to determine the position of said imager plane relative to said
reference surface;
combining said delay signal and said reference signal to form an
interference signal;
collecting said interference signal; and
analyzing said interference signal to determine (i) said second
depth from said reference surface to said optically transparent
plate front surface, (ii) said optical thickness of said optically
transparent plate, and (iii) said third depth from said optically
transparent plate back surface to said imager plane.
5. The method according to claim 4, wherein said variable optical
path delay element is displaced with a predetermined periodic displacement
profile, said variable optical path delay element being movable
over a distance sufficient to determine the position of the imager
plane.
6. The method according to claim 4, wherein the step of providing
a non-coherent light source emitting a non-coherent light signal
incident on said optical probe includes coupling said non-coherent
light source to said optical probe by means of a single mode optical
fiber cable.
7. The method according to claim 4, wherein the step of collecting
said reflected signal is accomplished by means of a single mode
optical fiber cable.
8. The method according to claim 4, wherein the step of analyzing
said interference signal includes:
sampling said interference signal;
digitizing said sampled interference signal;
storing said sampled interference signal in a data array; and
analyzing said stored data array to determine (i) said depth from
said reference surface to said optically transparent plate front
surface, (ii) said optical thickness of said optically transparent
plate, and (iii) said depth from said optically transparent plate
back surface to said imager plane.
9. The method according to claim 1 wherein the step of utilizing
non-coherent light interferometry includes the steps of:
providing a non-coherent light source emitting a non-coherent light
signal incident on said optical probe;
transmitting a portion of said non-coherent light signal through
said pellicle;
applying a portion of said non-coherent light signal to said image
sensor, a portion of said non-coherent light signal being reflected
by (i) said pellicle, (ii) said optically transparent plate front
surface, (iii) said optically transparent plate back surface, and
(iv) said imager plane;
collecting said reflected signal;
dividing said reflected signal into a first and second light signal;
applying said first and second light signals to said variable optical
path delay element to form first and second delay light signals
respectively, said variable optical path delay element varying the
optical path length of said first and second delay light signals;
displacing said variable optical path delay element with a predetermined
displacement profile as a function of time over a distance sufficient
to determine the position of said imager plane relative to said
reference surface;
combining said first and second delay light signals to form an
interference signal;
collecting said interference signal;
analyzing said interference signal to determine (i) said second
depth from said reference surface to said optically transparent
plate front surface, (ii) said optical thickness of said optically
transparent plate, and (iii) said third depth from said optically
transparent plate back surface to said imager plane.
10. The method according to claim 9, wherein said variable optical
path delay element is displaced with a predetermined periodic displacement
profile, said variable optical path delay element being movable
over a distance sufficient to determine the position of the imager
plane.
11. The method according to claim 9, wherein the step of providing
a non-coherent light source emitting a non-coherent light signal
incident on said optical probe includes coupling said non-coherent
light source to said optical probe by means of a single mode optical
fiber cable.
12. The method according to claim 9, wherein the step of collecting
said reflected signal is accomplished by means of a single mode
optical fiber cable.
13. The method according to claim 9, wherein the step of analyzing
said interference signal includes:
sampling said interference signal;
digitizing said sampled interference signal;
storing said sampled interference signal in a data array; and
analyzing said stored data array to determine (i) said depth from
said reference surface to said optically transparent plate front
surface, (ii) said optical thickness of said optically transparent
plate, and (iii) said depth from said optically transparent plate
back surface to said imager plane.
14. The method according to claim 1 wherein the step of utilizing
non-coherent light interferometry includes the steps of:
providing a non-coherent light source emitting a non-coherent light
signal incident on a 2.times.2 optical coupler;
dividing said non-coherent light signal into a first and second
non-coherent light signal;
transmitting a portion of said first non-coherent light signal
through said pellicle;
applying a portion of said first non-coherent light signal to said
image sensor, a portion of said non-coherent light signal being
reflected by (i) said pellicle, (ii) said optically transparent
plate front surface, (iii) said optically transparent plate back
surface, and (iv) said imager plane;
collecting said reflected first light signal to form an object
signal;
applying said second light signal to said variable optical path
delay element to form a delay signal, said variable optical path
delay element varying the optical path length of said delay light
signal;
collecting said delay signal;
displacing said variable optical path delay element with a predetermined
displacement profile as a function of time over a distance sufficient
to determine the position of said imager plane relative to said
reference surface;
combining said delay signal and said object signal to form an interference
signal;
collecting said interference signal;
analyzing said interference signal to determine (i) said second
depth from said reference surface to said optically transparent
plate front surface, (ii) said optical thickness of said optically
transparent plate, and (iii) said third depth from said optically
transparent plate back surface to said imager plane.
15. The method according to claim 14, wherein said variable optical
path delay element is displaced with a predetermined periodic displacement
profile, said variable optical path delay element being movable
over a distance sufficient to determine the position of the imager
plane.
16. The method according to claim 14, wherein the step of providing
a non-coherent light source emitting a non-coherent light signal
incident on said optical probe includes coupling said non-coherent
light source to said optical probe by means of a single mode optical
fiber cable.
17. The method according to claim 14, wherein the step of collecting
said reflected signal is accomplished by means of a single mode
optical fiber cable.
18. The method according to claim 14 wherein the step of analyzing
said interference signal includes:
sampling said interference signal;
digitizing said sampled interference signal;
storing said sampled interference signal in a data array; and
analyzing said stored data array to determine (i) said depth from
said reference surface to said optically transparent plate front
surface, (ii) said optical thickness of said optically transparent
plate, and (iii) said depth from said optically transparent plate
back surface to said imager plane.
19. In a digital camera, a method for determining a position of
an image sensor relative to an in-focus position, the image sensor
comprising an imager plane and an optically transparent plate, the
imager plane spaced from the plate to define an interstice, the
plate being disposed intermediate the reference surface and the
imager plane, the digital camera having a predetermined focal depth
in air relative to a reference surface on the digital camera, comprising
the steps of:
mounting a removable optical probe assembly having an optical probe
and a pellicle to said digital camera, said pellicle being disposed
intermediate said reference surface and said optically transparent
plate;
locking said optical probe assembly to said digital camera in a
predetermined orientation such that said pellicle is disposed at
a first depth relative to said reference surface;
utilizing non-coherent light interferometry to determine (i) a
second depth from said reference surface to a front surface of said
optically transparent plate, (ii) an optical thickness of said optically
transparent plate, and (iii) a third depth from a back surface of
said optically transparent plate to said imager plane;
calculating said in-focus position from said predetermined focal
depth in air and said optical thickness of said optically transparent
plate;
calculating a position of said imager plane relative to said reference
surface; and
determining the position of said imager plane relative to said
in-focus position.
20. In a digital camera, a method for positioning an image sensor
relative to an in-focus position, said image sensor comprising an
imager plane and an optically transparent plate, the imager plane
spaced from the plate to define an interstice, said digital camera
having a predetermined focal depth in air relative to a reference
surface on said digital camera, said optically transparent plate
being disposed intermediate said reference surface and said imager
plane, comprising the steps of:
mounting a removable optical probe assembly having an optical probe
and a pellicle to said digital camera, said pellicle being disposed
intermediate said reference surface and said optically transparent
plate;
locking said optical probe assembly to said digital camera in a
predetermined orientation such that said pellicle is disposed at
a first depth relative to said reference surface;
utilizing non-coherent light interferometry to determine (i) a
second depth from said reference surface to a front surface of said
optically transparent plate, (ii) an optical thickness of said optically
transparent plate, and (iii) a third depth from a back surface of
said optically transparent plate to said imager plane;
calculating a position of the imager plane relative to said reference
surface;
calculating said in-focus position;
calculating the position of said imager plane relative to said
in-focus position; and
moving said image sensor to align said imager plane at said in-focus
position.
21. In a digital camera, a method for determining whether an imager
plane of an image sensor is within a desired specification, the
image sensor including an optically transparent plate, the imager
plane spaced from the plate to define an interstice, the plate being
disposed intermediate the reference surface and the imager plane,
the digital camera having a predetermined focal depth in air relative
to a reference surface on the digital camera, comprising the steps
of:
mounting a removable optical probe assembly having an optical probe
and a pellicle to said digital camera, said pellicle being disposed
intermediate said reference surface and said optically transparent
plate;
locking said optical probe assembly to said digital camera in a
predetermined orientation such that said pellicle is disposed at
a first depth relative to said reference surface;
utilizing non-coherent light interferometry to determine (i) a
second depth from said reference surface to a front surface of said
optically transparent plate, (ii) an optical thickness of said optically
transparent plate, and (iii) a third depth from a back surface of
said optically transparent plate to said imager plane;
calculating a position of said imager plane relative to said reference
surface;
comparing the position of said imager plane relative to the desired
specification; and
determining whether the position of said imager plane is within
the desired specification.
22. In a digital camera, a method for determining whether an imager
plane of an image sensor is parallel to a reference surface, the
image sensor including an optically transparent plate, the imager
plane being spaced from the plate to define an interstice, the plate
being disposed intermediate the reference surface and the imager
plane, comprising the steps of:
mounting a removable optical probe assembly having an optical probe
and a pellicle to said digital camera, said pellicle being disposed
intermediate said reference surface and said optically transparent
plate;
locking said optical probe assembly to said digital camera in a
predetermined orientation such that said pellicle is disposed at
a first depth relative to said reference surface;
utilizing non-coherent light interferometry to determine (i) a
second depth from said reference surface to a front surface of said
optically transparent plate, (ii) an optical thickness of said optically
transparent plate, and (iii) a third depth from a back surface of
said optically transparent plate to said imager plane;
selecting three locations on said imager plane;
calculating the position of each of the three locations relative
to said reference surface; and
determining whether said positions of said three locations defines
a plane parallel to said reference surface.
23. In a digital camera, a method for determining the flatness
of an imager plane of an image sensor relative to a reference surface,
the image sensor including an optically transparent plate, the imager
plane being spaced from the plate to define an interstice, the plate
being disposed intermediate the reference surface and the imager
plane, comprising the steps of:
mounting a removable optical probe assembly having an optical probe
and a pellicle to said digital camera, said pellicle being disposed
intermediate said reference surface and said optically transparent
plate;
locking said optical probe assembly to said digital camera in a
predetermined orientation such that said pellicle is disposed at
a first depth relative to said reference surface;
utilizing non-coherent light interferometry to determine (i) a
second depth from said reference surface to a front surface of said
optically transparent plate, (ii) an optical thickness of said optically
transparent plate, and (iii) a third depth from a back surface of
said optically transparent plate to said imager plane;
selecting three locations on said imager plane;
calculating the position of each of the three locations relative
to said reference surface; and
determining whether said positions of said three locations defines
a plane parallel to said reference surface.
24. In a digital camera, a method for positioning an image sensor
relative to an in-focus position and parallel to a reference surface,
said image sensor comprising an imager plane and an optically transparent
plate, the imager plane spaced from the plate to define an interstice,
said digital camera having a predetermined focal depth in air relative
to a reference surface on said digital camera, said optically transparent
plate being disposed intermediate said reference surface and said
imager plane, comprising the steps of:
mounting a removable optical probe assembly having a plurality
of optical probes and a pellicle to said digital camera, said pellicle
being disposed intermediate said reference surface and said optically
transparent plate, each of said plurality of optical probes being
incident on a measurement location on said imager plane;
locking said optical probe assembly to said digital camera in a
predetermined orientation such that said pellicle is disposed at
a first depth relative to said reference surface;
utilizing non-coherent light interferometry to determine (i) a
second depth from said reference surface to a front surface of said
optically transparent plate, (ii) an optical thickness of said optically
transparent plate, and (iii) a third depth from a back surface of
said optically transparent plate to said imager plane for each optical
probe location;
calculating a position of the imager plane relative to said reference
surface for each measurement location;
calculating the parallelism of the imager plane relative to said
reference surface;
calculating the position of said imager plane relative to said
in-focus position at each measurement location; and
moving said image sensor to align said imager plane at said in-focus
position for each measurement location.
Digital Camera Patent Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to commonly assigned U.S. Ser. No.
08/408,871, titled ASSOCIATED INTERFEROMETRIC MEASUREMENT APPARATUS
FOR DETERMINING A PHYSICAL PROPERTY OF AN OBJECT, by Marcus et al,
filed on Mar. 22, 1995, and issued as U.S. Pat. No. 5,659,392, to
commonly assigned U.S. Ser. No. 08/408,770, titled ASSOCIATED INTERFEROMETRIC
MEASUREMENT METHOD FOR DETERMINING A PHYSICAL PROPERTY OF AN OBJECT,
by Marcus et al, filed on Mar. 22, 1995, and issued as U.S. Pat.
No. 5,596,409, and to commonly assigned U.S. Ser. No. 08/756,097,
titled APPARATUS FOR DETERMINING A POSITION OF AN IMAGE SENSOR IN
A DIGITAL CAMERA, by Marcus et al, filed on Nov. 22, 1996.
FIELD OF THE INVENTION
The invention relates to an apparatus and method for determining
a position of an image sensor in a digital camera, whereby the image
sensor can be positioned to provide a focused image.
BACKGROUND OF THE INVENTION
In a typical digital camera, an image beam is directed through
a lens and onto an imager or image sensor, for example a CCD (Charge
Coupled Device), comprised of an array of sensing elements. The
lens and the CCD need to be properly positioned relative to each
other within the digital camera to provide a focused image. In order
to properly position the CCD, the position of the CCD needs to be
determined. Such a position can be determined relative to a reference
surface or reference plane.
A Coordinate Measuring Machine (CMM) is an example of an apparatus
employed to determine the position of an object relative to a reference
plane. Typically, the object is retained in a suitable holder on
an optical bench. In one method to determine the position of an
object, three points on a reference plane approximately 120 degrees
apart are measured to define the reference plane; the coordinates
of the three points being tracked in the x, y and z locations. A
point on the object is then measured relative to the reference plane,
and the distance from the reference plane is calculated. Typical
coordinate measurement machines include contact probes for contacting
each of the points defining the reference plane and the object,
such as those described in U.S. Pat. No. 5,428,446 (Ziegart et al.),
U.S. Pat. No. 5,446,545 (Taylor) and U.S. Pat. No. 4,929,082 (Webber).
These references include interferometers which monitor the displacement
of the machine axes. Non-contact methods include optical triangulation
as described in U.S. Pat. No. 4,373,804 (Pryor) and U.S. Pat. No.
5,510,625 (Pryor).
While such apparatus and methods may have achieved a certain level
of success, the apparatus are not readily transportable and simple
to use. Further, the method is time consuming and often dependent
on the skill of the operator.
Accordingly, a need continues to exist for an apparatus and method
for determining the position of an image sensor in a digital camera.
The apparatus needs to be robust, transportable, simple to use,
and readily mounted and dismounted to the digital camera. The method
must be fast, provide objective results independent of the operator,
and provide accurate consistent results.
SUMMARY OF THE INVENTION
An object of the invention is to provide an apparatus and method
for determining a position of an image sensor in a digital camera.
Another object of the invention is to provide such an apparatus
which is robust, transportable, simple to use, and readily mounted
to the digital camera.
Yet another object of the invention is to provide such an apparatus
and method for determining the degree of flatness of an image sensor.
A further object of the invention is to provide such an apparatus
and method for determining the parallelism of a plane of an image
sensor relative to a reference surface.
Still another object of the invention is to provide such a method
which is fast, provides objective results independent of the operator,
and provides accurate, consistent results.
These objects are given only by way of illustrative example. Thus,
other desirable objectives and advantages inherently achieved by
the disclosed invention may occur or become apparent to those skilled
in the art. The invention is defined by the appended claims.
According to one aspect of the invention, there is provided a method,
in a digital camera, for determining a position of an image sensor
relative to a reference surface on the digital camera, wherein the
image sensor includes an imager plane and an optically transparent
plate, and the imager plane is spaced from the plate to define an
air gap and the plate is disposed intermediate the reference surface
and the imager plane. The method includes the steps of (1) mounting
a removable optical probe assembly having an optical probe and a
pellicle to the digital camera, such that the pellicle is disposed
intermediate the reference surface and the optically transparent
plate; (2) locking the optical probe assembly to the digital camera
in a predetermined orientation such that the pellicle is disposed
at a first depth relative to the reference surface; (3) utilizing
non-coherent light interferometry to determine (i) a second depth
from the reference surface to a front surface of the optically transparent
plate, (ii) an optical thickness of the optically transparent plate,
and (iii) a third depth from a back surface of the optically transparent
plate to the imager plane; and (4) calculating the position of the
imager plane relative to the reference surface.
According to yet another aspect of the invention, there is provided
in a digital camera, a method for determining a position of an image
sensor relative to an in-focus position. The method includes the
steps of mounting a removable optical probe assembly having an optical
probe and a pellicle to the digital camera, such that the pellicle
is disposed intermediate the reference surface and the optically
transparent plate; locking the optical probe assembly to the digital
camera in a predetermined orientation such that the pellicle is
disposed at a first depth relative to the reference surface; utilizing
non-coherent light interferometry to determine (i) a second depth
from the reference surface to a front surface of the optically transparent
plate, (ii) an optical thickness of the optically transparent plate,
and (iii) a third depth from a back surface of the optically transparent
plate to the imager plane; calculating the in-focus position from
the predetermined focal depth in air and the optical thickness of
the optically transparent plate; calculating a position of the imager
plane relative to the reference surface; and determining the position
of the imager plane relative to the in-focus position.
According to still yet another aspect of the invention, there is
provided a method in a digital camera for positioning an image sensor
relative to an in-focus position. The method further comprises the
step of moving the image sensor to align the imager plane at the
in-focus position.
According to further aspects of the invention, there are provided
methods for determining whether an imager plane of an image sensor
is within a desired specification; for determining whether an imager
plane of an image sensor is parallel to a reference surface; for
determining the flatness of an imager plane of an image sensor relative
to a reference surface; and for positioning an image sensor relative
to an in-focus position and parallel to a reference surface.
The present invention provides an apparatus and method for determining
the position of an image sensor in a digital camera. The apparatus
is robust, transportable, simple to use, and readily mounted to
the digital camera. The method is fast, provides objective results
independent of the operator, and provides accurate and consistent
results.
BRIEF DESCRIPTION OF THE DRAWING
The foregoing and other objects, features, and advantages of the
invention will be apparent from the following more particular description
of the preferred embodiments of the invention, as illustrated in
the accompanying drawings.
FIG. 1 shows a lens mounted within a camera body.
FIG. 2 shows a CCD mounted within a camera body.
FIG. 3 shows a non-coherent light interferometric measurement apparatus
according to the present invention.
FIG. 4(a) shows a cross-sectional view of the optical probe assembly
of FIG. 3 while FIG. 4(b) shows an exploded view of the optical
probe assembly of FIG. 3.
FIG. 5 shows a schematic view of the relationship of the optical
probe assembly and the CCD when the optical probe assembly is mounted
to the camera body.
FIG. 6 shows a first embodiment of a non-coherent light interferometric
measurement apparatus in accordance with the present invention.
FIG. 7 shows a second embodiment of a non-coherent light interferometric
measurement apparatus in accordance with the present invention.
FIG. 8 shows reflections corresponding to the schematic view illustrated
in FIG. 5.
FIG. 9 shows an interferogram obtained with the present invention
and corresponding to the reflections illustrated in FIG. 8.
FIG. 10 shows a series of reflections observed as the optical path
delay element is moved in accordance with the present invention.
FIG. 11 shows a non-coherent light interferometer of FIG. 3 having
a standard mode configuration in accordance with the present invention.
FIG. 12 shows a flow chart illustrating one method according to
the present invention utilizing an autocorrelation mode configuration
of a non-coherent light interferometer.
FIG. 13 shows a flow chart illustrating another method according
to the present invention utilizing an autocorrelation mode configuration
of a non-coherent light interferometer.
FIG. 14 shows a flow chart illustrating a further method according
to the present invention utilizing a standard mode configuration
of a non-coherent light interferometer.
FIG. 15 shows a multi-point non-coherent light interferometric
measurement apparatus according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The following is a detailed description of the preferred embodiments
of the invention, reference being made to the drawings in which
the same reference numerals identify the same elements of structure
in each of the several figures.
Referring to FIG. 1, a lens 12 is mounted to a camera body 10 by
means of a lens mount 14. The distance at which lens 12 focuses
light in the camera body is referred to as the focal depth F.sub.depth
of the lens in air, measured along the z-axis. The focal depth F.sub.depth
can be determined by means known to those skilled in the art. In
a conventional camera employing photographic film 13 as the image
media, the film is positioned at the focal depth F.sub.depth of
the lens.
FIG. 1 illustrates film 13 positioned at the focal depth of the
lens, to provide a focused image. Means (not shown) such as film
rails, may be employed to position film 13 at the focal depth F.sub.depth.
Hereinafter, the terminology "in-focus position" refers
to a position at which an imaging media is positioned to provide
an in-focus image.
Referring to FIG. 2, in a digital camera body 15, lens 12 is typically
mounted to the digital camera by means of lens mount 14. An image
sensor 16 is employed as the imaging media. Image sensor 16 comprises
an imager plane 18 referring to an active surface of the image sensor.
Image sensor 16 generally further comprises an optically transparent
plate 20 having an index of refraction n. Imager plane 18 is spaced
from plate 20 such that a gap or interstice 22 is interposed intermediate
the imager plane and the plate. Typically, the imager plane and
the transparent plate are hermetically sealed with a defined interstice
22.
As indicated above with regard to a conventional camera, the distance
at which lens 12 focuses light is referred to as the focal depth
F.sub.depth of the lens. However, in a digital camera, the presence
of plate 20 (disposed intermediate the lens and the image sensor)
affects the in-focus position. Accordingly, an in-focus position
F.sub.I for imager plane 18, taking into account the presence of
plate 20, is determined by: ##EQU1## t being the thickness of plate
20 and n being the index of refraction of plate 20.
FIG. 3 provides a general illustration of a non-coherent light
interferometric measurement apparatus 21 according to the present
invention for determining the position of image sensor 16 relative
to a reference surface on the digital camera. The apparatus allows
a user to verify that the position of imager plane 18 is within
a desired tolerance. If the position of imager plane 18 is outside
the desired tolerance, the image sensor can be re-positioned to
move imager plane 18 to a desired position, such as the in-focus
position F.sub.I.
Lens 12 is generally mounted to digital camera body 15 by lens
mount 14. Conventional means (not shown) are known to facilitate
mounting of the lens to the camera body. Such means may include:
a screw lens mount wherein screw threads allows installation of
the lens to the camera body; a bayonet lens mount wherein the lens
slips over a mating flange on the camera body with a twist of about
45 degrees; or a breechlock lens mount wherein a knurled collar
on the back of the lens fits over a mating flange on the camera
body. To secure the mounting, the lens mount may include a locking
means to lock the lens to the lens mount. Similarly, to unlock the
lens from the lens mount, an unlocking means may be included.
In the present invention, to determine the position of imager plane
18, lens 12 is removed from digital camera body 15. Since lens 12
is removed, the position of imager plane 18 is determined relative
to a reference surface on digital camera body 15. While various
surfaces (including, but not limited to, components of the digital
camera body) may be used as a reference surface, for ease of discussion,
lens mount 14 will provide the reference surface. Therefore, the
position of imager plane 18 will be discussed as being determined
relative to lens mount 14.
Referring to FIG. 3, an optical probe assembly 24 is removably
mounted to lens mount 14 to securely, but temporarily, attach optical
probe assembly 24 to digital camera body 15. Preferably, optical
probe assembly 24 incorporates the means to allow the assembly to
be mounted to digital camera body 15 by means of the bayonet lens
mount wherein the assembly would slip over a mating flange on the
camera body with a twist of about 45 degrees. To temporarily secure
the mounting, a locking means 23, for example a spring loaded locking
pin, locks optical probe assembly 24 to lens mount 14. To unlock
optical probe assembly 24 from lens mount 14, an unlocking means
25 such as a spring loaded release pin, is employed thereby allowing
optical probe assembly 24 to be readily released from digital camera
body 15. Note that such locking means 23 and unlocking means 25
may optionally be employed to lock and unlock, respectively, lens
12 to lens mount 14.
Optical probe assembly 24 includes a single mode optical fiber
cable 26 coupled to a 1.times.2 optical coupler 28. Preferably,
connectors and patchcords (not shown) of variable length (preferably
patchcords which terminate with a low back reflection connector
such an FC connector) are disposed between optical coupler 28 and
optical probe assembly 24 to permit portability for the apparatus
and to permit remote location mounting. A non-coherent light source
30, such as an LED (Light Emitting Diode), is coupled into a single
mode fiber 32 and passes through 1.times.2 optical coupler 28. A
suitable non-coherent light source 30 is a high bandwidth (approximately
40-120 nm) 1300 nm center wavelength LED having 1-10 microwatts
of coupled power.
Generally, light from non-coherent light source 30 travels along
optical fiber 26, transmitted through optical probe assembly 24,
and is incident on image sensor 16. The light is reflected from
each of the surface of image sensor 16, and passes back through
optical probe assembly 24 into optical fiber 26. The reflected light
then passes through 1.times.2 optical coupler 28 into an optical
fiber 34, which is introduced into a non-coherent light interferometer
36. Preferably, interferometer 36 is of a Michelson configuration,
though a non-Michelson configuration has been found suitable. Associated
with non-coherent light interferometer 36 is a computing means 38,
such a computer, for controlling, collecting, manipulating, analyzing,
and storing data.
Optical probe assembly 24 is further illustrated in FIGS. 4(a)
and 4(b). Optical probe assembly 24 comprises a probe mounting surface
40, an optical probe 42 such as a collimating lens or fiber collimator,
probe assembly mounting means 44, probe housing 43 having a probe
assembly gripping means 45, a probe recess 46 (not shown), a pellicle
48 disposed at a predetermined depth P.sub.depth from probe mounting
surface 40, and a pellicle mount 49.
Probe assembly mounting means 44 allows optical probe assembly
24 to mate with a camera body wherein optical probe assembly 24
is removably mounted to camera body 15 in a predetermined orientation.
As noted above, probe assembly mounting means 44 preferably incorporates
the conventional bayonet lens mount wherein optical probe assembly
24 would slip over a mating flange on the camera body with a twist
of about 45 degrees. As such, an operator would grasp optical probe
assembly 24 by gripping means 45, and mount the optical probe assembly
24 to camera body 15 by positioning optical probe assembly 24 such
that probe mounting surface 40 abuts lens mount 14. Optical probe
assembly 24 would then be rotated (for example, by approximately
45 degrees) to dispose locking means 23 into recess 46, thereby
locking optical probe assembly 24 to lens mount 14 in a predetermined
orientation. In a preferred predetermined orientation, probe mounting
surface 40 abuts lens mount 14 (i.e., the reference surface) to
provide a predetermined depth of pellicle 48 from lens mount 14.
The plane defined by abutting probe mounting surface 40 and lens
mount 14 will hereinafter be referred to as Reference Plane A.
To release optical probe assembly 24 from camera body 15, an operator
would grasp optical probe assembly 24 by gripping means 45, and
depress unlocking means 25 to release locking means 23 from probe
recess 46. If the conventional bayonet lens mount is utilized (wherein
optical probe assembly 24 mounts to the camera body with a twist
of about 45 degrees), optical probe assembly 24 can be rotated in
an opposite direction (of approximately 45 degrees) to dismount
the assembly from the camera body.
Pellicle 48 is comprised of an optically transparent material which
is sufficiently thin so as to not affect the focal depth measurement.
A suitable pellicle can be made of a polyester film material of
approximately 1.5 .mu.m. Means are provided for securely mounting
the pellicle to optical probe assembly 24. For example, as illustrated,
an O-ring 50 held in an O-ring groove provides secure mounting to
pellicle mount 49. Other means may be known, such as setting pellicle
48 in place with a suitable adhesive. Pellicle mount 49 predeterminedly
disposes pellicle 48 from probe mounting surface 40. A suitable
pellicle mount 49 includes an open aperture of approximately 0.5
inches in diameter and an outer diameter of approximately 0.75 inches.
As indicated above, optical probe assembly 24 provides parallel
collimated light to image sensor 16 when mounted in camera body
15, and collect light reflected back from image sensor 16. Accordingly,
referring now to FIG. 4(b), optical probe assembly 24 includes fiber
collimator 42, an optional fiber collimator collar 92, an adjustment
collar 94, at least one adjustment means 96 (for example, a screw),
a ball pivot 98, a ball pivot receiving plate 100 having an opening
102 to receive ball pivot 98, a mounting plate 104, an optional
fiber collimator collar mounting means 106, at least one ball pivot
receiving plate mounting means 108 (for example, a screw), and at
least one optical probe assembly means 110. In the preferred embodiment,
fiber collimator 42 is an active optical element including a single
mode fiber pigtailed, quarter-pitch Gradient index (GRIN) lens assembly.
A suitable fiber collimator 42 is a Dicon Fiber collimator Part
Number FC-9-1.3 FC-3.0-L. Such a fiber collimator includes a lens
having a beam diameter of about 0.45 mm and a beam divergence of
less than 0.25 degrees.
If optional fiber collimator collar 92 is employed, it surrounds
fiber collimator 42 and is securely mounted by optional fiber collimator
mounting means 106. Such a suitable mounting means 106 is a 2-56
screw. Fiber collimator 42, with collar 92 securely mounted, is
inserted into a recess 112 in adjustment collar 94. As represented
in FIG. 4(b), the left-most end of fiber collimator 42 is inserted
into rotatable ball pivot 98. Ball pivot 98, together with fiber
collimator 42 attached, is inserted into opening 102 of ball pivot
receiving plate 100. Adjustment collar 94 is then mounted to ball
pivot receiver plate 100 using collar mounting screws (not shown).
Adjustment means 96 allow adjustment of adjustment collar 94 to
provide orientation adjustment capability of fiber collimator 42.
Suitable adjustment means 96 is a 0-80 screw. These adjustment means
96 preferably contact a flat surface oriented approximately 120
degrees apart on fiber collimator collar 92. If optional fiber collimator
is not employed, adjustment means 96 contact the outer surface of
fiber collimator 42.
Ball pivot receiver plate 100 is fastened and centered with respect
to mounting plate 104 by mounting means 108, thereby locking ball
pivot 98 in position yet allowing free rotation of fiber collimator
42's optical axis by means of adjustment means 96. When assembly
is complete, orientation of fiber collimator 42 is preferably performed
by means of three 120 degree oriented adjustment screws 96. Adjustment
is performed with optical probe assembly 24 mounted in the camera
body. Adjustments are made until the non-coherent light interference
signal from non-coherent light interferometer 36 is optimized. This
occurs when the optical axis of fiber collimator 42 is normal to
the reference surface of the camera body.
Adjustment means 96 are intentionally not readily accessible once
gripping means 45 is attached to the assembly so that further adjustments
can not be readily made to fiber collimator 42 during a measurement
cycle. Rather, gripping means 45 is attached to optical probe assembly
24 by optical probe assembly means 110. In a preferred embodiment,
three access holes (one illustrated as element 116 in FIG. 4(b))
in probe housing 43 are provided to permit fine-tuning adjustments
to fiber collimator 42 via screws 96, if needed. During typical
usage, fiber collimator 42 is positioned approximately at the plane
of probe mounting surface 40 (i.e., the reference surface). During
assembly, fiber collimator 42 is adjusted such that maximum signals
from pellicle 48 are provided when connected to the non-coherent
light interferometer. In a preferred embodiment, when mounted in
camera body 15, pellicle 48 is mounted approximately 40 mm from
lens mount 14 with a tight tolerance (of about +/-2 microns). Hereinafter,
for ease of discussion, a collimating lens orientation and locking
means 114 will comprise fiber collimator collar 92 (optional), adjustment
collar 94, adjustment means 96, ball pivot 98, ball pivot receiving
plate 100, opening 102, fiber collimator collar mounting means 106
(optional), collar mounting screws (not shown), and recess 112.
With optical probe assembly 24 mounted to digital camera body 15,
the orientation of optical probe 42 and image sensor 16 are as illustrated
in FIG. 5. Pellicle 48 is disposed at a predetermined depth P.sub.depth
from probe mounting surface 40 and, accordingly, lens mount 14 (i.e.,
the reference surface positioned at Reference Plane A). The depth
from pellicle 48 to a front surface 52 of transparent plate 20 is
referred to as PG.sub.depth, while the depth from pellicle 48 to
a back surface 54 of transparent plate 20 to imager plane 18 is
referred to as GS.sub.depth. Accordingly, the actual depth D.sub.actual
of imager plane 18 relative to the reference surface (i.e., lens
mount 14 at Reference Plane A) is:
where P.sub.depth is the predetermined depth from Reference Plane
A to pellicle 48; PG.sub.depth is the depth from pellicle 48 to
front surface 52; GS.sub.depth is the depth from back surface 54
to imager plane 18; and t is the thickness of transparent plate
20;
The corresponding total optical depth D.sub.optical from pellicle
48 relative to the reference surface (i.e., lens mount 14 at Reference
Plane A) is given by the equation:
where n is the index of refraction of plate 20.
Substituting Equation 3 into Equation 4 results in:
Accordingly, the difference between where imager plane 18 is actually
positioned (i.e., D.sub.actual) and the desired position, for example
the in-focus position F.sub.I, is referred to as the difference
in focus D.sub.F :
With the optical probe assembly locked to the digital camera in
a predetermined orientation whereby the pellicle is disposed at
a known reference depth relative to the reference surface, non-coherent
light interferometry can be utilized to determine (i) the depth
from the reference surface to the front surface of the optically
transparent plate, (ii) the optical thickness of the optically transparent
plate, and (iii) the depth from the back surface of the optically
transparent plate to the image sensor. From this information, the
position of imager plane 18 relative to the reference surface can
be determined.
The depth D.sub.RF from the reference surface to front surface
52 of plate 20 is determined from the relationship:
The value P.sub.depth is a predetermined value, and the value of
PG.sub.depth is measured using the apparatus of the present invention.
FIG. 6 illustrates non-coherent light interferometric measurement
apparatus 21 having an optional coherent light interferometer 55
in mechanical association with non-coherent light interferometer
36 in an autocorrelation mode configuration. The autocorrelation
mode configuration allows the interference spectrum from image sensor
16 to be independent of the length of fiber disposed between imager
sensor 16 and non-coherent interferometer 36. Applicants note that
single mode optical fibers up to 10 km in length have been successfully
utilized between optical probe assembly 24 and non-coherent light
interferometer 36 without adverse affect to the resolution of the
measurements.
As illustrated in FIG. 6, non-coherent light interferometric measurement
apparatus 21 includes non-coherent light interferometer 36, optional
coherent light interferometer 55, optical probe assembly 24, computer
38, and various electronics for signal processing and motor control.
While non-coherent light interferometer 36 can be of a standard
mode configuration (described below), preferably, non-coherent light
interferometer 36 is of an autocorrelation mode as illustrated in
FIG. 6. The autocorrelation mode of non-coherent light interferometer
36 transforms a series of non-coherent light constructive interference
peaks occurring at optical interfaces in the digital camera body
to distances from a central autocorrelation peak.
Still referring to FIG. 6, coherent light interferometer 55 provides
a known displacement profile as a function of time. Coherent light
interferometer 55 includes a coherent light source 56, preferably
a HeNe laser, emitting a collimated coherent light signal which
is split by a splitting means 58, such as a beam splitter 58, into
first and second coherent light signals of approximately equal intensity.
The first coherent light signal is incident on a stationarily mounted
retroreflector 60, while the second coherent light signal is incident
onto a movable optical element, such as a retroreflector 62. The
first and second coherent light signals are retro-reflected back
to beam splitter 58 where they recombine and interfere with each
other. This recombined, coherent light interference signal is detected
by a photodetector 64 which is fed into and processed by signal
processing electronics 65 and sent to computer 38. The coherent
light signal is periodic, with a constant amplitude and provides
constant distance interval data acquisition sampling capability.
Accordingly, coherent light interferometer 55 monitors the motion
of retroreflector 62; the position of retroreflector 62 being controlled
by a common variable path delay element 80.
Non-coherent light source 30 emits a non-coherent light signal
along single mode fiber 32 which passes through 1.times.2 optical
coupler 28 to optical probe assembly 24. Within the optical probe
assembly, a portion of the light signal is transmitted through the
pellicle and applied to image sensor 16 positioned within camera
body 15. A portion of the light signal is reflected by the pellicle,
the front surface 52 of plate 20, the back surface 54 of plate 20,
and imager plane 18. The light from non-coherent light source 30,
which is reflected from image sensor 16 and pellicle 48, is collected
by optical probe assembly 24 and passes through optical coupler
28 into optical fiber 34 to be introduced into non-coherent light
interferometer 36.
The signal passing through optical fiber 34 (herein called the
object signal) is split into first and second non-coherent light
signals at 2.times.2 optical coupler 66. The first non-coherent
light signal is directed along a single mode optical fiber 68 to
a stationary reference reflector 70 through a collimating applying
and collecting means 72. Alternatively, single mode optical fiber
68 can be terminated with a normal cleaved mirrored surface at its
tip (not shown) in place of collimating applying and collecting
means 72 and stationary reference reflector 70. A portion of the
first non-coherent light signal is reflected back from stationary
reference reflector 70 into collimating applying and collecting
means 72, and is coupled back into single mode optical fiber 68.
This signal is referred to as the reference signal. The second non-coherent
light signal, traveling along single mode optical fiber 74, is incident
on collimating applying and collecting means 76, which collimates
the second non-coherent light signal. Collimating applying and collecting
means 76 applies the second non-coherent light signal to a mirror
78 mounted onto retroreflector 62, of a common variable optical
path delay element 80. Common variable optical path delay element
80 is mounted for precision movement by a motor 82 in a direction
shown by arrow A. A portion of the second non-coherent light signal
is reflected back from mirror 78 into collimating applying and collecting
means 76 and is coupled back into single mode optical fiber 74,
forming a delay signal. The optical path from 2.times.2 optical
coupler 66 to stationary reference reflector 70 and back to 2.times.2
optical coupler 66 is defined as the optical path length of the
stationary reference branch of non-coherent light interferometer
36.
While alternative configurations for variable optical path delay
element 80 may be known to those skilled in the art, preferably,
variable optical path delay element 80 includes a prism retroreflector
and a mirror mounted on the diaphragm cone of a moving-coil loudspeaker;
the mirror being mounted on a portion of the surface of the prism
retroreflector. Therefore, as the loudspeaker cone moves to the
left (as illustrated in FIG. 6), the optical path lengths of non-coherent
light source 30 and coherent light source 56 increases by the same
amount. Conversely, the optical path lengths of non-coherent light
source 26 and coherent light source 56 decreases by the same amount
as the loudspeaker cone moves to the right (as illustrated in FIG.
6). The optical path length of common variable optical path delay
element 80 is preferably varied using a motor drive control electronic
module 84 comprising a function generator and power amplifier, thereby
controlling the current to the loudspeaker coil. Optical path delay
element 80 may be displaced with a predetermined periodic velocity
profile (such as a sine wave, saw tooth, or arbitrary wave form)
though the distance over which optical path delay element 80 must
be sufficient to determine the position of imager plane 18.
In operation, the first and second non-coherent light signals traveling
along single mode optical fibers 68 and 74, respectively, are reflected
back to 2.times.2 optical coupler 66 (as the reference signal and
the delay signal, respectively) where they recombine and interfere
with each other to form a non-coherent light interference signal.
A portion of the recombined reference signal and delay signal is
directed into a photodetector 86 by a single mode optical fiber
87. The analog output of photodetector 86 is amplified and filtered
by an electronic module 88, and then sampled, digitized, and analyzed
by computer 38.
Note that a first and second branch of non-coherent light interferometer
36 is defined as the respective path lengths between the location
at which 2.times.2 optical coupler 66 splits the light signal into
two beams and the location at which the two beams are recombined
and made to interfere with each other. In FIG. 6, the first branch
is referred to as the stationary branch while the second branch
is referred to as the movable branch.
In typical operation, the coherent light interference signal is
utilized to sample the non-coherent light interference signal at
constant intervals of common variable optical path delay element
80's displacement. Applicants note that alternative configurations
for non-coherent light interferometric measurement apparatus 21
are possible. For example, a constant velocity common variable optical
path delay element may replace coherent light interferometer 55.
Alternatively, non-coherent light interferometer 36 may not have
a stationary reference branch. Further, the two branches may be
arranged such that the path length of one branch increases while
the path length of the second branch decreases by a corresponding
amount, as illustrated in FIG. 7. As illustrated, a mirror 78' is
mounted on common variable optical path delay element 80. In each
alternative configuration, the function of the common variable path
delay element is to provide a relative path delay between the two
branches of the non-coherent light interferometer.
The reflections shown in FIG. 8 further explain the interferogram
illustrated in FIG. 9. FIG. 9 shows an optimized interferogram obtained
during a measurement of a digital camera's imager plane location.
During alignment of the optical probe, the interference signal is
optimized when the peak amplitudes of the interferogram are at a
maximum. Reflections occur at each optical interface in the focal
region of optical probe 42, that is, the locations of pellicle 48,
the front 52 and back 54 surface of plate 20, and at imager plane
18. The interferogram measures optical path, so the apparent depth
between front surface 52 and back surface 54 is a value of nt.
FIG. 10 illustrates locations wherein constructive interference
will occur during the measurement of the imager plane location.
A series of reflections are observed as the variable optical path
delay element is moved. Traces (a)-(j) show the optical signals
input into the two branches of the non-coherent light interferometer.
Traces (b)-(j) show distance delayed traces in the second branch
or movable branch of the non-coherent light interferometer (that
is, the second non-coherent light signal) as the optical path difference
between the two branches of the non-coherent light interferometer
monitonically increases from left to right. Trace (a) shows the
first non-coherent light signal which can be considered to be the
locations of the reflections observed in the stationary reference
branch of non-coherent light interferometer 36. The vertical dashed
lines in FIG. 10 emanating from positions of optical reflections
in the stationary reference branch of non-coherent light interferometer
36 and represent reference markers where constructive interference
will occur. Constructive interference will occur when the pulse
trains of the two branches have peaks which are aligned vertically
as illustrated. The nine locations of constructive interference
shown in Traces (b)-(j) are sequential and define the zero path
delay condition (Trace f) and the closest four surrounding locations
of constructive interference. These locations are symmetrical around
the zero path delay condition. Trace (b) of FIG. 10 illustrates
the constructive interference wherein common variable optical path
delay element 80 is at a position -PG.sub.depth, and pellicle 48
interferes with the reflection from front surface 52 of plate 20.
Similarly, Trace (c) illustrates the constructive interference wherein
common variable optical path delay element 80 is at a position -(GS.sub.depth
+nt) and front surface 52 of plate 20 interferes with the reflection
from imager plane 18. Correspondingly, in Trace (d), constructive
interference occurs when common variable optical path delay element
80 is at a position -(nt) and front surface 52 of plate 20 interferes
with back surface 54 of plate 20. Trace (e) shows the location of
constructive interference at location -GS.sub.depth in which back
surface 54 of plate 20 interferes with imager plane 18. Trace (f)
shows the condition of zero path delay in which all reflections
from the two branches of the non-coherent light interferometer constructively
interfere with each other. This results in the largest intensity
amplitude peak of the interferogram as illustrated in FIG. 9. Continuing
to increase the optical path delay of the second branch of the non-coherent
light interferometer results in Traces (g)-(j) occurring at +GS.sub.depth,+
nt,+GS.sub.depth +nt and PG.sub.depth respectively. These locations
are symmetric with respect to the zero path delay condition and
are due to the same set of reflections but traveling down opposite
branches of the non-coherent light interferometer than those in
Traces (b)-(e). The autocorrelation spectrum is in general symmetrical
about the origin.
In FIG. 9, the large peak at the left of the figure is the self
correlation peak at the location where the path difference between
the two branches in the non-coherent light interferometer equals
zero. The first set of doublets (moving from left to right in FIG.
8) is due to interstice 22 (i.e., the gap between back surface 54
of plate 20 and imager plane 18) and the optical path of plate 20
(nt), respectively. The second set of doublets (continuing to the
right in FIG. 9) occurs when the path difference between the two
branches in the non-coherent light interferometer equals GS.sub.depth
+nt and PG.sub.depth, respectively. Intermediate the second and
third set of doublets, motor 82 changes directions and the third
set of doublets is due to PG.sub.depth and GS.sub.depth +nt respectively.
The fourth set of doublets is due to nt and GS.sub.depth respectively.
After the fourth set of doublets the interferometer crosses the
zero path delay condition, resulting in the second large peak. After
passing the zero path delay location, motor 82 is made to switch
directions again and another zero path delay peak occurs (the third
large peak from the left of FIG. 9). The pattern then repeats in
the right-hand side of the figure. Accordingly, from this interferogram,
the following information can be determined: the depth from the
reference surface to front surface 52 of plate 20, the optical thickness
of plate 20, and the depth from back surface 54 of plate 20 to imager
plane 18.
In a preferred embodiment, measurements are performed at 20 Hz,
the reference surface is selected as lens mount 14 (i.e., Reference
Plane A), and, in a particular camera body 15, the focal depth F.sub.depth
of the lens from the reference surface is determined to be 44 mm.
The value of Delta, dependent on the thickness and index of refraction
of plate 20, is determined to be between 242.5-277.1 microns for
a glass plate having an index of refraction of 1.5174 and a thickness
of about 30 mils +/-2 mils. Accordingly, the in-focus position F.sub.I
of imager plane 18 relative to lens mount surface 14, accounting
for optically transparent plate 20, varies between about 44.2425
to about 44.2771 mm. Pellicle 48 of a 1.5 .mu.m polyester film is
mounted to optical probe assembly 24 so as to be located 40 mm relative
to lens mount 14 when optical probe assembly is locked to digital
camera body 18. Optical probe assembly 24 is removably mounted to
digital camera body 18. A one-second measurement time is standard,
and data for twenty measurement cycles are typically calculated.
Applicants have noted that the measurement reproducibility for fifty
mount/dismount measurement cycles has been better than 2.5 microns
for the measurement of the actual depth of the imager plane relative
to the reference surface.
A corresponding non-coherent light interferometer 36 having a standard
mode configuration is illustrated in FIG. 11. In this configuration,
the optical probe assembly is inserted into one of the interfering
branches of the non-coherent light interferometer, thus requiring
the length of optical fiber 26 to be substantially equal to optical
fiber 74.
The apparatus of the present invention may be employed in several
methods: (i) to determine the position of an imager plane of an
imager sensor within a camera body, (ii) verify that the position
of an imager plane of an image sensor is within a desired tolerance,
such as in-focus position F.sub.I, (iii) determine the difference
between the current position of the imager plane and the desired
position of the imager plane, and, optionally, moving the imager
plane to the desired position, and (iv) to position the imager plane
at a desired position. Translation means, either manual or mechanical,
may be employed to align the imager plane to the desired position.
Such translation means are well known to those skilled in the art.
As indicated above, non-coherent light interferometry may be employed
in several ways to these methods: in a standard mode configuration,
and in an autocorrelation mode. FIGS. 12 and 13 illustrate a method
flow chart incorporating an autocorrelation mode configuration,
while FIG. 14 illustrates a method flow chart incorporating a standard
mode configuration of a non-coherent light interferometer. In each
method illustrated in FIGS. 12 and 13, the apparatus of the present
invention is employed to determine the position of the imager plane
within a camera body (i.e., method (i) above). FIG. 12 illustrates
a method incorporating an interferometer having a stationary and
movable branch controllable by common variable optical path delay
element 80. FIG. 13 illustrates a method incorporating an interferometer
wherein common variable optical path delay element 80 causes the
path length of the first branch to increase in length while the
path length of the second branch to decrease by a corresponding
amount.
As illustrated in FIG. 12 (an autocorrelation mode configuration),
optical probe assembly 24 is mounted and locked to camera body 15
(step 200). Non-coherent light source 30 emits a continuous wave
non-coherent light signal incident on optical probe assembly 24
(step 202). A portion of the non-coherent light signal is transmitted
through pellicle 48 (step 204) to imager plane 18 (step 206). A
portion of the non-coherent light signal is reflected by pellicle
48, front surface 52, back surface 54, and imager plane 18 (step
208). These reflected signals are collected (step 210) and divided
into a first and second light signals (step 212). The first light
signal is applied to stationary reference reflector 70 (step 214)
wherein a portion of the first light signal is reflected by stationary
reference reflector 70 to form a reference signal which is collected
(step 216). The second light signal is applied to variable optical
path delay element 80 to form a delay signal (step 218) which is
collected (step 220). This application of the second light signal
to the variable optical path delay element causes the optical path
length of the second light signal to be varied when the variable
optical path delay element is displaced with a predetermined distance
or velocity profile (step 222). The delay signal and the reference
signal are combined to form a non-coherent light interference signal
(step 224) which is collected (step 226). The interference signal
is analyzed (step 228) to determine the depth from the reference
surface to front surface 52 of plate 20, the optical thickness of
optically transparent plate 20, and the depth from back surface
54 of plate 20 to imager plane 18 (step 230). From this information,
the position of imager plane 18 relative to the reference surface
can be calculated (step 232).
To analyze the non-coherent light interference signal (step 228),
the non-coherent light interference signal is sampled, digitized,
and stored in a data array. The stored array is then analyzed to
determine the depth from the reference surface to front surface
52, the optical thickness of optically transparent plate 20, and
the depth from back surface 54 to imager plane 18. From these values,
the position of the image sensor relative to the reference surface
can be determined using Equation 1. Knowing the focal depth of the
camera lens, the difference between the position of the image sensor
and the in-focus position can be determined, whereby image sensor
16 can be moved by the difference value to locate the imager plane
at the in-focus position.
As illustrated in FIG. 13 (an autocorrelation mode configuration),
optical probe assembly 24 is mounted and locked to camera body 15
(step 250). Non-coherent light source 30 emits a continuous wave
non-coherent light signal incident on optical probe assembly 24
(step 252). A portion of the non-coherent light signal is transmitted
through pellicle 48 (step 254) to imager plane 18 (step 256). A
portion of the non-coherent light signal is reflected by pellicle
48, front surface 52, back surface 54, and imager plane 18 (step
258). These reflected signals are collected (step 260) and divided
into a first and second light signals (step 262). The first and
second light signals are applied to variable optical path delay
element 80 (step 264) to form first and second delay signals, respectively,
(step 266). This application of the first and second light signals
to the variable optical path delay element causes the optical path
lengths to be varied when the variable optical path delay element
is displaced with a predetermined distance or velocity profile (step
268). The first and second delay signals are combined to form a
non-coherent light interference signal (step 270) which is collected
(step 272). The interference signal is analyzed (step 274) to determine
the depth from the reference surface to front surface 52 of plate
20, the optical thickness of optically transparent plate 20, and
the depth from back surface 54 of plate 20 to imager plane 18 (step
276). From this information, the position of imager plane 18 relative
to the reference surface can be calculated (step 278).
To analyze the non-coherent light interference signal (step 274),
the non-coherent light interference signal is sampled, digitized,
and stored in a data array. The stored array is then analyzed to
determine the depth from the reference surface to front surface
52, the optical thickness of optically transparent plate 20, and
the depth from back surface 54 to imager plane 18. From these values,
the position of the image sensor relative to the reference surface
can be determined using Equation 1. Knowing the focal depth of the
camera lens, the difference between the position of the image sensor
and the in-focus position can be determined, whereby image sensor
16 can be moved by the difference value to locate the imager plane
at the in-focus position.
FIG. 14 illustrates a method incorporating the standard mode configuration.
Optical probe assembly 24 is mounted and locked to camera body 15
(step 300). Non-coherent light source 30 emits a non-coherent light
signal incident on 2.times.2 optical coupler 66 (step 302) which
is divided into a first and second non-coherent light signal (step
304) traveling along optical fibers 26 and 74, respectively. The
first non-coherent light signal is transmitted through pellicle
48 (step 306) to imager plane 18 of image sensor 16 (step 308).
A portion of the first non-coherent light signal is reflected by
pellicle 48, front surface 52, back surface 54, and imager plane
18 (step 310). These reflected signals are collected and form an
object signal (step 312). The second non-coherent light signal is
applied to variable optical path delay element 80, forming a delay
signal (step 314), which is collected (step 316). Optical path delay
element 80 is displaced (step 318), and the delay signal and object
signal are combined to form a non-coherent interference signal (step
320). The corresponding interference signal is analyzed (step 322)
to determine the values of the depth from the reference surface
to front surface 52, the optical thickness of optically transparent
plate 20, and the depth from back surface 54 to imager plane 18
(step 326). From this information, the position of imager plane
18 relative to the reference surface can be calculated (step 328).
To analyze the non-coherent light interference signal (step 324),
the non-coherent light interference signal is sampled, digitized,
and stored in a data array. The stored array is then analyzed to
determine the depth from the reference surface to front surface
52, the optical thickness of optically transparent plate 20, and
the depth from back surface 54 to imager plane 18. From these values,
the position of the image sensor relative to the reference surface
can be determined using Equation 1. Knowing the focal depth, the
difference between the position of the image sensor and the focal
depth can be determined, whereby image sensor 16 can be moved by
the difference value to locate the imager plane at the in-focus
position.
Additional steps may be required to accomplish the other methods
described above. Specifically, to verify that the position of an
imager plane of an image sensor is within a desired tolerance (i.e.,
method (ii) above), an additional calculation step of determining
whether the position of the imager plane is within the desired tolerance
is required. To determine the difference between the current position
of the imager plane and the desired position of the imager plane
(i.e., method (iii) above), an additional calculation step of Equation
6 is employed. to position the imager plane at a desired position
(i.e., method (iv) above), the difference between the current position
and the desired position of the imager plane is determined, and
means are provided to move the imager plane by the difference.
While the above discussion refers to a single measurement location,
a plurality of locations on the surface of image sensor 16 may be
measured to provide (a) an indication of parallelism of imager plane
18 with respect to a reference surface and/or plate 20, and (b)
a measure of the degree of flatness or bowing of imager plane 18,
thereby providing spatial information about the focal position of
the image sensor. For example, FIG. 15 illustrates the use of a
1.times.N optical switch 120, configured as a 1:5 optical switch,
which allows five locations to be measured by means of a five measurement
location fixture containing five individual optical probes mounted
in a single optical probe assembly. The depth direction is defined
as the z-axis. Each of the five measurement locations in the x,y
plane (i.e., the locations of each of the optical probes) are measured
sequentially using optical switch 120. Operation of the multiple-location
measurement apparatus is similar to the single-location measurement
apparatus described above with reference to FIGS. 12-14. However,
the measurement process is repeated for each location. For example,
in the instant case the measurement would be repeated five times
with a switch being toggled to each of the individual five measurement
locations. Additional information is provided about the flatness
of the focal plane of the image sensor, the parallelism of imager
plane 18 to plate 20 and the reference surface, the thickness of
uniformity of plate 20, and the uniformity of interstice 22. A minimum
of three measurement locations is required to obtain information
about the parallelism of the imager plane relative to the reference
surface.
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