Digital Camera Patent Abstract
A compact digital camera is formed with a lenslet array comprised
of a plurality of lenslets, each lenslet has a decentration corresponding
to its radial position in the lenslet array so that the axial ray
of each lenslet views a different segment of a total field of view.
A photosensor array comprised of a plurality of sub-groups of photodetectors
is positioned such that each sub-group is located along the axial
ray of a respective lenslet. A field limiting baffle comprised of
at least one aperture plate is positioned such that the center of
the apertures are located along the axial ray of a respective lenslet.
Digital Camera Patent Claims
I claim:
1. A compact digital camera, comprising:
a lenslet array comprised of a plurality of lenslets, each lenslet
having a decentration corresponding to its radial position in the
lenslet array so that the axial ray of each lenslet views a different
segment of a total field of view;
a photosensor array comprised of a plurality of sub-groups of photodetectors,
each sub-group comprising a plurality of photodetectors and being
positioned along the axial ray of a respective lenslet;
a plurality of electronic arrays dispersed in the spaces between
the spaced apart sub-groups of photodetectors, each of the arrays
configured to one of the sub-groups for receiving the provided output
signals from the sub-group that the array is associated with, the
array having electronics to amplify, digitize, invert, and store,
the provided output signals to facilitate image capture; and
a field limiting baffle comprised of at least one aperture plate
where the center of the apertures are positioned to lie along the
axial ray of a respective lenslet;
array electronics comprising:
a plurality of current multipliers, corresponding in number to
the number of sub-groups of radiation sensors, each connected to
a respective sub-group for receiving the outputs of the radiation
sensors in the sub-group;
a corresponding plurality of analog-to-digital converters, each
connected to an associated current multiplier;
a corresponding plurality of inverters for digitally inverting
the sub-images from said analog-to-digital converters; and
a corresponding plurality of storage devices connected to receive
the digitally inverted sub-images from corresponding inverters for
storing the provided digitally inverted sub-images.
2. A compact digital camera, according to claim 1 wherein said
lenslets have an a spherical surface.
3. A compact digital camera according to claim 2 wherein said lenslets
additionally have an a spherical surface whose profile varies as
a function of its radial position in the lenslet array.
4. A compact digital camera, comprising:
a lenslet array comprised of a plurality of lenslets, each lenslet
having a decentration corresponding to its radial position in the
lenslet array so that the axial ray of each lenslet views a different
segment of a total field of view;
a color photosensor array comprised of a plurality of sub-groups
of photodetectors wherein each sub-group of photodetectors is comprised
of sets of Red, Green, and Blue color sensitive photodetectors,
each sub-group of photodetectors being positioned along the axial
ray of a respective lenslet;
a plurality of electronic arrays dispersed in the spaces between
the spaced apart sub-groups of photodetectors, each of the arrays
configured to one of the sub-groups for receiving the provided output
signals from the sub-group that the array is associated with, the
array having electronics to amplify, digitize, invert, and store,
the provided output signals to facilitate image capture; and
a field limiting baffle comprised of at least one aperture plate
where the center of the apertures are positioned to lie along the
axial ray of a respective lenslet;
array electronics comprising:
a plurality of current multipliers, corresponding in number to
the number of sub-groups of radiation sensors, each connected to
a respective sub-group for receiving the outputs of the radiation
sensors in the sub-group;
a corresponding plurality of analog-to-digital converters, each
connected to an associated current multiplier;
a corresponding plurality of inverters for digitally inverting
the sub-images from said analog-to-digital converters; and
a corresponding plurality of storage devices connected to receive
the digitally inverted sub-images from corresponding inverters for
storing the provided digitally inverted sub-images.
5. A compact digital camera, according to claim 4 and further comprising:
a plurality of baffles extending between each lenslet and a corresponding
sub-group of photodetectors so as to prevent stray light from one
lenslet impinging on one or more of the photodetectors in an adjacent
sub-group of photodetectors.
6. A compact digital camera according to claim 2 wherein each of
said lenslets has a surface that is a diffractive/refractive hybrid
to eliminate chromatic aberration.
7. A compact digital camera according to claim 6 wherein said lenslets
have a diffractive/refractive surface whose profile varies as a
function of its radial position in the lenslet array.
8. A compact image capture device comprising:
an array of spaced apart sub-groups of radiation sensors for providing
output signals that arc a function of the incident radiation from
an image onto each radiation sensor of a sub-group, each sub-group
having a plurality of radiation sensors;
a plurality of electronic arrays dispersed in the spaces between
the spaced apart sub-groups of radiation sensors, each of the arrays
configured to one of the sub-groups for receiving the provided output
signals from the sub-group that the array is associated with, the
array having electronics to amplify, digitize, invert, and store,
the provided output signals to facilitate image capture;
a lens array positioned so as to focus the radiation of an image
to be captured onto said radiation sensors;
array electronics comprising:
a plurality of current multipliers, corresponding in number to
the number of sub-groups of radiation sensors, each connected to
a respective sub-group for receiving the outputs of the radiation
sensors in the sub-group;
a corresponding plurality of analog-to-digital converters, each
connected to an associated current multiplier;
a corresponding plurality of inverters for digitally inverting
the sub-images from said analog-to-digital converters; and
a corresponding plurality of storage devices connected to receive
the digitally inverted sub-images from corresponding inverters for
storing the provided digitally inverted sub-images.
9. The compact image capture device according to claim 8 further
comprising:
a plurality of multiplexing means each connecting the radiation
sensors of an associated sub-group to an associated current multiplier.
10. The compact image capture device according to claim 8 and further
comprising:
an associated multiplying current mirror for each of the sub-groups;
a plurality of multiplexing means each connecting the radiation
sensors of a sub-group to the associated multiplying current mirror.
11. The compact image capture device according to claim 8 and further
comprising:
means for stitching together the sub-image pixel signals from each
sub-group of radiation sensors so as to form a group of pixel signals
that may be used to replicate the captured image.
12. An image capturing device comprising:
an array of spaced apart groups of radiation sensors for providing
output signals that are a function of the color of incident radiation
from an image onto each radiation sensor;
a plurality of electronic arrays dispersed in the spaces between
the spaced apart groups of radiation sensors, each of the arrays
configured to one of the groups for receiving from the group that
the array is associated with, the array having electronics for amplifying,
digitizing, inverting, and storing the provided output signals;
and
a lens array positioned so as to focus the radiation from an image
to be captured onto associated ones of the radiation sensors in
each of the groups of said radiation sensors such that each radiation
sensor in a group provides an output signal that is stored as a
function of the color of the sensed radiation;
array electronics comprising
a plurality of current multipliers, corresponding in number to
the number of sub-groups of radiation sensors, each connected to
a respective sub-group for receiving the outputs of the radiation
sensors in the sub-group;
a corresponding plurality of analog-to-digital converters, each
connected to an associated current multiplier;
a corresponding plurality of inverters for digitally inverting
the sub-images from said analog-to-digital converters; and
a corresponding plurality of storage devices connected to receive
the digitally inverted sub-images from corresponding inverters for
storing the provided digitally inverted sub-images.
13. A compact digital camera, comprising:
a lenslet array comprised of a plurality of lenslets, each lenslet
having a decentration corresponding to its radial position in the
lenslet array so that the axial ray of each lenslet views a different
segment of a total field of view;
a photosensor array comprised of a plurality of spaced apart sub-groups
of photodetectors such that there are a plurality of photodetectors
associated with each sub-group, each sub-group positioned along
the axial ray of a respective lenslet for providing output signals
as a function of incident light;
a plurality of electronic circuits formed in the space between
the spaced apart sub-groups, each of the electronics circuits being
dedicated to one of the sub-groups and connected to receive, amplify,
and invert the output signals provided by the photodetectors of
that respective sub-group;
a field limiting baffle comprised of at least one aperture plate
having a plurality of apertures, corresponding in number to the
number of lenslets, where the center of each aperture is positioned
to lie along the axial ray of a respective lenslet;
means for receiving the amplified and inverted output signals from
said electronic circuits and for stitching the output signals together
so as to enable the viewing of a continuous image when the stitched
output signals are used to drive an output display device;
array electronics comprising:
a plurality of current multipliers, corresponding in number to
the number of sub-groups of radiation sensors, each connected to
a respective sub-group for receiving the outputs of the radiation
sensors in the sub-group;
a corresponding plurality of analog-to-digital converters, each
connected to an associated current multiplier;
a corresponding plurality of inverters for digitally inverting
the sub-images from said analog-to-digital converters; and
a corresponding plurality of storage devices connected to receive
the digitally inverted sub-images from corresponding inverters for
storing the provided digitally inverted sub-images.
14. The compact digital camera according to claim 13 herein said
electronic circuits further comprise:
an inverter for inverting the output signals from said photosensor
array.
15. The compact digital camera according to claim 13 and further
comprising:
storage means formed in the space between the spaced apart sub-groups
of photodetectors for storing the amplified and inverted output
signals from said electronic circuits.
Digital Camera Patent Description
FIELD OF THE INVENTION
The invention relates generally to the field of digital cameras
and more particularly to an improved image sensor with a lenslet
array, having a smaller number of lenslets and a larger number of
photodetectors per lenslet than here-to-fore used, along with associated
electronics being inserted in the unused spaces between spaced apart
sub-groups of photodetectors.
BACKGROUND OF THE INVENTION
Lenslet arrays have been used to concentrate light, imaged on a
photodetector plane by a photographic objective, into smaller areas
to allow more of the incident light to fall on the photosensitive
area of the photodetector array and less on the insensitive areas
between the pixels. This has been described in papers such as "Visible
Array Detectors" by Timothy J. Tredwell, from Handbook of Optics,
Vol. 1, Fundamentals Techniques, & Design, Second Edition, Chapter
22, pp. 32-34. These lenslet arrays are centered directly above
the corresponding photosensor and are not designed to look at different
portions of the field of view independently. Rather, they concentrate
the light from an existing image, formed by the photographic objective,
into the pixel aperture.
In U.S. Pat. No. 4,994,664, entitled, "Optically Coupled Focal
Plane Arrays Using Lenslets And Multiplexers" by Veldkamp,
an array of diffractive lenslets is used to concentrate incident
light onto an array of photosensors in order to allow for location
of amplifying circuitry in areas between photosensor sites. These
lenslets are centered over the photosensitive sites and are formed
on the opposite side of the photosensor array on a silicon substrate,
the use of the silicon substrate prevents them from imaging visible
light onto the photosensors since silicon does not transmit in the
visible wavelengths. That invention is not be able to work over
the visible wavelength range since the element is composed of all
diffractive optical power and suffers from severe chromatic aberrations.
In U.S. Pat. No. 5,233,174, entitled, "Wavefront Sensor Having
A Lenslet Array As A Null Corrector" by Zmek, teaches an array
of diffractive lenslets with decenters that are adjusted to eliminate
the local monochromatic wavefront tilt from a specific optic under
test in an interferometric or Hartman type test. A Hartman test
is used to certify the surface quality of various optics. If the
optics under test falls within the acceptance criteria, the wavefront
incident on the sensor array will form centered light spots on predetermined
pixels. If the wavefront is other than the desired wavefront, the
light spots will be incident on different pixel elements. That invention
is not applicable to the current invention since the centered lenslets
are not looking at angularly displaced sections of a larger field
of view. It is also not applicable to white light applications due
to the chromatic aberrations of the diffractive lenslets.
U.S. Pat. No. 5,340,978, entitled, "Image-Sensing Display
With LCD Display Panel And Photosensitive Element Array" Rostoker
et al., briefly describes an array of decentered lenses which form
an image of a segment of the field of view. These lenses are widely
separated and do not include a method for limiting a field of view
seen by a group of pixels. The use of the widely separated pixels
will greatly increase costs of the sensor since there will be fewer
sensor arrays fabricated on a given size substrate or wafer which
will cause a decreased yield of finished sensor arrays for a given
manufacturing process. There is no discussion of the trade-off between
the focal length of the lens array and the angular subtense of the
pixel's field of view. If the focal length of the lenslets is too
short, light from one angular location which is incident on one
group of pixels will also be within the field of view of an adjacent
group. For very short focal lengths the corresponding pixel dimension
required for an equivalent angular resolution will be so small as
to not be able to be fabricated with lithographic processes. If
the pixel dimension is reduced too much the light gathering area
of the pixel will be so small as to not generate a reliably measurable
number of electrons for a given incident intensity. Rostoker does
not envision the use of diffractive/refractive hybrids for achromatization.
That patent uses an array of uniformly shaped lenslets, while in
the present invention the utility of varying the surface profile
of the lenslets as a function of their radial position in the lenslet
array allows for a higher level of aberration correction at any
given location. In the current invention the lenses are abutted
to each other and an opaque baffle is placed over the photodetector
to limit the field of view of each pixel. The referenced patent
uses one lenslet per group of three color pixels. In the current
invention it is shown to be advantageous to form a small array of
pixels for each lenslet if the focal length of each lenslet is adjusted
appropriately (increased).
The invention disclosed in U.S. Pat. No. 5,471,515, to Fossum,
et. al., entitled "Active Pixel Sensor with Intra-Pixel Charge
Transfer," converts the photogenerated charge stored under
the photogate of a semiconductor photosensor into a voltage by transferring
the charge to a sense node (typically a capacitor) located within
the active pixel unit cell. Fossum then utilizes dual sample correlated
double sampling of the voltage based signal to reduce signal noise
and eliminate the effect of dark current from the photosensor. The
voltage associated with the image exposure is then subtracted from
the voltage associated with a read during a dark sample by a voltage
differencing amplifier located at the end of the row or column of
the photosensors. By using appropriate row and column select data
lines, a subsection of the array can be read out without the need
to read out the entire image array. The Fossum invention does not
however enable an increase in the overall sensitivity of the photosensor
(CCD detector) elements, nor does it envision the utilization of
an array optic type structure to form an image of different segments
of a field of view, although the patent does disclose the use of
a lens array for concentrating light on the active pixel. Fossum
is performing most of the signal processing in a voltage amplification
mode, whereas the present invention utilizes the advantages of a
current mode signal processing. In addition, the present invention
provides for the digitization and storage of the digital image data
at each photosensor site.
In U.S. Pat. No. 5,004,901, entitled "Current Mirror Amplifier
for use in an Optical Data Medium Driving Apparatus and Servo Circuit"
by Yoshimoto, et. al., a photogenerated current from an optical
disk tracking and read sensor is amplified in fixed steps by a switchable
series of current mirrors where the current mirrors achieve current
multiplication through the use of output stages that incorporate
either multiple output transistors with the bases of the output
transistors connected in parallel, or by the use of output transistors
with emitter areas that are integral multiples of the emitter areas
of the input side transistor. The purpose of Yoshimoto's invention
is to allow the utilization of received photocurrents with a large
dynamic range by multiplying the input current by an adjustable
ratio where the multiple current ratios are selected through a switchable
network of differential amplifiers. Yoshimoto's invention is not
related to the field of array image sensors and requires the use
of a switchable array of differencing amplifiers. Yoshimoto's invention
does not integrate the current from the photosensor and the current
is continuously generated by received light from the laser light
emitted by the optical disk head. The sensor is not exposed to an
image as in the current invention, but is used in a continuous optical
disk position monitoring mode. Yoshimoto does not utilize dual slope
correlated double sampling for noise reduction as disclosed in the
present invention. Yoshimoto does not make any mention of the use
of array optics with a field of view which varies as a function
of radial position in the sensor array.
SUMMARY OF THE INVENTION
The present invention is directed to overcoming one or more of
the problems set forth above. Briefly summarized, according to one
aspect of the present invention there is provided a compact digital
camera, comprising:
a lenslet array comprised of a plurality of lenslets, each lenslet
having a decentration corresponding to its radial position in the
lenslet array so that the axial ray of each lenslet views a different
segment of a total field of view;
a photosensor array comprised of a plurality of sub-groups of photodetectors,
each sub-group positioned along the axial ray of a respective lenslet;
and
a field limiting baffle comprised of at least one aperture plate
where the center of the apertures are positioned to lie along the
axial ray of a respective lenslet.
ADVANTAGEOUS EFFECT OF THE INVENTION
The present invention has the following advantages:
An extremely compact digital camera with a lenslet array in close
proximity to a photodetector array is formed. Due to the larger
sub-image size a reduced number of lenslets are needed to construct
the full image. By utilizing the space between sub-groups of photodetectors
for signal processing electronics, the digital camera can be formed
on a single substrate. In addition, a large high resolution sensor
can be synthesized by the use of sub-groups of photodetectors.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are, a top view of lens contours and baffles for
the lens array, and a block diagram of a photodetector array incorporating
the unit pixel subassemblies and associated memories at each site,
and an exploded view of a portion of the arrays of FIG. 1A, respectively;
FIG. 2 is a sectioned view taken along the section lines A--A of
the lens array of FIG. 1A;
FIGS. 3A, 3B, and 3C, illustrate in cross section a central lenslet
and the associated tangential and sagittal ray aberration curves,
respectively;
FIGS. 4A, 4B, and 4C, illustrate in cross section a lenslet at
a 16 degree field angle and the associated tangential and sagittal
ray aberration curves, respectively;
FIGS. 5A, 5B, and 5C, illustrate in cross section a lenslet at
a 26 degree field angle and the associated tangential and sagittal
ray aberration curves, respectively;
FIG. 6 is a schematic diagram of one unit photodetector element
of a sub-group connected to common circuits and used in a device
such as a camera to capture an image;
FIG. 7 is a circuit diagram of an alternate embodiment of common
circuits that may be used with the unit photodetector elements forming
a sub-group of photodetectors;
FIG. 8 is a circuit diagram of another alternate embodiment of
common circuits that may be used with the unit photodetector elements
forming a sub-group of photodetectors; and
FIG. 9 is a pictorial representation of the operation of a portion
of the compact digital camera with segmented fields of view.
To facilitate understanding, identical reference numerals have
been used, where possible, to designate identical elements that
are common to the figures.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIGS. 1A and 1B, a lenslet array 10 is formed with
an array of achromatized refractive/diffractive lenslets 12 or refractive
lenslets which are centered over sub-groups 22 of photodetector
24 formed into a photosensitive array 20 (shown more clearly in
the section view of FIG. 2). To be observed in FIG. 1A is that the
center of the mechanical optical axis 14 of each lenslet 12 is displaced
relative to the fixed sensor-to-sensor distance as a function of
its radial distance from the optical axis of the central lenslet.
The lines 15 appearing around the mechanical optical axis 14 of
each lenslet 12 are topographical lines indicating changes in height
of the lenslet's surface. An opaque mask 16 fills the areas between
the lenslets 12 to prevent light that would by-pass the lenslets
from reaching the photosensitive array 20. The lenslet array 10,
depicted in FIG. 1A represents only a small portion of an array
that will be used in an actual camera. In an actual implementation
approximately 150.times.100 lenslets are used to form the lenslet
array 10 each having an associated sub-group of photodetectors 22
formed with 10.times.10 photodetector elements 24.
In order for the lenslet array 10 to see different fields of view,
the optical axis 14 of the lenslets 12 in the lenslet array 10 are
located at a distance which becomes progressively larger than the
center-to-center distance of the sub-groups of photodetectors 22
in the photosensor array 20. The displacement of the lenslets optical
axis 14 increases radially from the center of the lenslet array
10. Decentering a lenslet's optical axis tends to bend rays from
off-axis field angles into the center of the lenslet's field of
view. By moving the optical axis of the lenslet further out radially
with increasing distance from the center of the lenslet array 10,
the angular location of an object at the center of the field of
view for a given lenslet/sub-group of photodetectors originates
from increasingly off-axis segments of the total field of view (shown
more clearly in FIG. 2).
For instance, the required decenter for a lenslet of focal length
FL.sub.i necessary to deflect a ray from the desired field angle
into the center of the lenslets associated sub-group of photodetectors
22 can be determined from the paraxial ray tracing equations. The
paraxial equations are
where
y'=height after propagation to next surface
y.sub.o =height at previous surface
u=paraxial slope angle (radians)
u.sub.o =slope angle before refraction
.phi.=power of lenslet (.phi.=1/f)
n=refractive index of the lenslet's medium
Therefore, the displacement d for a given lenslet with power .phi.
(where .phi.=1/f) that is necessary to bend the central ray from
a given angle of
incidence u.sub.o to a desired angle u', after refraction is given
by
where f=focal length of the lenslet
The invention utilizes an array of lenslets where the local displacement
d of each lenslet's optical axis varies as a function of its radial
position r relative to the optical axis of the lenslet positioned
in the center of the lenslet array; so that, to first order
The invention consists of adjusting the lenslet decenters so that
u' (r)=0 for the central ray within a given lenslet's field of view
(see FIG. 2). In this case the decenter necessary for a given lenslet
is approximately a linear function of the lenslet's radial distance
from the center lenslet's optical axis.
Each sub-group of photodetectors 22 may be provided with associated
array electronics dispersed in the open spaces on or in a supporting
substrate 100. The array electronics may contain a multiplexing
and clocking circuit 80, current mirror 30, a correlated double
sampling circuit 40, an analog-to-digital converter 56, a sub-image
pixel inverter 48, and a non-volatile memory cell 58. Each sub-group
of photodetectors 22 is attached to a data bus 50 in response to
signals from a row address decoder 60 and a column address decoder
62. FIG. 1B illustrates a preferred positioning of green, blue,
and red photodetector elements 24 for a color sensitive array.
Referring now to the cross-section view of FIG. 2, the lenslet
array 10 is positioned over a photosensor array 20 that may be a
CCD array having a number of sub-groups of photodetectors 22 corresponding
in number to at least the number of lenslets 12 forming the lenslet
array 10. In a three color environment up to one third of the photodetector
elements 24 in each sub-group of photodetectors 22 would be provided
with a given color filter (red, green, or blue). The lenslet array
10 is maintained a distance apart from the surfaces of the photodetector
elements 24 by spacers 70 that may also serve the function of being
baffles. The opaque masks 16 on the lenslet array 10 combined with
a field stop aperture plate 72 limits the field of view of any particular
sub-group of photodetectors 22 so that it does not overlap the field
of view of it neighbors by a large amount. The field stop aperture
plate 72 is positioned approximately 0.5 mm to 3 mm from the surface
of the lenslet array 10. The field stop aperture plate 72 may be
a layer of clear glass having an opaque mask pattern formed on one
of it's surfaces.
The center of the apertures in the aperture plate 72 are aligned
to the center of the field of view of a corresponding lenslet. The
spacing of the centers increases as a function of each lenslet's
radial position radially from the center of the lenslet array causing
the aperture plate to be slightly larger than the associated lens
array. The combination of the opaque masks 16 with the aperture
plate 72 and a given lenslet focal length determines the field of
view for a sub-group of photodetectors 22. The lenslet array 10
can be formed of etched quartz, injection molded plastic, or an
epoxy replica on a glass substrate.
The lenslets 12, combined with the appropriate field stop aperture
plate 72, form images of a small segment of the field of view on
each sub-group of photodetectors 22. By forming the lenslets 12
with decentrations of the mechanical optical axis 14 which increase
radially across the CCD array, the angle the axial ray (which is
incident on a respective sub-group of photodetectors), makes with
the surface normal to the photosensor array 20 will increase as
a function of the radial position of a sub-group of photodetectors
22 with respect to the center of the photosensor array 20. Therefore,
appropriately adjusting the decenters of each lenslet associated
with a sub-group of photodetectors will allow the sub-group of photodetectors
to view a different segment of a scene. The sub-images from each
sub-group of photodetectors will be digitally inverted and stitched
with the other sub-images to form a composite image as will be explained
in the discussion of FIG. 9. Therefore, any camera system incorporating
the present invention can be extremely compact and flat. The camera
can work in black and white or in color if color filters are used
on an appropriate number of photodetector elements in the sub-group
of photodetectors.
An array of refractive a spheric lenslets can also be used to form
images on the photosensor array. However, the aforementioned embodiment
does not correct for the variation in focal length as a function
of wavelength since the lens element is formed from a single refractive
material, therefore the spot size of the incident light varies as
a function of color. An improved optical design, including a diffractive/refractive
hybrid lens is used to correct the chromatic aberration in a single
material.
The imaging properties of diffractive optics are strongly wavelength
dependent. When modeling a diffractive optic this phenomena can
be represented as a direct dependence of equivalent refractive index
on wavelength:
Diffractive elements impart all of their wavefront bending in an
extremely thin layer. This is accounted for in the Sweat model by
modeling the diffractive surface as a very high index material (n.sub.c=
10,000) with very low curvatures (weak surfaces) on each surface.
The corresponding focal length can then be determined from:
so that
where .lambda..sub.c =design center wavelength
The Sweat model is disclosed in "Designing and Constructing
Thick Holographic Optical Elements," by W. C. Sweat, Applied
Optics/vol. 17, No. 8, Apr. 15, 1978, pp. 1220-1227.
The resultant dispersion .nu..sub.diff of the diffractive element
is: ##EQU1## which reduces to: ##EQU2## For designs using: ##EQU3##
For other wavelength bands of interest an appropriate .nu..sub.diff,
and element power distribution can be calculated. The direct dependence
of the equivalent refractive index on wavelength leads to a small,
negative .nu..sub.diff, and a high level of wavelength dispersion
associated with a first order (m=1) diffractive optical element.
Due to the variation of refractive index with wavelength, a single
element lens has a variation of focal length with wavelength. Two
materials with different dispersions can be used to form a doublet
lens which has the same focal length at two wavelengths and reduced
variation over the whole spectrum. The relative distribution of
focal powers required to achieve this is given by ##EQU4##
The negative .nu..sub.diff of the diffractive surface allows for
achromatization of single element hybrid refractive/diffractive
lenses utilizing a positive focal length diffractive and refractive
component. It also decreases the focal length and F/# required of
the doublet component lenses, because a refractive doublet consists
of a positive crown (low dispersion) lens with a shorter focal length
and smaller F/# than is required for a singlet with an equivalent
focal length, and a negative flint element which increases the doublet
focal length to the correct value and compensates for the dispersion
of the positive lens. This effect also decreases the size and weight
of the positive element of a hybrid refractive/diffractive element.
For traditional achromatization in the visible spectrum (d-e-f
lines) with PMMA plastic lenses, the fraction of power in the refractive
and diffractive portions would be
It is obvious to a person skilled in the art that one can fabricate
the lens array as a quartz master, an epoxy replica on a glass substrate,
or as an injection molded part.
The use of diffractive/refractive hybrid achromatization enables
the use of diffractive surfaces with longer focal lengths and higher
F/#'s. The high F/# makes the diffractive element easier to manufacture
due to the large zone spacings. For instance, for a lens with FL=3.0
mm and a F/#=2.0 the first order analysis of he refractive and diffractive
portions would provide focal lengths and F/#'s of ##EQU5## Assuming
the diffractive portion of the lenslet was used purely for first
order chromatic correction.
The diffractive lenslets 12 are shown in FIGS. 3A, 4A, and 5A,
for the on-axis, 16 degrees, and full field of view. To be noted
in a comparison of these figures is that the mechanical optical
axis 14 is moving radially outwards as a function of field angle
while the local optical axis 18 is incident normal to the plane
of the photosensor array 20. To be noted is that the opaque mask
16 acts as the aperture stop for the system as well as limiting
the field of view of a given photosensor
FIGS. 3B, 3C, 4B, 4C, 5B, and 5C, represent the tangential and
sagittal ray aberration curves for their respective lenslets. The
solid lines represent a wavelength of 546.1 nm, the dotted lines
represent a wavelength of 656.1 nm, and the dotted and dashed lines
represent a wavelength of 460.0 nm which represent light in the
green, red, and blue, wavelengths.
The focal length of the lenslet must be long enough so that when
an image is formed on a given number of photodetector elements the
angular resolution in the sub-group of photodetectors is comparable
to the angular resolution achieved by prior art digital cameras.
This establishes the lower bounds for the focal length of the lenslet
in the 2-5 .mu.m range for pixels of 3-10 microns in width.
The following table lists the optical design parameters for three
lenslets with an axial ray incident at 0, 16, and 24 degrees. Other
lenslet surface profiles at intermediate field angles are derived
by interpolating surface data between the profiles at these field
angles.
EXAMPLE A
0 Degree Field Lens
EFL=3.0 mm
F/#=2.0
______________________________________ Surface No. Radius Thickness
Glass ______________________________________ Object Infinity Air
1 Aperture 2.0 BK7 Stop 2 -1.639 3.0 Air Aspheric k = 0.973 Diffractive
DF3 = -8.2 .times. 10.sup.-3 Coeff. AD = 1.04 .times. 10.sup.-1
Coeff. DF5 = -8.4 .times. 10.sup.-3 AE = -3.35 .times. 10.sup.-2
Element DF10 = -8.3 .times. 10.sup.-5 AF = 1.17 .times. 10.sup.-1
Decenter DF14 = 1.4 .times. 10.sup.-3 AG = -7.75 .times. 10.sup.-2
DC = 0.0 DF21 = 4.5 .times. 10.sup.-4 DF27 = -2.3 .times. 10.sup.-3
Image Air ______________________________________
EXAMPLE B
16 Degree Field Lens
5 EFL=3.0 mm
F/#=2.0
______________________________________ Surface No. Radius Thickness
Glass ______________________________________ Object Infinity Air
1 Aperture 2.0 BK7 Stop 2 -1.602 3.0 Air 2 k = -0.670 Diffractive
DF3 = -4.8 .times. 10.sup.-4 Aspheric AD = -7.0 .times. 10.sup.-3
Coeff. DF5 = -4.7 .times. 10.sup.-3 Coeff. AE = 2.1 .times. 10.sup.-2
DF10 = 1.4 .times. 10.sup.-2 AF = -5.9 .times. 10.sup.-3 DF14 =
-3.6 .times. 10.sup.-3 AG = 6.5 .times. 10.sup.-4 DF21 = -5.2 .times.
10.sup.-3 DF27 = 6.2 .times. 10.sup.-4 Element DC = -0.513 Decenter
Image Air ______________________________________
EXAMPLE C
24 Degree Field Lens
EFL=3.0 mm
F/# =2.0
______________________________________ Surface No. Radius Thickness
Glass ______________________________________ Object Infinity Air
1 Aperture 2.0 BK7 Stop 2 -1.509 3.0 Air Aspheric k = -0.570 Diffractive
DF3 = 7.8 .times. 10.sup.-3 Coeff. AE = -4.6 .times. 10.sup.-3 Coeff.
DF5 = 5.2 .times. 10.sup.-3 AF = 9.5 .times. 10.sup.-3 DF10 = 1.0
.times. 10.sup.-2 AG = -1.2 .times. 10.sup.-3 DF14 = -6.3 .times.
10.sup.-2 AH = 1.4 .times. 10.sup.-4 DF21 = -2.9 .times. 10.sup.-3
DF27 = 7.3 .times. 10.sup.-4 Element DC = -0.872 Decenter Image
Air ______________________________________
where the a spheric surface profile is defined by ##EQU6## and
the diffractive phase profile is defined as ##EQU7## where
Although the lenslet array can operate at low F/#'s (F/2.0), it
is advantageous to minimize the lenslet's diameter (increasing F/#)
so as to minimize the space between lens elements and therefore
the image sensor's overall dimensions. If one can achieve higher
photo-signal gain with local signal processing electronics, lenslets
with smaller numerical apertures can be used to generate the same
level of photocurrent. This is accomplished by utilizing a multiplying
current mirror.
The illumination incident on the photodetector array from a given
lenslet is proportional to the (F/#).sup.2. Therefore, if the sensitivity
is increased by x, the F/# can be reduced by x.sup.1/2. For instance,
if an array optic camera, without a multiplying current mirror,
is used with a lenslet having a F/#=4.0 and a FL=3.0 mm, the lenslet's
diameter would be 750 .mu.m. Therefore, if an array optic camera
is formed with 780 by 640 pixels the length of the long dimension
of the array would be 58.5 mm, assuming an array of 10.times.10
photodetector elements in each sub-array. This would require a large
area of silicon per photosensor array which would increase part
costs and result in lower yields of photosensors from a given wafer
size. By incorporating a current mirror with a multiplication factor
of 16 at each sub-array of photodetectors, the lenslet diameters
can be reduced by 4.times. to 187 .mu.m and the length of the array
will be reduced to 14.6 mm, resulting in higher photosensor yields
and lower part costs. A digital camera can utilize this technique
with little decrease in usable photosensitive surface areas since
the space between each sub-group of photodetectors is not utilized
for light detection. In optimizing the design of the camera, the
area between sub-groups of photodetectors can be minimized to allow
space only for the necessary support circuitry.
The extra area between the sub-groups of photodetectors may be
used for the incorporation of analog circuitry, sub-image pixel
inverters, and from 1 to n non-volatile memory cells. The larger
the number of memory cells, the larger the number of images that
can be stored. The use of appropriate memory (for instance, flash
memory) allows for the storing of image data even when the power
to the camera is turned off. It is also possible to use DRAM memory
at the photosites given that it is acceptable to include refresh
circuitry. DRAM memory can be constructed using less surface area
which allows for either a smaller distance between the sub-groups
of photodetectors or more area for the storage of more image data.
Referring to FIG. 6, the photodetector element 24 may incorporate
a CCD device and/or a photodiode. The output of the photosensitive
detector 64 is connected to a transfer gate 65 and to a reset gate
66. The output of the transfer gate 65 is connected to the current
mirror 30A through multiplexing and clocking circuitry 80. The transfer
gate 65 allows the charge, accumulated by the photosensitive detector
64 during the exposure period, to be transferred to the multiplying
current mirror 30A at the desired time by the multiplexing and clocking
circuit 80. When closed, the reset gate 66 allows for the emptying
of the photosensitive element's accumulated charge from the previously
completed exposure. When the reset gate 66 is opened and the camera's
mechanical shutter is closed (or current from an opaquely masked
photodetector is used), the output from the photosensitive detector
64 is integrated for a time equivalent to the previous exposure
time so as to cancel dark current and noise. This cancellation occurs
within a correlated double sampling circuit (CDS) 40. The integrated
signal from the CDS circuit 40 is digitized by an analog-to-digital
converter 56 and the resulting digital value is processed by the
sub-image pixel inverter 48 and then stored in a non-volatile memory
58, which may be a SRAM memory. The digital value can be output
to the main data bus 50 by accessing the unit sub-array through
row and column address decoders 60 and 62, respectively. The image
data may be stored at site and/or downloaded after each capture.
The photosensor array 20 is then ready for exposure to a new image.
Each photodetector element 24 in a sub-group of photodetectors 22
is connected to an individually assigned input to the multiplexing
and clocking circuit 80. In operation then, the outputs from each
photodetector element are serially connected (one at a time) to
the current mirror 30A.
As is well known, the output of the current mirror 30A is a multiple
of the current produced by the photodetector element 24. The current
multiplying effect is traditionally accomplished either by connecting
the bases or gates of the transistors 32.sub.1 through 32.sub.n
in parallel with each other or by making the emitter (or source)
areas larger by an integral multiple of the emitter area of an input
side transistor 34. Current mirrors of this type operate on the
principle that the emitter base voltage (or the gate source voltage)
for all the transistors in the current mirror are the same so that
each of the collector (drain) currents is the same and therefore
the sum of the currents from the output side T.sub.o is a multiple
of either the number of transistors in the output side or the ratio
of the area. This current multiplication is represented mathematically
as follows:
where
n=number of transistors on the output side "T.sub.o "
of the current mirror
or
n=A.sub.out /A.sub.in =the ratio of emitter areas
With detailed analysis it can be shown that output current is not
as simple as the above equation and is more accurately represented
by
where
.beta.=transistor current gain (typically between 50 and 200)
In another embodiment of the current mirror 30A, a transistor 36
can be added to the input side T.sub.i of the current mirror to
drive the bases (or gates) of the output side T.sub.o of the current
mirror and thereby reduce the effects of leakage current. This is
known to reduce the non-ideality of the above equation to
The output of the current mirror 30A is switched between two current
amplifiers, amplifiers 42 and 44, with gains of +1 and -1, respectively.
When the photosensitive detector 64 is receiving incident light
its amplified current is switched through the +1 gain of amplifier
42 into an integrating amplifier 46 for a given amount of time and
then after the shutter is closed the current is integrated through
the -1 gain of amplifier 44 for an equal amount of time. This operation
is known as dual slope, correlated double sampling, and it eliminates
the effects of dark current and KTC noise. It differs from dual
sample CDS in that the signal is switched through two different
amplifiers depending on whether the signal represents the image
data or the background data wherever dual time CDS uses the same
amplification chain. Correlated double sampling also reduces Johnson
noise, flicker noise, and 1/f noise. A similar effect can be achieved
by integrating the current from an opaquely masked photodetector
through the -1 gain of amplifier 44. Using an opaquely masked photosensor
adjacent to the active photosensitive detector 64 allows the correlated
double sampling technique to be implemented in parallel and decreases
readout time. The integration of the outputs of the current amplifiers
42 and 44 occurs simultaneously, thus eliminating the need to wait
for two integration periods to have output data. However, since
separate photosensors are used, small differences in dark current
and noise can arise between them.
The integrated signal from the photosensors is then digitized by
an analog-to-digital converter (ADC) circuit 56.
The output of the ADC is stored in the memory cells 58 after inversion
by the sub-image pixel inverter 48. It is also possible to assign
a group of memory cells to each sub-group of photodetectors 22.
The image at each sub-array of hotodetectors is inverted with respect
to the other sub-arrays, therefore, means for inverting the image
at the sub-array are incorporated in one embodiment as a sub-image
pixel inverter 48. This means will consist of mapping the data from
one photodetector to a memory location corresponding to a photodetector
at a point which is a mirror image of the object reflected about
the center of the sub-array. It is also an embodiment of the invention
to have the sub-array images inverted by external circuitry or by
means of digital image processing.
This allows the resulting sub-images from each sub-group of photodetectors
to be stitched together to form a larger composite image. The invention
utilizes digital image capture and processing to eliminate the need
for an additional array of relay lenslets to invert the sub-images
formed by each lenslet. It also eliminates the need for a third
array of field lenses that would be used to maintain illumination
uniformity across the lenslet's field of view.
The stored digital pixel data is accessed by strobing the memory
cells 58 via row and column address decoders, in order to send data
to the data bus 50. The incorporation of the row and column address
data lines allows for addressing of subsections of the photosensor
array 20 for more rapid image readout in a specific area of interest
in the image. This is useful in digital cameras which monitor a
scene and are only interested in updating the section of the image
associated with moving objects.
Referring now to FIG. 7, a current mirror circuit 30B that may
be substituted for the current mirror 30A, provides a second variant
of the current mirror function by utilizing an output transistor
To emitter or source whose area is n times the area of the emitter
or source of the input transistor T.sub.i. This forms a smaller
circuit which in turn allocates more area to the sub-group of photodetector
22. Current mirror 30B would be substituted for the current mirror
30A at the connection point marked with a circled A.
FIG. 8 illustrates another variant of a current mirror labeled
30C that is connectable to an associated multiplexing and clocking
circuit 80 of FIG. 7 at the A labeled point marked with a circle
in place of either current mirrors 30A or 30B. The current mirror
30C achieves current multiplication by biasing the emitter base
(or gate source) voltage higher on the input side of the current
mirror than on the output side. This, more specifically, is accomplished
by adjusting the value of R1 and R2. Alternately, a diode junction
can be substituted into the emitter or source of the input side
of the current mirror 30C in order to provide a fixed voltage bias.
For a bipolar implementation of the multiplying current mirror,
this technique can be illustrated by the ebers-moll relation, which
is given by
where
allows for a current multiplication of approximately 10.times.at
300 k.
In operation, data from a subsection of an array can be accessed
by using only those addresses that represent the location of data
of interest. In this fashion segments of a captured image may be
clocked out more rapidly in order to view areas that are capturing
rapid scene changes while sampling less active areas less frequently.
If more than one image is stored at each sub-group of photodetectors
22, the length of the addresses are increased accordingly to allow
individual access to each of the multiple image data words (pixel
image data).
Referring next to FIG. 9, an object 90 (arrow) is imaged onto sub-groups
of photodetectors 22. Each lenslet 12 views a different segment
of the field of view. As a result, the segment 1-3 of the arrow
forms an inverted image 92A that impinges on its respective sub-group
of photodetectors 22. Likewise the segment 3-5 of the arrow 90 forms
an inverted image 92B that impinges on its respective sub-group
of photodetectors 22. The same holds true for the segment 5-7 forming
the inverted image 92C. Noted in the drawing is that the images
92A through 92C do not form, from left to right, a continuous image
of the arrow 90. More specifically, note that point 1 is adjacent
point 5 and that point 3 is adjacent point 7 at the surfaces of
the sub-group of photodetectors 22. That is, if the sub-images of
the arrow 90 were simply stitched together, the overall mosaic (composite)
image would be scrambled and unusable.
To arrange the pixel image data in proper order the charges form
the matrix of photodetector elements 24 in each sub-group 22 are
first clocked out and multiplexed through their respective current
multiplying circuit, correlated double sampling circuitry, and digitized
by the ADC. The digital data from a pixel, representing an image,
in each sub-group of photodetectors 22 is positionally inverted
with respect to the center of each sub-group of photodetectors 22.
Therefore, a sub-image pixel inverter 48 is needed to take the pixel
data from a pixel located at point A and to store it in a memory
cell 58 which is inverted with respect to the central pixel of the
sub-group of photodetectors. For example, pixel A would be stored
in the memory 58 at a location corresponding to D. The overall mosaic
composite image is then built up for use by an output device 110,
such as a printer or CRT display, by reading the first row from
the first sub-image and then adding the first row of the next sub-image
till the first rows have all been clocked out. Next the second row
from the first image is clocked out and the process continues until
the mosaic image is completed with the last pixel being stored in
memory location I.
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