Digital Camera Patent Abstract
An infrared illumination system for a digital camera having a taking
lens, includes: a plurality of infrared emitting diodes arranged
around the taking lens, each diode being contained in a can having
an infrared transparent window; a first diffuser located over the
windows of the cans; and a second diffuser spaced apart from the
first diffusers. The first diffuser is a new type of diffuser called
a Fourier transform holographic diffuser. Digital Camera Patent Claims
We claim:
1. An infrared illumination system for a digital camera having
a taking lens, comprising:
a) a plurality of infrared emitting diodes arranged around the
taking lens, each diode being contained in a package having an infrared
transparent window;
b) first diffuser means located over the windows of the packages
for diffusing the light from the infrared light emitting diodes;
and
c) second diffuser means spaced apart from the first diffuser means
for further diffusing the light from the light emitting diodes.
2. The infrared illumination system claimed in claim 1, wherein
the first diffuser means has stronger diffusing properties than
the second diffuser means.
3. The infrared illumination system claimed in claim 1, wherein
the infrared light emitting diodes are arranged symmetrically around
the taking lens.
4. The infrared illumination system claimed in claim 1, wherein
the packages containing the infrared light emitting diodes are cans.
5. The infrared illumination system claimed in claim 1, wherein
the first diffuser means comprise two sheets each sheet having a
thickness of 0.3mm and making of SPECTRALON.TM. diffusing material
and the second diffuser means comprises one sheet having a thickness
of 0.5mm and making of SPECTRALON.TM. diffusing material, and wherein
the first and second diffuser means are spaced apart by 4.75 mm.
6. The infrared illumination system claimed in claim 1, further
comprising a layer of infrared transmissive glass over the second
diffuser means.
7. The infrared illumination system claimed in claim 1, wherein
the second diffuser means is a ring shaped diffuser surrounding
the taking lens.
8. The infrared illumination system claimed in claim 1, wherein
the first diffuser means is a Fourier transform holographic diffuser.
9. The infrared illumination system claimed in claim 8, wherein
the Fourier transform holographic diffuser is located at a light
emitting surface of the infrared light emitting diode.
Digital Camera Patent Description
FIELD OF THE INVENTION
The invention relates generally to the field of digital photography,
and in particular to infrared illumination systems for digital infrared
cameras
BACKGROUND OF THE INVENTION
It is known to employ digital cameras to capture infrared information
from a scene. In prior art digital infrared image capture systems,
the illumination source has been ambient infrared light either reflected
from or emitted by the subject. In applications such as surveillance
or machine vision, an additional source of illumination is desirable.
In applications such as surveillance, it would be preferable if
the source of such illumination remained unnoticed by the subject.
Conventional flash systems capable of producing substantial infrared
light also produce appreciable quantifies of visible light. If the
visible light from such a flash is filtered out to produce an infrared
flash, the resulting flash apparatus is inefficient, since a considerable
portion of the energy produced by the flash is wasted. In applications
such as machine vision, it is preferable that the source of illumination
be highly uniform.
U.S. Pat. No. 4,866,285 issued Sep. 12, 1989 to Simms discloses
a light source for an infrared film camera having one or more infrared
diode light sources. It has been found that due to the point source
nature of the infrared diodes, the illumination from such a light
source is non-uniform at the subject. There is therefore a need
for an energy efficient uniform source of infrared illumination
for infrared electronic photography.
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, an infrared illumination system
for a digital electronic camera having a taking lens, includes:
a plurality of infrared emitting diodes arranged around the taking
lens, each diode being contained in a package having an infrared
transparent window; a set of two diffusers is located over the windows
of the packages; and a single diffuser spaced apart from the first
diffusers. We have discovered that a single diffuser placed over
the windows of the packages of the infrared light emitting diodes
is not effective to sufficiently diffuse the light from the diodes
to eliminate "hot spots" in the illumination from the
diodes. The addition of the second diffuser over the window as well
as the second "spaced" diffuser significantly improved
the uniformity of illumination. According to a further aspect of
the present invention, the first diffuser is a new type of diffuser
called a Fourier transform holographic diffuser.
These and other aspects, objects, features and advantages of the
present invention will be more clearly understood and appreciated
from a review of the following detailed description of the preferred
embodiments and appended claims, and by reference to the accompanying
drawings.
ADVANTAGEOUS EFFECT OF THE INVENTION
The present invention has the advantage of providing a more uniform,
energy efficient illumination of the photographic subject.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a exploded perspective view of an infrared illumination
system according to the present invention;
FIG. 2 is an exploded perspective view of one of the infrared light
emitting diodes and associated diffusers shown in FIG. 1;
FIG. 3 is a schematic diagram illustrating one of the infrared
light emitting diodes and an alternative embodiment of the first
diffuser as a Fourier transform holographic diffuser; and
FIG. 4 is a schematic diagram illustrating one way of making the
Fourier transform holographic diffuser of FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, a digital infrared camera 10 of the type known
in the prior art, such as custom miniaturized Infrared Cameras,
sold by the Eastman Kodak Company, Rochester, N.Y., has a taking
lens 12. An infrared illumination system according to the present
invention surrounds the taking lens 12 and includes a plurality
of infrared light emitting diodes 14, such as HE8812 infrared light
emitting diodes from the Hitachi Corporation, Japan. As shown in
FIG. 2, the light emitting diodes 14 are packaged in cans 16 having
fiat infrared transmissive windows 18. The infrared light emitting
diodes 14 are arranged symmetrically around taking lens 12 in a
collar 20 having nests 22 for receiving the diodes 14.
A plurality of diffusers made up of pairs of diffusing disks 24
and 26 are located over the windows 18 of the infrared light emitting
diodes 14. A locating ring 28 defines a plurality of apertures 30
for holding the infrared light emitting diodes 14 and the diffusing
disks (24, 26) into position around taking lens 12. The ring 28
also defines a central aperture 32 for receiving the taking lens
12. The locating ring 28 includes a recess for receiving a ring
shaped second diffuser 34. The second diffuser 34 is sandwiched
between first and second rings of infrared transmissive glass 36
and 38 respectively. The first ring of infrared transmissive glass
36 spaces first diffusing discs 24, 26 by a predetermined amount
from the second diffuser 34. The second ring of infrared transmissive
glass 38 provides a protective cover for the second diffuser 34.
Preferably, the first diffuser is made from two disks 24, 26 of
0.3mm thick SPECTRALON.TM. diffusing material available from Labsphere
Corporation, North Sutton, N.H. The second diffuser 34 is made from
one sheet of 0.5mm thick SPECTRALON.TM., and the two diffusers are
spaced apart by 4.75mm. Alternatively, rather than separate discs,
the first dffuser can be made of a ring of diffuser material similar
to the second diffuser.
Referring to FIG. 3, the light emitting diode 14, having a light
emitting surface 15 inside can 16 is shown with an alternative embodiment
of a first diffuser. In this embodiment, the first diffuser is a
Fourier transform holographic diffuser 40. The Fourier transform
holographic diffuser 40 contains a holographic pattern that is effective
to diffuse a substantially monochrome beam of light from a point
source located at a given distance from the diffuser. The diffuser
40 acts as if it were a conventional diffuser located precisely
at the point source, in this case, the light emitting surface 15.
This diffuser 40 has the advantages of being more transmissive than
conventional diffusers and is less sensitive to errors in locating
the diffuser with respect to the light source.
Referring to FIG. 4, a method for constructing the holographic
diffuser 40 of FIG. 3 will be described. A source of coherent light,
such as a laser diode 42, produces a beam of light 44 that is collimated
by a collimator 46. The laser diode 42 may be selected to emit substantially
the same wavelength of light as the light emitting diodes 14, but
this is not a strict requirement. The collimated beam is split by
a variable beam splitter 48 into an object beam 50 and a reference
beam 52. The variable beam splitter 48 may be adjusted to vary the
amount of light into the object and reference beams to optimize
the formation of the interference pattern described below. The object
beam 50 is directed to a beam expander 54 by a mirror 56. The expanded
beam 58 illuminates a conventional diffuser 60, such as a sheet
of SPECTRALON.TM. diffusing material, or frosted glass to form a
diffuse object wavefront 62.
The reference beam 52 is coupled into an optical fiber 64. The
optical fiber 64 is routed through a pinhole in the conventional
diffuser 60 at a location corresponding to the eventual position
of the light emitting surface of the infrared light emitting diode,
to emit a reference wavefront 66 at that location. The interference
pattern formed by the diffuse object wavefront 62 and the reference
wavefront 66 is recorded by a photosensitive plate 68 located at
a recording plane representing the eventual location (i.e. over
the window 18 of the can 16) of the holographic diffuser relative
to the light emitting surface of the infrared light emitting diode.
The exposed photosensitive plate 68 is developed to produce the
Fourier transform holographic diffuser 40. The resulting Fourier
transform holographic diffuser has the effect of a conventional
diffuser located at the light emitting surface 15 of the light emitting
diode 14. The Fourier transform holographic diffuser produced by
this method is useful in any optical application where it is desirable
to diffuse substantially monochromatic light from a substantially
point source.
The electronics employed to power the light emitting diodes 14
in the present invention are conventional and not shown. The electronics
apply a current pulse of variable width, depending on the amount
of illumination desired, simultaneously to all of the light emitting
diodes when the camera is actuated. |