Digital Camera Patent Abstract
An illuminator for a digital camera includes a body of transparent
material defining a cone shaped cavity with an opening at the apex
of the cone for a taking lens of the camera and an opening at the
base of the cone for locating an object to be photographed with
the digital camera. A plurality of light emitting diodes (LEDs)
are uniformly distributed throughout the body of transparent material
with respect to the surface of the cone. A power supply applies
power to the LEDs so that their brightness' are equal. Digital Camera Patent Claims
What is claimed is:
1. An illuminator for a digital camera, comprising:
a) a body of transparent material defining a cavity with an opening
for a taking lens of the camera and an opening for locating an object
to be photographed with the digital camera;
b) a plurality of light emitting diodes (LEDs) uniformly distributed
in a plurality of layers throughout the body of transparent material
with respect to the surface of the cavity; and
c) a power supply for applying power to the LEDs so that their
brightnesses are equal, by arranging the LEDs in a plurality of
legs wherein the LEDs in each leg are in series connection, and
wherein each layer contains diodes from more than one leg.
2. The illuminator claimed in claim 1, wherein the cavity is a
cone shaped cavity, the opening for the taking lens of the camera
is located at the apex of the cone shaped cavity and the opening
for the object is located at the base of the cavity.
3. The illuminator claimed in claim 2 for an infrared sensitive
digital camera, wherein the body of transparent material is transparent
to infrared light, the LEDs are infrared emitting diodes, and the
surface finish of the cone shaped cavity is a flat finish.
4. The illuminator claimed in claim 3, further comprising an infrared
transmissive window over the window at the apex of the cone.
5. The illuminator claimed in claim 4, wherein the infrared transmissive
window is partially IR-reflecting.
6. The illuminator claimed in claim 3, wherein the body of infrared
transparent material is polycarbonate impregnated with IR transmissive
dye.
7. The illuminator claimed in claim 3, further comprising a coating
of black paint on the outside of the body of the transparent material.
8. The illuminator claimed in claim 3, having 12 infrared emitting
LEDs arranged in four levels around the cone shaped cavity.
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 used for example in surveillance, machine vision, and microscopy
to image objects having spectrally reflecting surfaces.
BACKGROUND OF THE INVENTION
It is known to employ digital cameras to capture infrared information
from a scene having specular surfaces, for example a part on an
assembly line, or a specimen in a microscope. It is also known to
use a source of artificial infrared illumination to aid in the capture
of such images. A problem occurs in such image capture situations
because the specular reflections of the source of infrared illumination
wash out the detail in the image in the neighborhood of the specular
reflection. Furthermore, 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 quantities
of visible light. If the visible light from such a flash is filtered
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 and microscopy,
it is desirable that the source of illumination be highly uniform
and extended in space (non point source) to minimize the intensity
of specular reflections. However, in applications where the object
needs to be close to the lens, there is scant room for an extended
light source. Therefore, it is also desirable for the light source
to be compact, a requirement that is in apparent conflict with the
need for the light source to be extended.
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 and the point source nature
of the diodes causes intense specular reflections from an object
having specularly reflecting surfaces. There is therefore a need
for an improved, 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 illuminator for a digital camera
includes a body of transparent material defining a cavity with an
opening for locating a taking lens of the camera and an opening
for locating an object to be photographed with the digital camera.
A plurality of light emitting diodes (LEDs) are uniformly distributed
throughout the body of transparent material with respect to the
surface of the cone. A power supply applies power to the LEDs so
that their brightnesses are equal. In a preferred embodiment of
the invention, the body of material is infrared transmissive, the
LEDs are infrared emitting diodes, the cavity is a cone shaped cavity,
and the surface finish of the cavity is a flat finish.
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 uniform,
energy efficient illumination of the photographic subject resulting
in few noticeable specular reflections.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a exploded perspective view, partially broken away, of
an infrared illumination system according to the present invention;
FIG. 2 is a cross sectional view of the infrared illumination system
taken along lines 2--2 in FIG. 1;
FIG. 3 is a cross sectional view of the infrared illumination system
taken along lines 3--3 in FIG. 2;
FIG. 4 is a cross sectional view of the infrared illumination system
taken along lines 4--4 in FIG. 2;
FIG. 5 is a cross sectional view of the infrared illumination system
taken along lines 5--5 in FIG. 2;
FIG. 6 is a cross sectional view of the infrared illumination system
taken along lines 6--6 in FIG. 2; and
FIG. 7 is a schematic diagram of a control circuit for the illumination
system of the present invention.
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 14 according to the present
invention surrounds the taking lens 12 and includes a plurality
of infrared light emitting diodes (IR-LEDs) 16.sub.a-l, such as
the TSUS 4400 infrared light emitting diodes from the Telefunken
Semiconductor Corp., Germany. As shown in FIG. 1, the IR-LEDs 16
are packaged in a light diffusing body 18 of infrared transmissive
plastic that defines a cone shaped cavity 15 with an opening 22
(see FIG. 2) at the apex of the cone for the taking lens 12 of the
camera 10 and an opening 23 at the base of the cone for locating
an object to be photographed with the digital camera 10. Alternatively,
the cavity can define a shape that substantially deviates from a
cone, such as a parabolic surface of revolution. The IR-LEDs 16
are arranged symmetrically around taking lens 12 with respect to
the surface of the cone shaped cavity 15. The body 18 is provided
with holes 17 and slots 19 for receiving the diodes 16. A sapphire
glass window 20, located between the camera lens 12 and the opening
22 at the apex of the cone shaped cavity provides a protective cover
for the taking lens 12. The sapphire glass window 20 is attached,
for example by cyanoacrylate super glue, to a mounting pad 24 formed
on the back of the body 18.
The light diffusing body 18 is made from polycarbonate plastic
impregnated with Filtron.TM. absorptive dye manufactured by the
Gentex Corp., Carbondale, Pa. The plastic material with the dye
is absorptive to visible light, but highly transmissive to infrared.
It was found that the light source performed best when the inside
surface of the cone shaped cavity had a flat finish. The flat finish
on the inside surface was provided by milling the cone shaped cavity
on a lath.
Referring to FIGS. 2-6, the locations and orientations of the IR-LEDs
16.sub.a-l within the light diffusing body 18 will be described.
As shown in FIGS. 3 and 6, IR-LEDs 16.sub.a,i,e,g,l and h are located
in holes that are bored in the diffusing body at an angle perpendicular
to the axis 25 of the cone shaped cavity. As shown in FIGS. 4 and
5, the IR-LEDs 16.sub.b,c,d,f,j, and k, are located in holes 17
that are bored parallel to the axis of the cone shaped cavity. The
IR-LEDs are secured in their respective holes 17 with cyanoacrylate
super glue. IR-LEDs 16.sub.e,g,h and l are installed in slots 19.
The slots 19 extend to the end of the body 18 to accommodate the
wires for their respective IR-LEDs. The plastic containers of IR-LEDs
16.sub.e,g,h and l were trimmed to reduce their thickness so that
they would fit in the slots 19 without protruding into the cavity,
or beyond the surface of the body 18. To prevent visible light from
entering the diffusing body 18, the outside surface of the body
was painted with flat black paint.
The infrared illumination system 14 is secured to the front of
camera 10 by a mounting ring 27 on the front of the camera that
receives the mounting pad 24 and secures the illumination system
14 to the front of the camera with detents 26.
Referring to FIG. 7, a flash control electrical circuit 28 located
in the camera 10 applies a control pulse of variable width, depending
on the amount of illumination desired, to enable a P channel MOSFET
30 to apply power from a flash power supply 32 simultaneously to
all of the IR-LEDs 16.sub.a-l when the camera 10 is actuated. The
IR-LEDs are wired in four parallel legs, each leg containing three
diodes in series with a current limiting resistor 34. The values
of the respective resistors 34 are chosen to adjust LED brightness
to a desired uniform level. The IR-LEDs are wired in the diffusing
body 18 such that each layer of LEDs (as shown in FIGS. 2-6) contains
IR-LEDs from more than one leg of the electrical circuit, where
wiring ability permits. This technique of distributing the LEDs
further enhances the ability of the diffusing cone to provide a
homogeneous source of illumination.
The extended source according to the present invention will appear
to the object as a nearly a uniform source of illumination extending
from horizon to horizon. There is only one flaw in the uniformity,
and that will be at the opening at the apex of the cavity near the
lens of the camera. This flaw is minimized by making the opening
as small as possible, and can be further minimized by placing a
partially reflective surface over the opening. For example, the
sapphire window 20 may be provided with a partially reflective coating
for this purpose.
The invention has been described with reference to a preferred
embodiment. However, it will be appreciated that variations and
modifications can be effected by a person of ordinary skill in the
art without departing from the scope of the invention. For example
although 12 IR-LEDs are shown in the preferred embodiment, more
or fewer LEDs can be used in the present invention. Also, although
the light source is shown as an infrared light source, a visible
light source can also be constructed according to the present invention
by using a body of material transparent to visible radiation and
visible light emitting LEDs. |