Digital Camera Patent Abstract
An optical system for detecting image light in a digital camera
includes an electronic sensor having an area array of pixels for
detecting the image light. A field lens is positioned adjacent the
electronic sensor so as to be substantially in the path of the image
light for directing the image light onto the area array of pixels.
The field lens straightens the image light so that the straightened
image light rays are substantially parallel to an optical axis of
the digital camera. Digital Camera Patent Claims
What is claimed is:
1. An optical system for imaging light in a digital camera, comprising:
an electronic sensor having an area array of pixels for detecting
said image light; a lens for focusing said image light onto said
electronic sensor, wherein said lens comprises a near-telecentric
lens; and one or more field lenses positioned adjacent said electronic
sensor so as to be substantially in the path of said image light
for directing said image light onto said area array of pixels.
2. The optical system of claim 1, wherein said one or more field
lenses redirect said image light onto said area array of pixels.
3. The optical system of claim 1, wherein said one or more field
lenses are constructed of plastic.
4. The optical system of claim 1, wherein said one or more field
lenses are constructed of glass.
5. The optical system of claim 1, wherein said one or more field
lenses reduces an angle of incidence of at least a portion of said
image light upon said area array of pixels.
6. The optical system of claim 1, further comprising one or more
color filters positioned in said image light between said area array
of pixels and said one or more field lenses.
7. The optical system of claim 6, further comprising an array of
microlenses positioned in said image light between said area array
of pixels and said one or more field lenses.
8. The optical system of claim 1, further comprising a substantially
flat cover mounted over said area array of pixels, wherein said
one or more field lenses are attached to said cover.
9. The optical system of claim 1, wherein said one or more field
lenses are integral to a package for said electronic sensor and
act as a cover to provide physical protection to said electronic
sensor.
10. The optical system of claim 1, wherein said area array of pixels
comprises a center region and a peripheral region, and wherein an
angle of incidence of said image light upon said area array of pixels
is substantially equal in said center region and said peripheral
region after said image light has passed through said one or more
field lenses.
11. A method for reducing the angle of incidence of image light
rays falling upon an electronic sensor in a digital camera, comprising:
providing a near-telecentric lens to focus said image light rays;
and mounting a field lens between said near-telecentric lens and
said electronic sensor so as to be substantially in the path of
said image light rays.
12. The method of claim 11, wherein said electronic sensor further
comprises a cover, and wherein mounting said field lens over said
electronic sensor comprises attaching said field lens to said cover.
13. An optical apparatus in a digital camera, the apparatus consisting
of: a two-dimensional array of light detecting pixels mounted in
said digital camera substantially in a light path of said optical
apparatus and substantially perpendicular to an optical axis of
said digital camera; and a field lens having a substantially flat
bottom surface and a convex upper surface and a positive focal length,
said field lens being mounted in said digital camera in front of
said two-dimensional array of light detecting pixels so that said
substantially flat bottom surface is parallel to and mounted adjacent
said two-dimensional array of light detecting pixels and the light
path passes through said field lens before reaching said two-dimensional
array of light detecting pixels.
14. The apparatus of claim 13, further comprising a substantially
transparent cover attached in a parallel fashion to said two-dimensional
array of light detecting pixels between said two-dimensional array
of light detecting pixels and said field lens.
15. The apparatus of claim 14, wherein said field lens is mounted
to said substantially transparent cover.
16. The apparatus of claim 14, wherein said field lens is mounted
above and parallel to said substantially transparent cover.
17. The apparatus of claim 13, wherein said field lens is mounted
to a light-facing side of said two-dimensional array of light detecting
pixels as a cover.
Digital Camera Patent Description
FIELD OF THE INVENTION
The present invention relates generally to the field of digital
cameras, and more particularly, to a system and method for directing
image light onto an array of optical sensors in a digital camera.
BACKGROUND OF THE INVENTION
A digital camera 102, FIGS. 1 and 2, typically includes a lens
system 106 for projecting and focusing the image of a subject onto
the surface of an electronic sensor 116. Digital cameras are described
in the following patents which are hereby incorporated by reference
for all that is disclosed therein. U.S. Pat. Nos. 4,131,919, 4,420,773,
and 4,541,010. The digital camera 102 may have a housing 104 with
elements such as a display 112 to indicate the status of the digital
camera 102, a button 110 which may be pushed to cause the digital
camera 102 to take a picture, and a flash 114 to illuminate a subject.
The electronic sensor 116 in a digital camera 102 comprises an area
array sensor, i.e., a two-dimensional array of individual optical
sensors, or pixels 126, 127, 129, 131, 133, etc., FIG. 12.
The image quality of a digital camera 102 is determined, in part,
by the "spacial resolution," or the number of pixels 126,
etc., in the electronic sensor 116. It is also determined by the
bit-depth and signal-to-noise ratio of the pixels 126, etc., or
the ability of the pixels to measure and quantify the image light,
e.g. 118, 120, and 122, incident upon it.
A pixel 126 may be constructed in various known ways. Generally
a pixel 126 is constructed of a material which converts image light
120 into electrical signals, which can then be processed and stored
in the circuitry of the digital camera 102. As best seen in FIG.
3, a pixel 126 contains a light sensitive region 128 and one or
more non-light sensitive regions 130 and 132. The ratio of light
sensitive, or active, regions 128 to non-light sensitive regions
130 and 132 is referred to as the fill factor. The light sensitive
region 128 may comprise a portion of a silicon wafer 134, which
is surrounded by support circuitry such as polysilicon gates 136,
138, 142, and 144, metal conductors, channel stops, light shields
140 and 146, etc, forming a pit 148. The image light 120 must travel
down through the pit 148 to the bottom where the light sensitive
region 128 is located.
As digital cameras 102 are designed with higher resolution, requiring
more pixels 126, the pixel size must necessarily be smaller to keep
the overall size and cost of the digital camera 102 down. However,
it is more difficult to scale down the electronic support circuitry
constituting the non-light sensitive regions 130 and 132 than it
is to scale down the light-sensitive region 128. Therefore, as pixels
(e.g., 126) become smaller, the fill factor becomes smaller, and
the ratio of the sizes of light sensitive 128 and non-light sensitive
regions 130 and 132 in the pixel 126 is reduced. In other words,
if a pixel 126 is scaled down to half the size, the scaled down
pixel is less than half as sensitive to light as the larger pixel
would be.
Microlenses 166, 168, FIGS. 3 and 4, have been employed to increase
the fill factor of very small pixels. A microlens e.g. 166 is a
small lens with approximately the same area as the entire associated
pixel 126, and may be formed with photolithographic processes. The
microlens 166 is positioned above the pixel 126, gathering nearly
all the image light 120 incident on the pixel 126 and directing
it to the light sensitive region 128 of the pixel 126, as best shown
by FIG. 3.
As a result of the non-linearity of fill factor versus size described
above, the light sensitive region 128 at the bottom of the pit 148
grows relatively smaller as pixel size decreases, and the height
150 of the pit wall increases, reducing the acceptable angle of
incidence (e.g., 162, FIG. 4) of the image light 120. If the image
light 120 is at too great an angle of incidence 162, it will terminate
on the wall of the pit, such as on the light shields 140 and 146,
rather than making it down to the light sensitive region 128 at
the bottom of the pit 148.
Referring now to FIGS. 2, 3, and 4, placing microlenses 168 over
the center of a pixel 152 has the disadvantage of only working well
near the center of the optical axis 108 of the digital camera's
102 lens system 106. If a pixel (e.g. 152) is located at the periphery
of the electronic sensor 116, remote from the optical axis 108 of
the lens system 106, FIG. 4, the angle of incidence 162 of the image
light 122 is larger than the angle of incidence 162 for pixels (e.g.,
126) near the optical axis 108, FIG. 3. In the peripheral pixel
location shown in FIG. 4, the image light 122 passes through the
microlens 168 and is focused not on the light sensitive region 128,
but on a non-light sensitive region 130 such as a light shield 140.
As a result, such pixels (e.g., 152) near the periphery of the electronic
sensor 116 detect less image light 122 than a more centered pixel
and the image quality of the digital camera 102 is degraded.
Referring now to FIG. 5, one prior solution to the problem described
above has been to shift the microlenses 264 and 268 at the periphery
of the electronic sensor 216 in towards the optical axis 208, so
that they are no longer centered over their respective pixels 224
and 252. The microlenses 264, 266, and 268 are shifted in towards
the optical axis 208 as a function of distance of the corresponding
pixel 224, 226, and 252 from the optical axis 208. For the pixels
226 near the optical axis 208, the corresponding microlenses 226
are not shifted or are not shifted very far towards the optical
axis 208. For the pixels 224 and 252 farther out from the optical
axis 208, the corresponding microlenses 264 and 268 are shifted
a relatively larger distance towards the optical axis 208. The microlenses
264, 266, and 268 are placed so that the greatest possible amount
of image light 218, 220, and 222 is focused and directed toward
the light sensitive regions 228, 254, and 270.
This approach of shifting the microlenses 264, 266, and 268 has
several disadvantages. First, the microlenses 264, 266, and 268
are less effective at focusing to a well defined spot at large angles
of incidence 262. Second, the height 150 of the walls of the pits
148 limit the angle of incidence, e.g., 262, of the image light
rays 218, 220, and 222 that allows the image light 218, 220, and
220 to reach the light sensitive regions 228, 254, and 270. A third
problem arises when color filters 272, 274, and 276 are placed in
the path of the image light 218, 220, and 222 in order to produce
a color image. Since the image light rays 218 and 222 with a relatively
high angle of incidence 262 pass through the color filters 272 and
276 at an angle, their path through the dye in the color filters
is longer, thus they are more heavily filtered. This can result
in undesirable color shifts from the center to the edges of the
resulting images.
Another prior solution to the problems described above, illustrated
in FIG. 6, is the use of a telecentric lens 378. Image light rays
318, 320, and 322 produced by a telecentric lens 378 are focused
downward on the pixels 324, 326, and 352 at a consistent angle of
incidence, independent of the original angles of incidence of the
image light 318, 320, and 322 before passing through the telecentric
lens 378. FIG. 6 illustrates how the image light 318, 320, and 322
is directed in paths which are substantially parallel to the optical
axis 308. However, using a telecentric lens 378 in a digital camera
102 makes it much larger, more complex, and expensive. Typical telecentric
lens designs for digital cameras 102 may have twice the length,
diameter and cost as a comparable non-telecentric lens design for
a conventional film camera.
A need therefore exists for a small, simple and inexpensive lens
and electronic sensor system for a digital camera which can focus
a substantial amount of image light on the light sensitive regions
of the electronic sensor.
SUMMARY
To assist in achieving the aforementioned needs, the inventor has
devised a system and method for using a field lens in a digital
camera to focus and direct image light toward the pixels in an electronic
sensor.
An optical system for detecting image light in a digital camera
having features of the present invention comprises an electronic
sensor having an area array of pixels for detecting the image light.
One or more field lenses are positioned adjacent or in front of
the electronic sensor so as to be substantially in the path of the
image light. The one or more field lenses straighten the image light
and direct it onto the area array of pixels. If more than one field
lenses are used, they are each aligned over the optical axis of
the optical system, forming a compound lens.
The electronic sensor is mounted in the digital camera in the path
of the image light perpendicularly to the optical axis of the optical
system. The field lens generally has a substantially flat bottom
surface mounted adjacent the electronic sensor, and a convex upper
surface.
The field lens is preferably mounted to the electronic sensor as
the cover to the electronic sensor, or alternatively, if the electronic
sensor is provided with a cover, the field lens may be mounted above
the cover of the electronic sensor or mounted directly to the cover
of the electronic sensor.
The present invention may also comprise a method for reducing the
angle of incidence of image light rays falling upon an electronic
sensor in a digital camera, including providing a field lens to
straighten the image light rays, and mounting the field lens over
the electronic sensor so as to be substantially in the path of the
image light rays.
BRIEF DESCRIPTION OF THE DRAWING
An illustrative and presently preferred embodiment of the invention
is illustrated in the drawing, in which:
FIG. 1 is a perspective view of a digital camera;
FIG. 2 is a cross-sectional top view of the digital camera of FIG.
1 taken along line 115;
FIG. 3 is a cross-sectional view of a pixel in an electronic sensor
of a digital camera;
FIG. 4 is a cross-sectional view of a pixel in an electronic sensor
of a digital camera illustrating an image light ray with a high
angle of incidence;
FIG. 5 is a cross-sectional view of a lens system and electronic
sensor in which microlenses have been shifted toward the optical
axis to correct for image light rays having high angles of incidence;
FIG. 6 is a cross-sectional view of a lens system and electronic
sensor in which a telecentric lens is used to correct for image
light rays having high angles of incidence;
FIG. 7 is a cross-sectional view of a lens system and electronic
sensor in a preferred embodiment comprising a field lens in use
with a near-telecentric lens, in which the field lens comprises
the cover of the electronic sensor;
FIG. 8 is a cross-sectional view of a pixel in the embodiment of
FIG. 7 with a field lens correcting for an image light ray with
a high angle of incidence;
FIG. 9 is a cross-sectional view of a lens system and electronic
sensor in which a field lens is mounted to the cover of the electronic
sensor;
FIG. 10 is a cross-sectional view of a lens system and electronic
sensor in which a field lens is mounted over the cover of the electronic
sensor; and
FIG. 11 is a schematic representation of a portion of a two dimensional
photosensor array.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A digital camera 102 is used to take pictures which are stored
digitally, in the form of electrical signals, rather than storing
the pictures visually using chemicals in photographic paper. Digital
images produced by a digital camera 102 must be processed by a computer
to view or print them. As digital cameras 102 become more common
and more sophisticated, lens systems 106 and electronic sensors
116 need to become smaller and less expensive.
A field lens 482 for a digital camera 102 significantly lowers
the size and cost of the digital camera 102. As illustrated in FIG.
7, a preferred embodiment comprises a near-telecentric lens 480
to focus and direct image light 418, 420, and 422 onto an electronic
sensor 416, and a field lens 482 placed between the near-telecentric
lens 480 and the electronic sensor 416. A field lens 482 is a positive,
or converging, lens, which changes the direction of a light path
through it by refraction. A positive lens has a positive focal length,
and thus causes light rays parallel to its axis to converge, and
light rays 418 and 422 with non-zero angles of incidence 462 to
emerge substantially parallel to its axis. The field lens 482 is
preferably mounted near the electronic sensor 416 so that it has
only a small effect on the performance of the rest of the optical
system. As a result, the field lens 482 can be manufactured as a
simple, single element molded plastic lens. The field lens 482 generally
comprises a flat or nearly flat bottom surface and a convex top
surface, and is nearly transparent to the desired wavelengths of
the image light 418, 420, and 422. Field lenses 482 may be manufactured
using any method now known or that may be developed in the future,
such as grinding and polishing glass or molding plastic.
The near-telecentric lens 480 of the preferred embodiment is less
expensive and smaller than a more fully telecentric lens 378 as
shown in FIG. 6, however, a near-telecentric lens 480 does not completely
straighten image light 418, 420, and 422, as a telecentric lens
378 would. The field lens 482 is placed between the near-telecentric
lens 480 and the electronic sensor 416 to substantially straighten
the image light 418, 420, and 422 as it passes through the field
lens 482.
The angle of incidence of a light path through the digital camera
102 is therefore somewhat reduced by the near-telecentric lens 480
and is further reduced to acceptable levels by the field lens 482.
The electronic sensor 416 comprises an area array, or two dimensional
array, of light detecting pixels (e.g., 424, 426, and 452) formed
on a silicon wafer 434. As will be described in more detail below
with respect to FIG. 8, the image light 418, 420, and 422 must be
substantially parallel to an optical axis 408 of the digital camera
102 in order to reach the light sensitive regions 470, 428, and
454 of the pixels 424, 426, and 452.
The electronic sensor 416 may further comprise color filters 472,
474, and 476 if a color image is desired. A color filter 472, 474,
and 476 is typically either red, blue or green, and absorbs incident
light having all wavelengths other than those in a given range.
For example, red color filters absorb nearly all light wavelengths
other than red, thus a red filter transmits the red portions of
the image light. To produce a color image, pixels (e.g., 424, 426,
and 452) in the electronic sensor 416 are covered by alternating
colors of filters 472, 474, and 476, to detect either the red, blue,
or green wavelengths of the image light 418, 420, and 422. The resulting
red, blue, and green images are then recombined in the processing
equipment used to display or print the image. The degree to which
unwanted wavelengths of light are filtered out in the color filters
472, 474, and 476 is related to the length of the light path through
the filters 472, 474, and 476. The field lens 482 greatly improves
the quality of the resulting color image by causing the length of
the light paths to be substantially the same through the filters
472, 474, and 476 in the center of the electronic sensor 416 and
at its periphery, thus avoiding unwanted color shifts at the periphery
of the resulting image.
The electronic sensor 416 may further comprise microlenses 464,
466, and 468 centered over each pixel 424, 426, and 452 to collect
the image light 418, 420, and 422 incident on the entire area of
each pixel 424, 426, and 452 and focus it on their respective light
sensitive regions 470, 428, and 454.
In a preferred embodiment, the field lens 482 is attached to the
top of the electronic sensor 416 to act as a protective cover to
the delicate electronic and optical components within, as well as
to fill the optical role of straightening the image light 418, 420,
and 422.
Referring now to FIG. 8, a pixel 452 in the electronic sensor 416
of FIG. 7 is shown in more detail. A field lens 482 is mounted at
the top of the electronic sensor 416. A color filter 472 having
a height 494 of about 1.5 .mu.m is centered over the pixel 452 between
the microlens 468 and the pixel 452 at a distance 492 of about 1.5
.mu.m below the microlens 468 and a distance 496 of about 1.5 .mu.m
above the pixel 452.
The pixel 452 comprises a light sensitive region 454 on a silicon
wafer 434, surrounded by non-light sensitive regions 430 and 432.
The non-light sensitive regions 430 and 432 comprise electronic
circuitry such as polysilicon gates 436, 438, 442, and 444, as well
as other elements of integrated circuits, not shown. The electronic
circuitry such as polysilicon gates 436, 438, 442, and 444 may be
covered by light shields 440 and 446. These elements (e.g., 440
and 446) form a pit 448 through which the image light 422 must pass
to reach the light sensitive region 454, necessitating a low angle
of incidence. In a preferred embodiment, the pixel height 450 may
be about 2 .mu.m, while the width 488 of the light sensitive region
454 is about 4 .mu.m. The width 486 of the pixel is about 6 .mu.m.
The microlens 468 thus gathers the image light 422 over the entire
width 486 of the pixel and focuses it onto the smaller width 488
of the light sensitive region 454. An image path 484 having a relatively
high angle of incidence is thus straightened by the field lens 482
and directed substantially vertically into the pixel 452.
In another embodiment illustrated in FIG. 9, the electronic sensor
516 further comprises a cover 598, to which the field lens 582 is
mounted. The field lens 582 may be mounted to the cover 598 of the
electronic sensor 516 in any suitable manner, such as with an adhesive
which transmits image light 518, 520, and 522, or by attaching it
at the edges with mechanical clips (not shown).
In this embodiment, image light 518, 520, and 522 passes through
a near-telecentric lens 580 and is focused onto the field lens 582.
The field lens 582 straightens the image light 518, 520, and 522
so that it travels nearly vertically down onto the pixels 524, 526,
and 552.
Alternatively, as illustrated in FIG. 10, a field lens 682 may
be mounted a relatively small distance 699 above an electronic sensor
616, ranging from a distance of about 0.1 mm to about 2 mm. The
field lens 682 may be mounted in the digital camera 102 above the
electronic sensor 616 in any suitable manner, such as by mounting
posts 697.
While illustrative and presently preferred embodiments of the invention
have been described in detail herein, it is to be understood that
the inventive concepts may be otherwise variously embodied and employed,
and that the appended claims are intended to be construed to include
such variations, except as limited by the prior art. |