Television – Camera – system and detail – Optics
Reexamination Certificate
1998-11-03
2003-03-18
Garber, Wendy R. (Department: 2612)
Television
Camera, system and detail
Optics
C348S272000, C359S663000
Reexamination Certificate
active
06535249
ABSTRACT:
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 no
Garber Wendy R.
Hewlett--Packard Development Company, L.P.
Ye Lin
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