Image analysis – Image sensing
Reexamination Certificate
2000-06-26
2004-03-16
Couso, Jose L. (Department: 2721)
Image analysis
Image sensing
Reexamination Certificate
active
06707955
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an image sensing device, an image processing apparatus and method, and a memory medium.
BACKGROUND OF THE INVENTION
FIG. 16
is a schematic block diagram showing the arrangement of a conventional digital still camera. In this digital camera, an overall control circuit
80
detects changes in state of operation switches (main switch, release switch, and the like)
91
, and starts power supply to other blocks.
An object image within the photographing frame range is formed on the image sensing surface of an image sensing unit
82
via a main photographing optical system
81
. The image sensing unit
82
converts this image into an analog electrical signal, and supplies it to an A/D converter
83
in turn in units of pixels. The A/D converter
83
converts the analog electrical signal into a digital signal, and supplies the digital signal to a process circuit
84
.
The process circuit
84
generates R, G, and B image signals on the basis of the input data. In the state before photographing, these image signals are periodically transferred to a video memory
89
via a memory controller
85
in units of frames, thus displaying an image on a display unit
90
. In this manner, a viewfinder display is implemented.
On the other hand, when the photographer operates one of operation switches
91
to instruct execution of photographing, pixel data for one frame output from the process circuit
84
are stored in a frame memory
86
in accordance with a control signal output from the overall control circuit
80
. The data in the frame memory
86
are compressed by the memory controller
85
and a work memory
87
on the basis of a predetermined compression format, and the compression result is stored in an external memory (e.g., a nonvolatile memory such as a flash memory or the like)
88
.
When the photographer reviews photographed images, the compressed data stored in the external memory
88
are read out, and are expanded by the memory controller
85
to be converted into normal data in units of photographing pixels. The conversion result is transferred to the video memory
89
, and an image is displayed on the display unit
90
.
In such digital camera, microlenses
31
shown in
FIGS. 2A and 2B
are provided to an image sensing element of the image sensing unit
32
in units of photosensitive pixels so as to improve optical sensitivity in units of pixels. In
FIGS. 2A and 2B
, reference numeral
30
denotes a photographing lens (main photographing optical system) as a field lens; and
32
, each photosensitive section (light-receiving section) of the image sensing element.
Since the microlenses
31
are provided in units of photosensitive pixels of the image sensing element, even when the image sensing element has a narrow effective sensitivity range, marginal light can be effectively focused on photosensitive pixels.
When light rays have passed through the photographing lens
30
strike the image sensing element in a direction nearly parallel to the optical axis, as shown in
FIG. 2A
, incoming light rays focus on the photosensitive section
31
without posing any serious problem. However, when light rays obliquely enter the photographing lens
30
, as shown in
FIG. 2B
, only some of the incoming light rays become incident on the photosensitive section
31
in an area (peripheral portion of the image sensing element) separated from the optical axis of the photographing lens
30
.
Such light amount drop is normally called white shading. This phenomenon becomes conspicuous as the pixel position on the image sensing element separates farther away from the optical axis of the photographing lens, and also as the focal length of the photographing lens
30
decreases (in practice, as the distance from the image sensing element to the pupil position of the photographing lens becomes shorter).
FIGS. 3A and 3B
are graphs showing changes in white shading when the stop of the photographing lens
30
has changed.
FIGS. 3A and 3B
respectively show the relationship between the image height and relative sensitivity on the image sensing element in a full-aperture state (FIG.
3
A), and a stop-down state (FIG.
3
B). In the full-aperture state of the stop, the relative sensitivity drops largely compared to the central portion with increasing height of an image formed on the image sensing element, since many light components that obliquely enter the photographing optical system are included, as shown in FIG.
2
B.
On the other hand, when the stop is stopped down, the relative sensitivity changes little even when the image height becomes larger, since light components that obliquely enter the photographing lens
30
are suppressed by the stop effect.
As a method of correcting white shading produced by a combination of the photographing lens
30
and microlenses on the image sensing element, a method disclosed in Japanese Patent Laid-Open No. 9-130603 is known. In this method, assuming that each pixel position on the image sensing element is expressed by a pointer indicated by a horizontal direction X and vertical direction Y, shading correction data H(x) for one horizontal line, and shading correction data V(y) for one vertical line are used. The shading correction data for one horizontal line and one vertical line assume larger values as the pixel position approaches an end (periphery). A pixel S(i, j) on the image sensing element is multiplied by horizontal correction data H(i) and vertical correction data V(j) as per:
S
(
i, j
)×
H
(
j
)×
V
(
j
)→
S
(
i, j
)
In this way, sensitivity drop in the peripheral portion of the image sensing element due to shading is apparently prevented.
When a digital camera is constructed using the same optical system (e.g., exchangeable lens system) as that of a conventional single-lens reflex silver halide camera, an image sensing element having an image sensing area considerably larger than that of a normal image sensing element is required.
However, an image sensing element having such a large image sensing area often suffers a problem of sensitivity nonuniformity of color filters and the like, which is produced in the manufacture process. In view of the whole image sensing element, only a local area suffers a decrease or increase in sensitivity, which cannot be corrected by simply setting higher gains toward the periphery.
In
FIG. 4
,
401
indicates an example of the sensitivity distribution of the image sensing element, in which the central sensitivity is higher than the peripheral one. Such sensitivity nonuniformity can be corrected to some extent using the aforementioned conventional method.
For example,
402
in
FIG. 4
expresses a function of horizontal sensitivity correction data H(i), which has smaller correction coefficient values at the central portion and larger ones at the periphery. On the other hand,
403
in
FIG. 4
expresses a function of vertical sensitivity correction data V(j), which has smaller correction coefficient values at the central portion and larger ones at the periphery.
Therefore, correction can be done using such functions by calculating a value S(i, j) at each pixel point as per:
S
(
i, j
)×
H
(
i
)×
V
(
j
)→
S
(
i, j
)
However, when local sensitivity nonuniformity is present in a plurality of areas on the frame, as indicated by
501
in
FIG. 5
, it cannot be perfectly removed by the above-mentioned correction method that simply uses shading correction data H(x) for one horizontal line and shading correction data V(y) for one vertical line.
On the other hand, in the device structure of an image sensing element indicated by
601
in
FIG. 6
, since aluminum interconnects and the like run to sandwich, e.g., two sensitivity areas, the level of incoming light to one (G sensitivity area) of two sensitivity areas lowers in areas on the left side in
FIG. 6
, while one (R sensitivity area) of two sensitivity areas lowers in areas on the right side in FIG.
6
. Therefore, the two sensitivity areas (G and R) have different sensitivity levels depending
Canon Kabushiki Kaisha
Couso Jose L.
Morgan & Finnegan , LLP
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