1. Field of the Invention
The present invention relates to an imaging device which uses a solid state image pickup element. More particularly, the present invention relates to an imaging device which (a) adjusts the ratio of color excitation values (for example, red/green/blue "RGB") of the image pickup element in correspondence with the exit pupil position of an associated optical unit, and (b) adjusts the ratio of the color excitation values in correspondence with the size of an aperture controlling light passing through the associated optical unit.
2. Description of the Related Art
A conventional imaging device converts optical images to image signals using a solid state image element, such as a CCD element (charge coupled device) or a MOS element (metal oxide semiconductor element). Moreover, recent technological developments have attempted to produce such an imaging device with a more compact solid state image element and to increase the number of pixels in the imaging device. Unfortunately, as an imaging device is made with a more compact solid state image element, the aperture efficiency of the image pickup element is reduced and the signal-to-noise (S/N) ratio of the image signals is decreased.
FIG. 1 is a diagram illustrating a cross section of a conventional CCD image pickup element. Referring now to FIG. 1, an image pickup element 1 includes a light receiving unit 1a (such as a "pixel") which converts light to an electric charge, and a transfer unit 1b which transfers the electric charge from light receiving unit 1a. Light receiving part 1a and transfer unit 1b are formed on the surface of image pickup element 1. An on-chip micro-lens 2 forms a condenser lens which corresponds to light receiving units 1a. On-chip micro-lens 2 is arranged on the surface of image pickup element 1.
With image pickup element 1, the light incident on-chip micro-lens 2 is focused on light receiving unit 1a. For example, as illustrated in FIG. 1, a light ray A and a light ray B are both focused by on-chip micro-lens 2 so that the respective light rays strike light receiving unit 1a. Therefore, the amount of light received by light receiving unit 1a is increased. Consequently, the level of the signals that are optoelectrically converted by light receiving unit 1a becomes larger, and the imaging device can output image signals with a high S/N ratio.
FIG. 2 is a diagram illustrating a conventional electronic still camera using a conventional CCD image pickup element. Referring now to FIG. 2, an aperture 4 and a mirror 5 are arranged in the optical axis of a photographic lens 3, and image pickup element 1 is arranged on the focal plane of photographic lens 3. On-chip micro-lens 2 is formed on the light receiving surface of image pickup element 1. Image pickup element 1 produces three excitation values (referred to as an "R output", a "G output" and a "B" output). A signal processing unit 7a processes the R output, G output and B output of image pickup element 1, and produces corresponding image signals. The G output of image pickup element 1 is connected "as is" (that is, "directly") to signal processing unit 7a. The R output and the B output of image pickup element 1 are connected to signal processing unit 7a through variable gain amplifiers 6a and 6b, respectively. A recording unit 7 records the image signals produced by signal processing unit 7a.
A light measurement unit 8a measures the subject brightness (that is, the brightness of a subject (not illustrated)) and is arranged in a position which is illuminated by the light reflected from mirror 5. An exposure calculation unit 8 is connected to light measurement unit 8a. Also, control terminals (not illustrated) of aperture 4 and image pickup element 1 are individually connected to output terminals of exposure calculation unit 8. A color measurement unit 9a measures the color of the ambient light. A white balance control unit 9 is connected to the control terminals of variable gain amplifiers 6a and 6b, and to color measurement unit 9a. In addition, a control unit 10 controls, and is connected to, photographic lens 3, image pickup element 1, signal processing unit 7a, recording unit 7, exposure calculation unit 8, and white balance control unit 9. A release button 10a is also connected to control unit 10. Release button has a half-push position and a full-push position and is pushed by a photographer to either the half-push position or the full-push position to initiate specific camera operations.
In an electronic still camera as illustrated in FIG. 2, when release button 10a is pushed to the half-push position, exposure calculation unit 8 incorporates the light measurement value of the subject brightness based on a measurement by light measurement unit 8a, and calculates the correct aperture value and exposure. Moreover, white balance control unit 9 incorporates the color measurement value of the ambient light based on a measurement by color measurement unit 9a, and controls the gain of variable gain amplifiers 6a and 6b in correspondence with the ratio of the three excitation values (RGB) of the ambient light. In this state, if release button 10a is pushed to the full-push position, mirror 5 flies up, and exposure calculation unit 8 adjusts aperture 4 to the correct aperture value.
The amount of light incident on image pickup element 1 from photographic lens 3 is restricted by aperture 4, and an optical image is focused on the light receiving plane of image pickup element 1. On-chip micro-lens 2 allows image pickup element 1 to raise the light receiving efficiency, and to produce image signals with a high S/N ratio. The white balance of the image signal produced by image pickup lens 1 is adjusted by variable gain amplifiers 6a and 6b. Signal processing unit 7a processes the image signal produced by image pickup lens 1 and adjusted by variable gain amplifiers 6a and 6b. Such processing performed by signal processing unit 7a can include, for example, gamma correction, and gain adjustment. The processed image signal produced by signal processing unit 7a is recorded in record unit 7.
As the exit pupil position of photographic lens 3 approaches image pickup element 1, the optical rays incident on on-chip micro-lens 2 from the side direction become more numerous. Because the light rays incident from the side direction are strongly affected by color aberrations on the axis of on-chip micro-lens 2, the size of spots focused on light receiving unit 1a change for every wavelength of light. For this reason, the amount of light output from light receiving unit 1a varies for every wavelength of light. This variation in the amount of light output from light receiving unit 1a causes the color phase of the light output from light receiving unit 1a to vary.
FIG. 3(A) is a graph indicating the output of an image pickup element versus the distance between the exit pupil position of an associated optical system and the image pickup element. As illustrated by FIG. 3(A), the ratio of the R output of an image pickup element to the G output of the image pickup element becomes large as the exit pupil position approaches (becomes "near") the image pickup element. As a result, an image produced by the image pickup element appears reddish as the exit pupil position approaches the image pickup element. Moreover, the ratio of B output of the image pickup element to the G output of the image pickup element becomes smaller as the exit pupil position approaches the image pickup element. As a result, the blueness of an image produced by the image pickup element becomes thin as the exit pupil position approaches the image pickup element.
In particular, if a zoom lens is used with an image pickup element, the exit pupil position is moved greatly forward and backward following the adjustment of the image angle. Therefore, there is a wide range of fluctuations in the ratio of RGB output from the image pickup element, and the color phase of the image varies greatly. This kind of color phase variation cannot be corrected by an external light white balance adjustment. Therefore, in a conventional imaging device, the color phase of an image produced by an image pickup element undesireably varies in correspondence with the exit pupil position of an associated photographic lens.
Moreover, referring to the imaging device illustrated in FIG. 2, if aperture 4 is set to the open aperture side (that is, near the fully open position), the light rays incident from the side in relation to on-chip micro-lens 2 become numerous. Therefore, there are changes in the color phase of the image produced by the image pickup element.
FIG. 3(B) is a graph indicating the output of an image pickup element versus the aperture value of an aperture controlling the light incident on the image pickup element. As illustrated by FIG. 3(B), the ratio of R output of an image pickup element to the G output of the image pickup element becomes large as the aperture approaches the open aperture side. As a result, the image produced by the image pickup element appears reddish as the aperture approaches the open aperture side. Moreover, the ratio of B output of the image pickup element to the G output of the image pickup element becomes small as the aperture approaches the open aperture side. As a result, the blueness of the image produced by the image pickup element becomes thin as the aperture approaches the open aperture side,
Moreover, if the white balance is manually selected by the photographer, the white balance is often set to the lowest permissible limit. Therefore, the ratio of RGB output in the state of the open aperture changes, and there is an increased probability that the white balance will become unnatural.
In view of the above, it is difficult to accurately reproduce the color phases of the subject because the ratio of RGB output of an image pickup element changes depending on the aperture value.