JP2006344880A - Imaging device with microlens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for improving color shading.
First, an imaging apparatus of the present invention includes an imaging region and a lens array. In this imaging area, light receiving elements having a flat light receiving area are arranged in a pixel unit, and an image signal is generated by photoelectric conversion for each light receiving element. On the other hand, the lens array is a surface array of microlenses that collect incident light with respect to a light receiving element. In the peripheral portion of the imaging region, the outer edge far from the center of the imaging region among the outer edges facing the short axis direction of the light receiving region is set darker than the outer edge near the center.
[Selection] Figure 2

Description

  The present invention relates to an imaging apparatus including a microlens.

2. Description of the Related Art Conventionally, there is known a technique in which a microlens is arranged for each pixel of an image pickup device and incident light is collected in a light receiving area of each pixel. With such a microlens arrangement, the light receiving efficiency of each pixel can be increased.
By the way, in the vicinity of the periphery of the imaging region, light that has passed through a photographing lens (excluding special optical systems such as a telecentric optical system) is incident obliquely. For this reason, vignetting or the like occurs in the peripheral pixels, and luminance shading that darkens the peripheral portions occurs in the image signal.

Therefore, there is known a technique in which the microlenses are shifted and arranged at a pitch smaller than the pitch of the pixel array as the distance from the center of the imaging region increases (Patent Document 1, etc.). This shifted arrangement makes it possible to collect obliquely incident light at the center of the light receiving area of each pixel. As a result, the light receiving efficiency in the peripheral portion is increased, and the luminance shading of the image signal can be improved.
JP-A-1-213079

  Incidentally, a phenomenon called color shading also occurs in the imaging apparatus. This color shading is a phenomenon in which the color balance of the image signal changes as it approaches the periphery of the imaging region, and color deviation occurs around the screen.

FIG. 10 shows data obtained by actually measuring the color shading of an image signal by projecting a uniform white light source onto a conventional apparatus with a conventional microlens shift.
In FIG. 10, the color shading of the R signal is digitized by the following evaluation value S (R).
S (R) = [(VRL / VGL) − (VRC / VGC)] / (VRC / VGC)
... [1]
(However, the G value of the central pixel in the imaging region is VGC and the R value is VRC. On the other hand, the G value of the peripheral pixels in the imaging region is VGL and the R value is VRL.)
In this case, the left and right edges of the landscape screen have a certain effect on suppressing color shading. However, color shading is prominent at the upper and lower ends of the screen.

As described above, with the microlens shift of Patent Document 1, a certain effect can be obtained in suppressing color shading in one direction of the screen, but correction is insufficient for color shading in the other direction.
Incidentally, the general idea is that the upper and lower ends near the center of the screen are superior in light receiving conditions for pixels, such as the inclination of incident light being smaller than the left and right ends, and are considered advantageous for color shading. However, FIG. 10 described above shows the opposite result, and this cause could not be explained.

  In view of the above-described problems, an object of the present invention is to provide a technique for satisfactorily improving color shading.

  First, a structure common to the imaging apparatus of the present invention is that an imaging area and a lens array are provided. In this imaging area, light receiving elements having a flat light receiving area are arranged in a pixel unit, and an image signal is generated by photoelectric conversion for each light receiving element. On the other hand, the lens array is a surface array of microlenses that collect incident light with respect to a light receiving element.

<< 1 >> In the first invention, the pitch of the microlenses in the minor axis direction of the light receiving area is made shorter than the pitch of the microlenses in the major axis direction.

<< 2 >> In the second aspect of the invention, the microlens is arranged so as to be shifted in the central direction of the imaging area with respect to the light receiving area at least in the periphery of the imaging area. In this case, the shift width of the microlens is set to be larger in the minor axis direction than in the major axis direction of the light receiving area.

<< 3 >> In the third invention, at least in the periphery of the imaging region, the outer edge far from the center of the imaging region out of the outer edges facing the short axis direction of the light receiving region is more concentrated than the outer edge closer to the center. The light distribution is dark.

<< 4 >> Preferably, a color filter array in which color filters for color-separating incident light to the imaging region are arranged is provided. The color filter is disposed on the optical path from the microlens to the light receiving area.

The light receiving area for each pixel of the imaging device has a flat shape, except for some cases such as a square and a circle. The present inventor has noticed that the short axis direction of the flat shape matches the direction in which color shading occurs. Therefore, the present inventor has studied to optimize the arrangement or shape of the microlenses in each of the flat major axis direction and the minor axis direction, and succeeded in correcting color shading satisfactorily.
Specifically, the present inventor has found that the color shading of the imaging apparatus can be improved by any one of the following conditions (1) to (3).

(1) The pitch of the microlenses that has been conventionally substantially equal in the major axis direction and the minor axis direction is intentionally changed. Specifically, the pitch of the microlenses in the minor axis direction is made substantially shorter than the pitch of the microlenses in the major axis direction. Experiments (described later) demonstrate that this new configuration can improve color shading.

(2) The shift width of the microlens, which has conventionally been substantially equal in the major axis direction and the minor axis direction, is intentionally changed. Specifically, the shift width of the microlens in the short axis direction is substantially larger than the shift width of the microlens in the long axis direction. Experiments (described later) demonstrate that this new configuration can improve color shading.

(3) Conventionally, the condensing center of the microlens has been set to be located at the center of the light receiving area in the pixel. The purpose of the shifted arrangement of Patent Document 1 is also to make the condensing center of the microlens substantially coincide with the center of the light receiving area in the periphery of the imaging area. In this case, the brightness of the opposing outer edges of the light receiving area is substantially equal due to the light collection distribution having the peak at the center of the light receiving area. This conventional balance is intentionally broken. Specifically, out of the outer edges facing in the minor axis direction, the outer edge closer to the imaging area is set to a light collection distribution substantially darker than the outer edge closer to the center of the imaging area. With such a new configuration, color shading and crosstalk can be improved.

  In addition, the arrangement | positioning difference and shape difference of a micro lens with respect to the short-axis direction mentioned above and a long-axis direction are not performed without an unlimited. Needless to say, the present invention is limited to a range in which the original function of the microlens for collecting incident light to the light receiving element is maintained.

<Description of pixel structure>
FIG. 1 is a diagram illustrating a pixel cross section of the imaging device 11.
First, a general pixel structure of the imaging device 11 will be described with reference to FIG.
In the imaging region of the imaging device 11, light receiving elements 12 a having a light receiving area 12 having a flat shape (in this case, a rectangle when viewed from above the imaging surface) are arranged in a plane with a predetermined pixel pitch. A lens array 13 a is arranged on the upper layer of these light receiving areas 12. The lens array 13a is configured by arranging microlenses 13 that condense incident light with respect to individual light receiving elements 12a.

A layer of the color filter array 15 is arranged between the microlens 13 and the light receiving element 12a. In this color filter array 15, RGB color filters 14 are arranged on an optical path connecting the microlens 13 and the light receiving area 12.
A light shielding film 16 or the like is provided between adjacent light receiving areas 12. Under this light shielding film 16, a transfer circuit (not shown), a circuit element (not shown) and the like for reading a signal from the light receiving element 12a are formed.

<< Characteristic conditions of microlens 13 >>
In the present embodiment, the arrangement and shape of the microlens 13 are designed so as to satisfy at least one of the following conditions 1 to 3. The conditions 1 to 3 are realized in an adjusted manner within a range in which the original function of the microlens 13 for condensing light in the light receiving area 12 is maintained.

[Condition 1]
As shown in FIG. 2, the lens pitch Lb in the short axis direction is set shorter than the lens pitch La in the long axis direction.
(Lens pitch direction lens pitch Lb) <(Long axis direction lens pitch La) [2]
When the pixel pitch Ra in the major axis direction is different from the pixel pitch Rb in the minor axis direction, this relationship can be defined by the following scaling value relationship.
(Scaling value Lb / Rb in the short axis direction) <(Scaling value La / Ra in the long axis direction)
... [3]
Incidentally, when the microlenses 13 are arranged at a constant pitch independently in the major axis direction and the minor axis direction in the entire imaging region, it is preferable to design so as to satisfy this condition 1.

[Condition 2]
In condition 2, the shift width of the microlens 13 with respect to the light receiving area 12 is set so that the minor axis direction is larger than the major axis direction at least in the periphery of the imaging area.
First, as shown in FIG. 3, the shift width in the major axis direction is set to Za in the peripheral pixels separated by Ka in the major axis direction from the center of the imaging region. On the other hand, the shift width in the short axis direction is Zb in the peripheral pixels separated by Kb in the short axis direction from the center of the imaging region.
At this time, if Ka≈Kb, the lens arrangement is set so as to satisfy the following expression.
(Mission width Zb in the minor axis direction)> (Mission width Za in the major axis direction) [4]
In the general case where Ka ≠ Kb, this relationship can be defined by the following normalized condition.
(Short axis direction shift width Zb / Kb)> (Long axis direction shift width Za / Ka) [5]
Condition 2 is a more general condition than condition 1. Therefore, the condition is applicable to a case where the pitch of the microlenses 13 is changed in the imaging region.

[Condition 3]
In the conventional microlens shift, the shift width of the microlens 13 is determined so that obliquely incident light in the peripheral portion is condensed and distributed in the center of the light receiving area 12 via the microlens 13. As a result, as shown in FIG. 4A, the light is distributed almost symmetrically with respect to the center of the light receiving area 12. In this case, the opposing outer edges of the light receiving area 12 have substantially the same brightness.
In condition 3, the brightness balance of the outer edge is intentionally shifted in the minor axis direction. That is, the outer edge on the outer side of the imaging region in the short axis direction is set to a darker light collection distribution than the outer edge near the center.
Such setting of Condition 3 can be realized by shifting the short-axis direction of the microlens 13 described above or by biasing the lens shape of the microlens 13 in the short-axis direction.

<< Design Example of Micro Lens 13 >>
Next, a specific design example of the microlens 13 will be described with specific experimental data.
Here, the imaging device 11 under the following conditions is designed.
(1) The light receiving area 12 has a rectangular shape with an aspect ratio of about 1: 1.7. (2) The pixel pitch is equal to the vertical and horizontal directions.

  5 to 7 are data obtained by actually measuring the color shading S (R) of the light receiving area 12 while changing the scaling value of the microlens 13. [A] in the figure indicates color shading S (R) in the major axis direction. [B] in the figure indicates color shading S (R) in the minor axis direction.

5-7, although the lens shape of the microlens 13 differs, other measurement conditions are equal and are as follows.
Light source: White light source with a color temperature of 5100K Lens: AiNikkor50mmf1.4
Use f value: 5.6

  The actually measured data of the color shading S (R) thus obtained is approximated by a quadratic function in each of the major axis direction and the minor axis direction. Then, it turns out that the secondary coefficient of a quadratic function changes notably according to the change of a scaling value. From this, it can be seen that the component corresponding to the quadratic term in the color change of the color shading S (R) can be corrected by shifting the microlens 13. In this case, color shading can be minimized by setting the secondary coefficient to zero.

  FIG. 8 shows data obtained by plotting the scaling value and the secondary coefficient at the scaling value from such a viewpoint. FIG. 8A shows data shown in the long axis direction. From this data, an appropriate scaling value in the major axis direction is about 0.99994. On the other hand, FIG. 8B is data shown for the minor axis direction. By extending this data, it can be estimated that an appropriate scaling value is about 0.99985.

From the above experiment, the following design data is obtained for the microlens 13.
Scaling value in the long axis direction of the microlens 13: 0.99994
Scaling value in the minor axis direction of the microlens 13: 0.99985

  By forming the microlens 13 (lens array 13a) based on this scaling value, it becomes possible to satisfactorily correct color shading over the entire screen. As a result, it is possible to improve the phenomenon that color shading occurs only at the top and bottom of the screen as shown in FIG.

Further, it is preferable that the color filter 14 is disposed again on the optical path connecting the microlens 13 and the light receiving area 12. For example, the scaling value of the color filter 14 may be set as follows based on the scaling value of the microlens 13.
Scaling value in the major axis direction of the color filter 14: 0.99997
Scaling value in the minor axis direction of the color filter 14: 0.999925
The design data of the microlens 13 here satisfies all of the above conditions 1 to 3.

<< Explanation of the hypothesis of this embodiment >>
In the following, a hypothetical explanation of the above-described color shading correction principle will be made.

[Cause of color shading]
Here, it is assumed that the main cause of color shading that occurs in the minor axis direction is stray light that enters the light shielding film 16 (including all layers that propagate light such as a wiring layer) from the outer edge of the light receiving region 12.
This stray light reaches an adjacent pixel and is photoelectrically converted as a different color component, thereby generating a different color crosstalk.
If this different color crosstalk occurs uniformly for all color signals, the color balance does not change and color shading does not occur.
However, in the color filter 14, the transmittance of the G filter is higher than the transmittance of the RB filter. In digital cameras and the like, an infrared cut filter is often inserted, and this tendency is further strengthened. Therefore, the G stray light mixed in the RB pixel is brighter than the RB stray light mixed in the G pixel, and the crosstalk due to the stray light tends to work in the direction of strengthening the RB signal.
As a result, in the stray light generation region, the color balance changes and color shading occurs.

[Cause of directionality of color shading]
Next, the relationship between the direction of color shading as shown in FIG. 10 and the shape of the light receiving area 12 will be described.
First, the light receiving area 12 has a flat shape. Therefore, as shown in FIG. 4A, the outer edge facing in the minor axis direction is relatively close to the condensing center of the microlens 13 and becomes brighter. On the other hand, the outer edge facing in the major axis direction is relatively far from the light collection center of the microlens 13 and darkens. For this reason, stray light is strongly generated in the bright short axis direction of the outer edge. Therefore, color shading is more likely to occur in the minor axis direction of the light receiving area 12 than in the major axis direction.

[Measures for color shading in the short axis direction]
Next, based on the above hypothesis, a measure for color shading in the minor axis direction will be considered.
As shown in FIG. 9A, oblique incident light tilted toward the outside of the imaging region arrives at the periphery of the imaging region. Depending on the direction of the incident light, stray light tends to be generated toward the outside of the imaging region.

  Therefore, as shown in FIG. 4B, the light collection distribution in the light receiving area 12 is biased toward the center line of the imaging area along the minor axis direction. As a result, of the outer edges facing in the minor axis direction, the outer edge is darkened. Therefore, the stray light generated at the outer edge is reduced, and the occurrence of crosstalk can be suppressed.

  By the way, when the condensing distribution is simply shifted, as shown in FIG. 4B, the outer edge on the center becomes bright instead of the outer edge on the outer side becoming dark. However, as shown in FIG. 9B, since the obliquely incident light hits the outer edge closer to the center in a direction in which it is difficult to sink, stray light does not increase so much. Therefore, the adverse effects of crosstalk and color shading are small.

  Of course, when the light collection distribution is excessively biased with respect to the light receiving area 12, luminance shading appears remarkably in the minor axis direction. Therefore, it is preferable to bias the light collection distribution within a range in which luminance shading and color shading are balanced.

  The above color shading countermeasures agree with the conditions 1 to 3 of this embodiment. Conditions 1 to 3 of the present embodiment are conditions heuristically obtained from a trial experiment of the shifted arrangement and shape design of the microlens 13. Therefore, regardless of whether the hypothesis described here is true or not, Conditions 1 to 3 of this embodiment are effective for correcting color shading.

<< Additional items of embodiment >>
Note that the present embodiment is not limited to the readout structure of the imaging device 11. For example, the imaging apparatus 11 can employ an XY address reading method such as a CMOS method, a CCD method, or a reading structure including an amplifying element for each pixel.

  In the present embodiment, the rectangular light receiving area 12 has been described. However, the embodiment may be a flat light receiving area 12. For example, the light receiving area 12 may be an elliptical shape, a flat cross shape, a T shape, or the like.

  In the present embodiment, the case where the major axis direction and the minor axis direction of the light receiving area 12 are orthogonal to each other has been described. However, the embodiment is not limited to this. Depending on the shape of the light receiving area 12, the major axis direction and the minor axis direction may not be orthogonal.

  In the present embodiment, the case where pixel arrangement is performed in a vertical and horizontal matrix has been described. However, the embodiment is not limited to this. For example, Conditions 1 to 3 of the present embodiment can be applied to a honeycomb array and other pixel arrays.

  In the present embodiment, the color filter array 15 having the RGB Bayer array has been described. However, the embodiment is not limited to this.

  Further, in the present embodiment, the case where the short axis direction of the light receiving area 12 matches the short side direction of the imaging area has been described. However, the embodiment is not limited to this. For example, in a general interline type CCD image pickup device, a vertical CCD is provided for each column of pixels, so that the light receiving area of the pixel has a vertically long shape. In this case, the short axis direction of the light receiving area coincides with the long side direction of the imaging area.

  In the design example of this embodiment, color shading S (R) between RGs is used. However, the embodiment is not limited to this. For example, the design may be performed using color shading S (B) between BGs. Further, for example, by verifying with an optical simulation of the microlens 13, it is possible to perform a design that satisfies any of the conditions 1 to 3.

  As described above, the present invention is a technique that can be used for an imaging apparatus including a microlens.

3 is a diagram illustrating a pixel cross section of the imaging device 11. FIG. It is a figure explaining the condition 1 of the micro lens. It is a figure explaining the condition 2 of the micro lens. It is a figure explaining the condition 3 of the micro lens 13. FIG. It is a figure which shows the actual measurement data of color shading. It is a figure which shows the actual measurement data of color shading. It is a figure which shows the actual measurement data of color shading. This is data in which the scaling value and the secondary coefficient of color shading are plotted. It is a figure explaining the crosstalk by a stray light. This is experimental data for conventional color shading.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Imaging device, 12 ... Light-receiving area, 12a ... Light receiving element, 13 ... Micro lens, 13a ... Lens array, 14 ... Color filter, 15 ... Color filter array, 16 ... Light-shielding film

Claims (4)

An imaging region in which light receiving elements having a flat light receiving area are arranged in units of pixels, and an image signal is generated by photoelectric conversion for each light receiving element;
A lens array in which microlenses that collect incident light with respect to the light receiving element are arranged in a plane;
The pitch of the micro lens in the short axis direction of the light receiving area is shorter than the pitch of the micro lens in the long axis direction. An imaging region in which light receiving elements having a flat light receiving area are arranged in units of pixels, and an image signal is generated by photoelectric conversion for each light receiving element;
A lens array in which microlenses that collect incident light with respect to the light receiving element are arranged in a plane;
In the peripheral part of the imaging area, the microlens is shifted from the light receiving area in the center direction of the imaging area,
The image pickup apparatus, wherein the shift width of the microlens is larger in the minor axis direction than in the major axis direction of the light receiving area. An imaging region in which light receiving elements having a flat light receiving area are arranged in units of pixels, and an image signal is generated by photoelectric conversion for each light receiving element;
A lens array in which microlenses that collect incident light with respect to the light receiving element are arranged in a plane;
In the periphery of the imaging region, the outer edge far from the center of the imaging region among the outer edges facing the short axis direction of the light receiving region has a light collection distribution of the microlens more than the outer edge near the center. An imaging device characterized by darkness. The imaging apparatus according to any one of claims 1 to 3,
A color filter array in which color filters for color-separating incident light on the imaging region are arranged;
An image pickup apparatus, wherein the color filter is disposed on an optical path from the microlens to the light receiving area.

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