JP2009141571A - Imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging apparatus capable of sufficiently exhibiting the correction effect in color shading correction, without determining a color temperature. <P>SOLUTION: The imaging apparatus includes an imaging element, a cut filter, a light quantity detection part, a correction coefficient calculation part, and a color shading correction part. The imaging element images an image formed by incident light passing through a photographing lens. The cut filter is disposed in front of an imaging plane of the imaging element and cuts a predetermined wavelength component in the incident light. The light quantity detection part detects the quantity of light in the vicinity of a cutoff wavelength of the cut filter in a detection area corresponding to at least a part of a photographing angle. The correction coefficient calculation part calculates, based on the quantity of light, a correction coefficient for correction color non-uniformity of the image caused by deviation of the cutoff wavelength of the cut filter that occurs in accordance with an incident angle of the incident light. The color shading correction part adjusts the correction coefficient in accordance with respective positions on the imaging element, thereby removing color non-uniformity. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an imaging apparatus having a color shading correction function.

Conventionally, an imaging apparatus having a color shading correction function has been proposed in order to correct color unevenness (color shading) occurring between the center and the periphery of an image (see, for example, Patent Document 1). Patent Document 1 discloses an imaging apparatus that determines a color temperature of a light source and performs color shading correction according to the color temperature.
JP 2005-278004 A

  By the way, it is known that color shading also occurs when a cut filter for cutting unnecessary infrared light is arranged in front of the image sensor. This is because the optical path length of the incident light when passing through the cut filter differs depending on the incident angle of the light, which causes the incident angle dependence of the spectral characteristics of the cut filter. In this case, the wavelength (cutoff wavelength) cut by the cut filter shifts to the short wavelength side as the incident angle of the incident light increases. For this reason, when the incident angle is large, the signal amount of the R (red) signal is reduced and color shading occurs. Therefore, it is desirable to sufficiently perform color shading correction based on incident angle dependency.

  However, the determination of the color temperature by the imaging apparatus of Patent Document 1 has a problem that it is difficult to sufficiently perform the color shading correction based on the incident angle dependency.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an imaging apparatus that can sufficiently bring out the correction effect of color shading correction.

  An imaging apparatus according to a first invention includes an imaging element, a cut filter, a light amount detection unit, a correction coefficient calculation unit, and a color shading correction unit. The imaging element captures an image of incident light that has passed through the photographing lens. The cut filter is disposed in front of the imaging surface of the imaging device and cuts a predetermined wavelength component of the incident light. The light amount detection unit detects a light amount in the vicinity of the cutoff wavelength of the cut filter in a detection region corresponding to at least a part of the shooting angle of view. The correction coefficient calculation unit obtains a correction coefficient for correcting the color unevenness of the image due to the shift of the cutoff wavelength of the cut filter generated according to the incident angle of the incident light based on the light amount. The color shading correction unit removes the color unevenness by adjusting the correction coefficient according to each position on the image sensor.

  In a second aspect based on the first aspect, the cut filter cuts the wavelength component on the long wavelength side including the infrared region of the incident light. The light amount detection unit detects the light amount on the shorter wavelength side than the cutoff wavelength.

  In a third aspect based on the first aspect, the cut filter cuts a wavelength component on the short wavelength side including the ultraviolet region in the incident light. The light amount detection unit detects the light amount on the longer wavelength side than the cutoff wavelength.

  According to the imaging apparatus of the present invention, the correction coefficient for correcting the color unevenness of the image due to the shift of the cutoff wavelength of the cut filter generated according to the incident angle of the incident light is obtained based on the light amount. Then, the color unevenness is removed by adjusting the correction coefficient according to each position on the image sensor. Thereby, the imaging device of the present invention can sufficiently bring out the correction effect of the color shading correction.

<Description of the configuration of the electronic camera>
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a schematic diagram showing a configuration of an optical system in the electronic camera of the present embodiment. As shown in FIG. 1, the electronic camera 1 includes a camera body 11 and a lens unit 13 that houses a photographing lens 12. The lens unit 13 is replaceably attached to the camera body 11 via a mount (not shown). In this state, the lens unit 13 is electrically connected to the camera body 11.

  In the camera body 11, a main mirror 14, a cut filter 15, an image sensor 16, a finder optical system (17 to 20), a re-imaging lens 21, and an infrared sensor 22 are arranged. The main mirror 14, the cut filter 15, and the image sensor 16 are disposed along the optical axis of the photographing lens 12. The finder optical system (17 to 20), the re-imaging lens 21 and the infrared sensor 22 are arranged in the upper region of the camera body 11.

  The main mirror 14 rotates around a rotation shaft (not shown), thereby switching between an observation state and a retracted state. The main mirror 14 in the observation state is inclined and disposed in front of the cut filter 15 and the image sensor 16. The main mirror 14 in the observation state reflects the incident light captured by the photographing lens 12 upward and guides it to the finder optical system (17 to 20).

  On the other hand, the retracted main mirror 14 is flipped upward and deviated from the optical axis of the taking lens 12. When the main mirror 14 is in the retracted state, incident light captured by the photographing lens 12 is guided to the cut filter 15 side.

  The finder optical system (17 to 20) includes a focusing screen 17, a condenser lens 18, a pentaprism 19, and an eyepiece lens 20.

  The focusing screen 17 is located above the main mirror 14 in the retracted state. Incident light imaged by the focusing screen 17 enters the lower surface of the pentaprism 19 via the condenser lens 18. Incident light incident on the lower surface of the pentaprism 19 first enters the pentaprism 19 and reflects the inner surface of the pentaprism 19. Subsequently, the reflected incident light is emitted to the outside of the pentaprism 19. Furthermore, the emitted incident light goes to the eyepiece lens 20 and to the infrared sensor 22 through the re-imaging lens 21.

  FIG. 2 is a block diagram of the inside of the electronic camera 1 shown in FIG. As shown in FIG. 2, on the camera body side of the electronic camera 1, a cut filter 15, an image sensor 16, an infrared sensor 22, an A / D converter 22a, a timing generator (TG) 23, and an analog front end are provided. Unit (hereinafter referred to as “AFE”) 24, image processing unit 25, buffer memory 26, recording interface (recording I / F) 27, operation switch (SW) 28, and CPU (Central processing Unit) 29. A RAM (Random Access Memory) 30, a ROM (Read Only Memory) 31, and a bus 32. Among these, the image processing unit 25, the buffer memory 26, the recording interface (recording I / F) 27, the CPU 29, the RAM 30, and the ROM 31 are connected to each other via a bus 32. The operation switch 28 is connected to the CPU 29.

  The cut filter 15 cuts the infrared wavelength component and the ultraviolet wavelength component. The cut filter 15 cuts the wavelength component on the long wavelength side including the infrared region of the incident light, and cuts the wavelength component on the short wavelength side including the ultraviolet region of the incident light.

  The cut filter 15 is formed by depositing an IR (InfraRed) cut filter for cutting an infrared component on an optical low-pass filter for cutting the wavelength of an ultraviolet component.

  FIG. 3 is a diagram illustrating an example of the transmittance of the cut filter 15 with respect to the wavelength. The cut-off wavelength on the red outer side is 670 nm, and the cut-off wavelength on the ultraviolet side is 400 nm. The transmittance characteristic indicated by the solid line represents the case where there is no influence of the incident angle dependency. As the incident angle of incident light increases, the transmittance characteristic shifts to the short wavelength side as a whole as shown by a broken line as an example. If this shift is large, the signal amount of the R signal decreases, and the degree of color shading also increases. The influence of the incident angle dependency on the cut filter 15 is large in the vicinity of 670 nm. For this reason, when the luminance of the light source is high at a wavelength that has an influence on the incident angle dependency, the signal amount of the R signal is decreased and the color unevenness is increased.

  The imaging element 16 generates an image signal (analog signal) of the main image by photoelectrically converting incident light that has passed through the cut filter 15.

  Note that three types of color filters of red (R), green (G), and blue (B) are arranged in, for example, a Bayer array on the imaging surface of the imaging element 16 in order to detect the color of the subject image. Thereby, three types of pixels of the R pixel, the G pixel, and the B pixel are arranged on the imaging surface of the imaging element 16. That is, the image signal of the main image is composed of three types of signals: R signal, G signal, and B signal.

  The timing generator (TG) 23 sends a drive signal to each of the image sensor 16 and the AFE 24 in accordance with an instruction from the CPU 29, thereby controlling the drive timing of both.

  The AFE 24 is an analog front-end circuit that performs signal processing on the image signal generated by the image sensor 16. The AFE 24 performs correlated double sampling of image signals, A / D conversion of image signals, and the like. The image signal (digital signal) output from the AFE 24 is input to the image processing unit 25 as image data of the main image.

  The image processing unit 25 performs various types of image processing on the image data of the main image output from the AFE 24. The image processing unit 25 includes a color shading correction unit 25a that performs color shading correction on image data, a white balance correction unit (WB) 25b that performs white balance correction on image data, and a color interpolation processing unit that performs color interpolation processing on image data. 25c is arranged.

  The buffer memory 26 is also a memory that temporarily stores image data of the main image.

  A connector for connecting the recording medium 33 is formed in the recording interface (recording I / F) 27. The recording interface (recording I / F) 27 accesses the recording medium 33 connected to the connector, and writes and reads image data of the main image.

  The operation switch 28 is a release button, a command dial, or the like, and gives a signal to the CPU 29 in accordance with the operation content by the user. For example, the user can give a shooting instruction to the CPU 29 by fully pressing the release button.

  In addition, the infrared sensor 22 detects the amount of light in the vicinity of the cutoff wavelength (670 nm) of the cut filter 15 in a detection region corresponding to at least a part of the shooting field angle of the image sensor 16.

  FIG. 4 is a diagram illustrating an example of a detection region by the infrared sensor 22. Here, when the detection area of the infrared sensor 22 is associated with the shooting angle of view of the image sensor 16, it is as follows. For simplicity, the shooting angle of view is a rectangular area indicated by a in the figure. The infrared sensor 22 has a detection region having the same size as the rectangular region indicated by b in the figure, and calculates the amount of light in the vicinity of the cutoff wavelength (670 nm) from the incident light that has passed through the main mirror 14 prior to imaging. To detect. The detection area in which the infrared sensor 22 detects the amount of light is an example, and is not limited thereto. Detection may be performed by providing detection areas corresponding to the four corners of the area indicated by a in the drawing.

  FIG. 5 is a diagram illustrating an example of the sensitivity of the infrared sensor 22 with respect to the wavelength. The distribution shown by the solid line and the broken line represents an example of the intensity distribution of sensitivity, and has a peak at the cutoff wavelength (670 nm).

  In the present embodiment, the amount of light in the vicinity of the cutoff wavelength (670 nm) among the incident light incident on the cut filter 15 can be estimated based on the amount of light detected by the infrared sensor 22.

  For example, when the amount of light near the cutoff wavelength (670 nm) increases, the amount of light detected by the infrared sensor 22 increases from the light amount of the intensity distribution indicated by the solid line in FIG. 5 to the light amount of the intensity distribution indicated by the broken line. As a result, it can be seen that when the amount of light detected by the infrared sensor 22 increases, the risk of color unevenness increases.

  An analog signal corresponding to the amount of light detected by the infrared sensor 22 is digitized via the A / D converter 22a and then input to the CPU 29 as a signal level of the amount of light.

  The CPU 29 is a processor that performs overall control of the electronic camera 1. The CPU 29 calculates a parameter for each process and controls each part of the electronic camera 1 by executing a sequence program stored in advance in the ROM 31.

  Further, the CPU 29 can capture lens information from a lens CPU (not shown) that controls the lens unit 13. The lens information includes information such as the focal length of the photographing lens 12, the subject distance, the aperture value (F value), and the exit pupil position (EPD: Exit Pupil Distance).

  The correction coefficient calculation unit 29 a of the CPU 29 estimates the amount of light in the region affected by the incident angle dependency of the cut filter 15 by the infrared sensor 22. Then, the correction coefficient calculation unit 29a obtains a correction coefficient for correcting the color unevenness (color shading) of the image based on the light amount detected by the infrared sensor 22.

Here, when the light amount detected by the infrared sensor 22 is small, the cut filter 15 reduces the light amount of the R component that is cut in the vicinity of the cutoff wavelength. In this case, the color shading correction unit 25a corrects weakly using the correction coefficient. On the other hand, when the amount of light detected by the infrared sensor 22 is large, the cut filter 15 increases the amount of R component light that is cut in the vicinity of the cutoff wavelength. In this case, the color shading correction unit 25a performs a strong correction using the correction coefficient. As a result, the color shading correction unit 25a can perform color shading correction without determining the color temperature of the light source. The color shading correction unit 25a performs color shading correction using a lookup table T _R (LUT) described below.

Figure 6 is a diagram showing an example of a look-up table T _R. The look-up table T _R is stored in the ROM31 as a LUT for the R signal. Color shading varies depending on the image height direction. Here, in order to correct this color shading, it is conceivable to use an R signal correction gain for the position of the image corresponding to each R pixel of the image sensor 16. In the present embodiment, in order to improve the processing speed, to set the gain value of each block is divided into blocks images in a look-up table T _R. Here, the block is a plurality of blocks formed by dividing an image. For example, the block is 750 blocks formed by dividing an image into 25 vertical and 30 horizontal. Hereinafter, the i-th block is referred to as "i-th block", put a "g _ri" gain g _r to be multiplied by the R signals of each pixel in the i-th block.

The division into 750 blocks is an example, and the number is not limited to this. The look-up table T _R, the gain group _{g r1, ..., g r750} are stored as a table.

In the present embodiment, it is prepared and a look-up table T _R for calculation correction coefficient, a look-up table T _R for each combination of the F value and the EPD advance.

<Description of electronic camera operation>
Next, the flow of operation of the CPU 29 relating to shooting will be described. FIG. 7 is a flowchart showing the operation of the CPU related to shooting.

  Step S101: The CPU 29 determines whether or not the release button has been pressed halfway. If the release button has been pressed halfway (step S101: Yes), the process proceeds to step S102. On the other hand, when the release button is not half-pressed (step S101: No), step S101 is repeated.

  Step S102: The CPU 29 drives the infrared sensor 22. Then, the infrared sensor 22 starts detecting the amount of light.

  Step S103: The CPU 29 fetches lens information (focal length, subject distance, F value, EPD, etc.) from the lens CPU (not shown) of the lens unit 13. Then, the CPU 29 sets shooting conditions used for shooting the main image based on the lens information.

Step S104: the correction coefficient calculating unit 29a in the CPU29 selects a look-up table T _R in combination with the F value and the EPD that has been set. The correction coefficient calculation unit 29a then compares the signal level of the light quantity detected by the infrared sensor 22 with the signal level of the reference light quantity ((light quantity signal level / reference light quantity signal level), hereinafter referred to as “light quantity ratio”. ) The signal level of the reference light amount will be described in the process of step S107.

  Step S105: The CPU 29 determines whether or not the release button has been fully pressed. If the release button has not been fully pressed (step S105: No), step S105 is repeated. When the release button is fully pressed (step S105: Yes), the process proceeds to step S106.

  Step S106: The CPU 29 performs shooting under the shooting conditions set in step S103. That is, the CPU 29 puts the main mirror 14 in the retracted state and drives the image sensor 16 via the timing generator 23. Thereby, the image data of the main image is acquired. Note that the main image data acquired by this shooting is buffered in the buffer memory 26 after passing through the AFE 24.

  Step S107: The color shading correction unit 25a performs color shading correction. Specific processing will be described below.

  FIG. 8 is a conceptual diagram illustrating color shading correction in the present embodiment. In this embodiment, a correction curve for correcting color shading (curve a shown in FIG. 8) is obtained in advance. In the horizontal axis, the center of the optical axis is 0%, and the maximum image height of the photographed image height is 100%. The vertical axis represents the degree of color shading when uniform incident light (R component) is incident from the optical axis center to the maximum image height. Here, 100% color shading means that no color shading (color unevenness) has occurred. The degree of color shading corresponds to the signal level of the R signal. That is, as the degree of dissociation from 100% of color shading increases, the signal level of the R signal decreases. It should be noted that the color shading varies depending on the F value and EPD, but it is assumed that the influence of the F value and EPD is removed from this curve data. The signal level of the reference light amount is the signal level of the light amount detected by the infrared sensor 22 when the correction curve in FIG. 8 is obtained. The signal level value of the reference light quantity is stored in the ROM 31 in advance.

  Here, a straight line b shown in FIG. 8 represents a case where the signal level (curve a) of the R signal for each image height is corrected to be the same signal level. The color shading correction unit 25a corrects the signal level (curve a) of the R signal for each image height to be the same signal level (straight line b) by using the correction coefficient.

FIG. 9 is a diagram for explaining the correction coefficient for color shading in the present embodiment. In FIG. 9, a curve a is a graph obtained by taking the reciprocal of the degree of color shading (the signal level of the R signal) for each image height in the correction curve of FIG. The reciprocal value is a correction coefficient. Therefore, when the correction coefficient for each image height shown in FIG. 9 and the color shading degree (signal level of the R signal) for each image height shown in FIG. 8 are correlated and multiplied, a straight line b shown in FIG. 8 is obtained. That is, color shading correction is performed. The curves b and c shown in FIG. 9 will be described later. In this embodiment, each block position in the look-up table T _R is previously associated with each image height correction curve shown in FIG. Each correction factor (gain group) is previously stored in the correction coefficient calculation of the look-up table T _R (LUT) within the ROM 31.

Correction coefficient calculating unit 29a of the CPU29, first, gain group _g r1 from the look-up table T _R for calculation correction coefficient, _..., acquires _{g R750.}

Here, from the light quantity ratio obtained in step S104, the correction coefficient curve at the time of photographing is obtained by multiplying the value of each correction coefficient of the curve a by the light quantity ratio. When the light quantity ratio is larger than 1, a curve b in FIG. 9 is obtained as an example. Further, when the light amount ratio is smaller than 1, a curve c in FIG. 9 is obtained as an example. This corresponds to applying a light amount ratio on the gains set of look-up table T _R for calculation correction coefficient. The correction coefficient calculation unit 29a multiplies the gain groups g _r1 ,..., G _r750 by the light amount ratio, thereby obtaining new gain groups g _{r1 ,.}

In the case where the infrared sensor 22 does not detect the light amount, the correction coefficient calculation unit 29a is the gain group g _r1 each gain set in the look-up table T _R for calculation correction factors as _1, ..., acquires g _R750. In this case, color shading is based on the F value and EPD. Further, obtaining the correction coefficient from the light amount ratio is an example, and the present invention is not limited to this.

The correction coefficient calculation unit 29a is the gain group _g r1 from the look-up table T _R of the combination of the set F values and EPD in step S103, _..., acquires _{g R750.}

Correction coefficient calculating unit 29a, respectively multiplies the respective gains set in the look-up table T _R after the light amount ratio is multiplied, the respective gains set in the lookup table T _R corresponding to the set F values and EPD during shooting Finally, the color shading gain group g _r1 ,..., G _r750 is acquired.

Then, the CPU 29 sets the acquired gain groups g _r1 ,..., G _r750 to the color shading correction unit 25a.

The color shading correction unit 25a uses the acquired gain groups g _r1 ,..., G _r750 to multiply the R signal of each pixel in each block. As a result, color shading correction is performed on the image data. Subsequently, the image processing unit 25 performs white balance correction and color interpolation processing based on the image data subjected to the color shading correction.

  Step S108: The CPU 29 compresses the image data of the main image after the image processing and stores it in the recording medium 33.

  As described above, in the electronic camera 1 of the present embodiment, the infrared sensor 22 detects the amount of light in the vicinity of the cutoff wavelength of the cut filter 15. Then, the correction coefficient calculation unit 29a obtains a correction coefficient based on the light amount. Next, the color shading correction unit 25a adjusts the correction coefficient according to each position on the image sensor 16, thereby generating image data from which the color unevenness has been removed.

Therefore, the electronic camera 1 of the present embodiment can sufficiently bring out the correction effect of color shading correction. Furthermore, the electronic camera according to the present invention does not need to determine the color temperature at the time of color shading correction. Further, in the full-size image sensor, the size of the image sensor increases, so that the color shading at the four corners of the image is worse than that of the APS size image sensor. However, even in such a case, the electronic camera 1 of the present embodiment can perform color shading correction by obtaining a correction coefficient based on the light amount.
<Supplementary items of the embodiment>
The electronic camera 1 of the present embodiment may perform the same color shading correction as the R signal described above on the B signal even on the ultraviolet side. However, since the cutoff wavelength differs between the ultraviolet side and the infrared side, the amount of incident light on the ultraviolet side that passes through the cut filter 15 increases due to a shift in wavelength characteristics. Therefore, it is only necessary to correct the influence of color shading due to this increase.

  In this embodiment, the infrared sensor 22 is used to detect the light amount. For example, a multi-photometric sensor that is used for white balance correction and an infrared sensor that detects the light amount are combined. May be used. The cut-off wavelength 670 nm is an example, and is not limited to this frequency. For example, the cut-off wavelength 670 nm may be in the range of 650 nm to 700 nm according to the specifications of the cut filter 15. Moreover, the cut-off wavelength of 400 nm on the ultraviolet side is an example, and is not limited to this wavelength.

  In the present embodiment, three types of RGB color filters are used as an example, but color shading correction may be performed on the complementary color of the R signal using a complementary color filter.

The schematic diagram which shows the structure of the optical system in the electronic camera of this embodiment. It is a block diagram inside the electronic camera 1 shown in FIG. The figure which shows an example of the transmittance | permeability of the cut filter with respect to a wavelength The figure which shows an example of the detection area | region by the infrared sensor 22 The figure which shows an example of the sensitivity of the infrared sensor 22 with respect to a wavelength It illustrates an example of a look-up table T _R Flow chart showing CPU operation related to shooting Conceptual diagram for explaining color shading correction in the present embodiment The figure explaining the correction coefficient for color shading in this embodiment

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Electronic camera, 15 ... Cut filter, 16 ... Image sensor, 25a ... Color shading correction part, 29a ... Correction coefficient calculating part, 22 ... Infrared sensor

Claims (3)

An image sensor that captures an image of incident light that has passed through the taking lens;
A cut filter that is disposed in front of an imaging surface of the imaging element and cuts a predetermined wavelength component of the incident light;
A light amount detection unit for detecting a light amount in the vicinity of a cutoff wavelength of the cut filter in a detection region corresponding to at least a part of the shooting angle of view;
A correction coefficient calculation unit for determining a correction coefficient for correcting color unevenness of the image based on the amount of light due to a shift in a cutoff wavelength of the cut filter that occurs according to an incident angle of the incident light;
A color shading correction unit that removes the color unevenness by adjusting the correction coefficient according to each position on the image sensor;
An imaging apparatus comprising: The imaging apparatus according to claim 1,
The cut filter is for cutting a wavelength component on the long wavelength side including the infrared region of the incident light,
The imaging apparatus according to claim 1, wherein the light quantity detection unit detects a light quantity on a shorter wavelength side than the cutoff wavelength. The imaging apparatus according to claim 1,
The cut filter is for cutting the wavelength component on the short wavelength side including the ultraviolet region of the incident light,
The imaging apparatus according to claim 1, wherein the light quantity detection unit detects a light quantity on a longer wavelength side than the cutoff wavelength.

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