JP2009296348A - Image capturing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem that in a monitor camera, a housing with an optical window to be protected or invisible from outside is mounted thereon and there is a case where the optical window is comprised of a lowered transmittance smoked color so as to be removable or invisible from outside depending on use environment but if spectral transmittance of the optical window is not properly determined, color balance of an image picked up by the monitor camera is lost when removably mounting the optical window on the housing. <P>SOLUTION: Spectral transmittance of an optical window is adjusted so that a change between transmissive light quantitative ratio of red and green determined when mounting the optical window in front of the optical system and the transmissive light quantitative ratio of blue and green is maintained within ≤5.0% on the basis of the transmissive light quantitative ratio of red and green and the transmissive light quantitative ratio of blue and green obtained by integrating from a visible wavelength region to an infrared wave length region based on composite transmittance of an image pick-up optical system, an infrared cut filter and image sensor color filter. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an adjustment range of a spectral transmittance of an optical window that stabilizes an influence on a color balance of a photographed image in an imaging apparatus including a housing having a detachable optical window, and an imaging apparatus including the adjustment range. .

  As a conventional example relating to the adjustment of the spectral transmittance of an optical filter provided in an imaging device, there is “aperture device and optical filter” described in Patent Document 1. This is because the transmittance variation in the infrared region of the imaging lens is corrected by the transmittance of the optical filter, and in the infrared region, a transmittance equivalent to the transmittance in the visible region can be obtained.

The optical filter is an optical filter that transmits light from the subject together with an imaging lens having a maximum peak wavelength in the visible region, and the imaging between the peak wavelength and the wavelength in the infrared region. The transmittance is set so that the combined transmittance with the lens is substantially constant.
Japanese Patent No. 3727934

  2. Description of the Related Art In a surveillance camera that monitors outdoors or indoors, a housing having an optical window may be attached to the surveillance camera for the purpose of protecting or making it difficult to see from the outside. In some surveillance cameras, the optical window may be configured with a smoke color with reduced transmittance for the purpose of detaching the housing having the optical window or making it difficult to see from outside.

  However, when the optical window has a smoke color with a reduced transmittance, the color balance of the photographing screen of the surveillance camera may be lost due to the influence if the spectral transmittance of the optical window is not set appropriately.

  When the surveillance camera has an automatic white balance adjustment function, color balance can be corrected by attaching and detaching the optical window. However, the preset white balance pull-in range for each imaging light source is optimized when the optical window is not installed, so automatic white balance adjustment is performed by the imaging light source when the optical window is installed. The function may not work sufficiently.

  The surveillance camera has an automatic infrared cut filter switching function, and when the infrared cut filter switching timing is determined based on the color of the photographed image, the preset infrared light color range is the optical window. Therefore, depending on the imaging light source, the automatic infrared cut filter switching function may malfunction.

The present invention is an image pickup apparatus including an image pickup optical system, an infrared cut filter, an image pickup element having a color filter disposed on the front surface, and a housing having a detachable optical window made of a light-transmitting resin. The ratio of the transmitted light amount of red and green obtained by integrating the combined spectral transmittance of the imaging optical system, the infrared cut filter, and the color filter from the visible wavelength region to the infrared wavelength region in each color, and blue and green Based on the transmitted light ratio of
Red and green transmitted light amount ratio obtained by integrating the spectral spectral transmittance of the imaging optical system, the infrared cut filter, the color filter, and the optical window from the visible wavelength region to the infrared wavelength region in each color, and blue An image pickup apparatus including a housing having a detachable optical window and adjusting a spectral transmittance of the optical window so that a sum of a difference between each light intensity ratio of green and green and each reference is within 5.0%. is there.

  When the spectral transmittance of the optical window is adjusted as described above, the ratio of transmitted light of each color component received by the image sensor is hardly affected by the attachment / detachment of the optical window. It is possible to stabilize at a level that is difficult to discriminate.

  Furthermore, the image pickup apparatus including the housing having the removable optical window adjusts the spectral transmittance so that the average transmittance in the infrared wavelength region of the optical window is higher than the average transmittance in the visible wavelength region. It is characterized by that.

  A surveillance camera capable of photographing up to an infrared wavelength region, and the optical window further includes a smoke-colored housing with reduced transmittance. Decreases the minimum illuminance performance that can be imaged. However, by setting the average transmittance in the infrared wavelength region high as described above, it is possible to prevent the minimum illuminance performance from being lowered while the color balance is stabilized.

  Furthermore, in the imaging apparatus including the housing having the removable optical window, the light-transmitting resin constituting the optical window is polycarbonate or acrylic, and the adjustment of the spectral transmittance of the optical window has light resistance. It is characterized by adjusting by a combination of resin toning pigments.

  There is no problem as long as the material constituting the optical window is an ND filter in which the spectral transmittance from the visible wavelength region to the infrared wavelength region is controlled to be constant, but in general, such an ND filter has a delicate content. It is often expensive due to adjustment and multilayer coating. Further, the housing having the optical window is increased in size because the image pickup apparatus is accommodated therein, and the optical window becomes more expensive. However, the optical window having the spectral transmittance adjustment range as described above is made of polycarbonate or acrylic, and can be sufficiently manufactured only by a combination of resin toning pigments and can be configured at low cost.

  Further, by using a resin toning pigment having light resistance by adjusting the spectral transmittance of the optical window, the color balance of the photographed image does not change even when the optical window is used for a long period of time.

  According to the present invention, based on the combined transmittance of the imaging optical system, the infrared cut filter, and the imaging element color filter, the spectral balance of the optical window is adjusted under a predetermined condition, so that the color balance of the photographed image that accompanies attachment and detachment. Can be provided.

  Next, details of the present invention will be described in accordance with the description of the embodiments.

  Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

  In an imaging apparatus including a housing having the detachable optical window, an embodiment of an adjustment range of the spectral transmittance of the optical window based on the combined transmittance of the imaging optical system, the infrared cut filter, and the image sensor color filter Will be described in detail. A schematic diagram of an embodiment of the present invention is shown in FIG.

  As shown in FIG. 1, an image pickup apparatus having a housing having a detachable optical window includes a detachable optical window 1 disposed in front of the image pickup optical system 2, an image pickup optical system 2, and an infrared cut filter 3. And the image sensor color filter 4 disposed in front of the image sensor 5.

  When the reflected light 6 from the subject does not have an optical window, the specific wavelength band is cut by first passing through the imaging optical system 2 and the infrared cut filter 3.

The spectral transmittance of the imaging optical system 2 is _assumed to be f _Lwns (x), and the spectral transmittance of the infrared cut filter 3 is assumed to be f _IR (x).

  x represents a wavelength.

  The reflected light 6 that has passed through the imaging optical system 2 and the infrared cut filter 3 is decomposed into red, green, and blue components by the image sensor color filter 4 and enters the image sensor 5.

The spectral transmittance of each color of the image sensor color filter 4 is assumed to be f _R (x), f _G (x), and f _B (x).

  FIG. 2 shows a spectral transmittance graph of each color of the imaging optical system 2, the infrared cut filter 3, and the image sensor color filter 4, and FIG. 4 shows a table of spectral transmittance.

The reflected light finally incident on the image sensor 5 is
Composite transmittance of imaging optical system 2, infrared cut filter 3, and image sensor color filter 4 F _R0 (x) = f _Lens (x), f _IR (x), f _R (x)
F _G0 (x) = f _Lens (x), f _IR (x), f _G (x)
F _B0 (x) = f _Lens (x), f _IR (x), f _B (x)
It is represented by

A graph of the spectral transmittance of the image sensor color filter 4 and the combined transmittances F _R0 (x), F _G0 (x), and F _B0 (x) is shown in FIG. 3, and a table of spectral transmittance is shown in FIG.

The red, green, and blue transmitted light received by the image sensor 5 is the combined transmittance F _R0 (x), F _G0 (x), and F _B0 (x) in the range from the visible wavelength region to the infrared wavelength region. Integrated value

It is represented by

  The color balance of the photographed image is based on the transmitted light amounts of red, green, and blue light received by the image sensor 5 and is based on the ratio of the transmitted light amount of red and blue with reference to the transmitted light amount of green that is a brightness component. It can be quantified.

  When the optical window is not installed, the color balance of the photographed image is the ratio of the transmitted light amount between red and green.

  Blue to green transmitted light ratio

It can be expressed as

When the spectral transmittance of the image sensor color filter 4 shown in FIG. 3 is compared with the combined transmittances F _R0 (x), F _G0 (x), and F _B0 (x), the imaging optical system 2 and the infrared cut filter 3 are compared. It can be seen that the combined transmittances F _R0 (x), F _G0 (x), and F _B0 (x) are attenuated due to the influence on the shorter wavelength side than 430 nm and on the longer wavelength side than 650 nm.

  The wavelength light in this band has a lower transmittance than the wavelength light in the band of 430 nm to 650 nm, and therefore has little effect on the red, green, and blue transmitted light received by the image sensor 5, that is, it affects the color balance of the photographed image. It can be said that there are few.

  Next, FIG. 5 shows spectral transmittances of two types of optical windows having different transmission characteristics.

The spectral transmittance of the optical window of sample 1 is g ₁ (x), and the spectral transmittance of the optical window of sample 2 is g ₂ (x).

  The optical window of sample 1 has a characteristic that the transmittance gradually increases as it becomes longer wavelength side with little variation in waveform. The optical window of the sample 2 has a characteristic that the transmittance is abruptly increased on the short wavelength side and the long wavelength side with many waveform variations.

When the optical window of the sample 1 is mounted, the reflected light incident on the image sensor 5 is the combined transmittance of the image pickup optical system 2, the infrared cut filter 3, the image sensor color filter 4, and the optical window of the sample 1.
F _R1 (x) = g ₁ (x) · F _R0 (x), F _G1 (x) = g ₁ (x) · F _G0 (x), F _B1 (x) = g ₁ (x) · F _B0 It is represented by (x).

  FIG. 6 shows a graph of the combined transmittance when the optical window is not mounted and the combined transmittance when the sample 1 is mounted, and FIG. 7 shows a table of spectral transmittance. In addition, the color balance of the photographed image when Sample 1 is installed is

It can be expressed as

When the optical window of the sample 2 is mounted, the reflected light incident on the image sensor 5 is the combined transmittance of the image pickup optical system 2, the infrared cut filter 3, the image sensor color filter 4, and the optical window of the sample 2.
F _R2 (x) = g ₂ (x) · F _R0 (x), F _G2 (x) = g ₂ (x) · F _G0 (x), F _B2 (x) = g ₂ (x) · F _B0 It is represented by (x).

  FIG. 8 is a graph of the combined transmittance when the optical window is not mounted and the combined transmittance when the sample 2 is mounted, and FIG. 9 is a table of spectral transmittance. In addition, the color balance of the captured image when Sample 2 is installed is

It can be expressed as

  The change in the color balance of the photographed image that accompanies the attachment / detachment of the optical window is based on the ratio of the transmitted light amount of red and green and the transmitted light amount ratio of blue and green when not mounted, and the transmitted light amount ratio of red and green when mounted. In addition, it is possible to evaluate quantitatively by adding the difference between the transmitted light amount ratio of blue and green and the respective standards.

ΔH ₁ = | H _R1 -H _R0 | / H _R0 + | H _B1 -H _B0 | / H _B0
ΔH ₂ = | H _R2 −H _R0 | / H _R0 + | H _B2 −H _B0 | / H _B0
It can be expressed as

  In order to suppress the change ΔH in the color balance of the photographed image due to the attachment / detachment of the optical window to such an extent that it can hardly be discerned by human eyes, the color difference ΔE * ab in the L * a * b color system is generally suppressed to 1.5 or less. It should be good. Although depending on the color and brightness of the subject to be imaged, 1.5 or less of the color difference ΔE * ab in the L * a * b color system should be approximately 5.0% or less when converted to the color balance change ΔH. Can be achieved.

As the change in the combined transmittance accompanying the attachment / detachment of the optical window of Sample 1, when focusing on FIG. 6, the blue transmittance F _B1 (x) is lower than F _B0 (x), and the red transmittance is F _R1 (x). It can be seen that is higher than F _R0 (x).

The difference between the blue and green transmitted light amount ratio reference is ΔH _B1 = | H _B1 -H _B0 | / H _B0, which is 7.55% as shown in FIG.
The difference between the red and green transmitted light amount ratio reference is ΔH _R1 = | H _R1 −H _R0 | / H _R0, which is 7.87% as shown in FIG.

The change ΔH _{1 in} the color balance of the photographed image, which is the sum of the differences from the transmitted light amount ratio reference, is 15.42%. Therefore, the image pickup apparatus including the housing having the optical window of the detachable sample 1 has a large change ΔH _{1 in} the color balance of the photographed image accompanying the attachment / detachment of the optical window, and is stable with respect to color reproducibility and control using color information. Sex cannot be maintained.

As the change in the combined transmittance associated with the attachment and detachment of the optical window of sample 2, the waveform varies for both F _B2 (x) which is blue transmittance and F _R2 (x) which is red transmittance when focusing on FIG.
The difference from the blue and green transmitted light amount ratio reference is ΔH _B2 = | H _B2 −H _B0 | / H _B0, which is 3.24% as shown in FIG.
The difference between the red and green transmitted light amount ratio reference is ΔH _R2 = | H _R2 −H _R0 | / H _R0, which is 0.46% as shown in FIG.

A change ΔH _{2 in} the color balance of the photographed image, which is the sum of the differences from the transmitted light ratio, is 3.70%. Therefore, the image pickup apparatus including the housing having the optical window of the detachable sample 2 has almost no change ΔH _{2 in} the color balance of the photographed image due to the attachment / detachment of the optical window, and is related to control using color reproducibility and color information. High stability.

The spectral transmittance of the optical window of sample 2 increases on the short wavelength side from 430 nm and on the long wavelength side from 650 nm, and this wavelength band is changed to the change ΔH ₂ in the color balance of the photographed image as described above. Has almost no effect. A high transmittance on the longer wavelength side than 650 nm is advantageous for night vision photography using infrared light. Therefore, in an imaging device capable of photographing up to the infrared wavelength region, the spectral transmittance of the optical window can be adjusted so that the average transmittance in the infrared wavelength region is higher than the average transmittance in the visible wavelength region. desirable.

The waveform characteristics of the spectral transmittance of the optical window of sample 2 have variations in the entire wavelength band, but the change ΔH _{2 in} the color balance of the captured image accompanying attachment / detachment is stable. Since such variations can be tolerated, the material constituting the optical window is, for example, a resin material such as polycarbonate or acrylic having light transmittance, and can be sufficiently adjusted only by a combination of general resin toning pigments. Further, it is desirable to use the resin toning pigment having light resistance so that the spectral transmittance does not change even when the optical window is used for a long time.

  If the spectral transmittance of the optical window is adjusted as in the present invention, the color reproducibility of the image pickup apparatus and the control stability using the color information are not impaired. Further, in an imaging device capable of imaging the infrared wavelength region, the minimum illuminance performance that can be captured with infrared light is not lowered. Furthermore, since the variation in the waveform characteristics of the spectral transmittance can be allowed, an optical window can be configured only by adjusting inexpensive resin and resin toning pigment.

Schematic of an embodiment of the invention Spectral transmittance of imaging optical system, infrared cut filter and color filter Spectral transmittance and combined transmittance of color filter Table of spectral transmittance and combined transmittance of imaging optical system, infrared cut filter and color filter Spectral transmittance of optical window Synthetic transmittance associated with attachment / detachment of the optical window of sample 1 Change in color balance of sampled image as the optical window of sample 1 is attached / detached Synthetic transmittance associated with attachment / detachment of the optical window of sample 2 Change in the color balance of the photographed image when the optical window of sample 2 is attached or detached

Claims (3)

An imaging apparatus comprising: an imaging optical system; an infrared cut filter; an imaging element having a color filter disposed on a front surface; and a housing having a removable optical window made of a light-transmitting resin,
Red and green transmitted light amount ratio obtained by integrating the combined spectral transmittance of the imaging optical system, the infrared cut filter and the color filter from the visible wavelength region to the infrared wavelength region in each color, and transmission of blue and green Based on the light intensity ratio,
Red and green transmitted light amount ratio obtained by integrating the spectral spectral transmittance of the imaging optical system, the infrared cut filter, the color filter, and the optical window from the visible wavelength region to the infrared wavelength region in each color, and blue So that the sum of the difference between the light intensity ratio of green and green and each standard falls within 5.0%
An image pickup apparatus comprising: a housing having a detachable optical window while adjusting a spectral transmittance of the optical window.   2. The imaging apparatus according to claim 1, wherein the optical window has a spectral transmittance adjusted so that an average transmittance in an infrared wavelength region is higher than an average transmittance in a visible wavelength region. The light-transmitting resin constituting the optical window is polycarbonate or acrylic,
3. The imaging apparatus according to claim 1, wherein the spectral transmittance of the optical window is adjusted by a combination of light-resistant resin toning pigments.

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