JP5771910B2 - Imaging optical device having antireflection structure and imaging optical system having antireflection structure - Google Patents

Description

  The present invention relates to an image pickup optical apparatus and an image pickup optical system having an antireflection structure, and more particularly to an image pickup optical apparatus and an image pickup optical system capable of suppressing the occurrence of ghosts.

  When a part of the light incident on the imaging optical system is reflected once or a plurality of times between the lens surfaces and then enters the imaging device, noise called ghost may occur in the image. In order to prevent the occurrence of this ghost, by providing a so-called antireflection structure such as providing a certain thin film on the lens surface of the imaging optical system by vacuum deposition or the like, or providing a minute unevenness on the lens surface, It is known to suppress causal reflection (Patent Documents 1 to 3).

JP 2000-275402 A JP 2006-215542 A JP 2008-233585 A

  As described above, a general ghost caused by reflection between the surfaces of the lens can be effectively suppressed by the above-described antireflection structure. In addition, a general ghost is a single circular or polygonal form, and although it is noise, it may be used more actively as a part of video expression. However, in a digital optical apparatus having an image pickup device, the light reflected from the surface or inside of the image pickup device re-enters the image pickup optical system, is further reflected by a lens in the image pickup optical system, and re-enters the image pickup device. A unique ghost appears. That is, in a digital optical apparatus having a solid-state image sensor such as a CCD, photoelectric conversion elements are periodically divided (two-dimensionally arranged) inside the image sensor, and reflected with respect to light incident on the surface of the photoelectric conversion element. Acts like a diffraction grating. Therefore, the reflected light has an intensity distribution that periodically repeats light and dark, and when the reflected light is incident on the image sensor again, a ghost with a polka dot pattern (dot pattern) in which the light spots are aligned at regular intervals is generated. To do.

  This polka dot ghost has a peculiar form, and therefore, with the same strength, it is more visible than a general ghost, and causes a significant deterioration of the image. For this reason, it is difficult to reliably prevent a ghost with a polka dot pattern by providing an anti-reflection structure on the lens surface in the dark cloud simply to prevent a ghost caused by general reflection between lens surfaces. That is, the conventional antireflection methods as shown in Patent Documents 1 to 3 are insufficient, and it is necessary to adopt a suppression method specialized for polka dot ghosts.

  An object of the present invention is to realize an imaging optical system and an imaging optical apparatus that effectively suppress a ghost generated by reflected diffracted light from deteriorating an image.

  The imaging optical device of the present invention includes an imaging optical system having an aperture stop, and an imaging element on which incident light from a subject that has passed through the imaging optical system is incident. The imaging element has a plurality of photoelectric conversion elements. Usually, it is arranged in two dimensions. In the imaging optical device, at least in the imaging optical system, the reflected diffracted light generated by reflection of incident light on the surface of the photoelectric conversion element is reflected in the imaging optical system and is again incident on the imaging element. An antireflection structure having an antireflection film in which a plurality of layers are laminated is formed on one surface of a photographic lens disposed between the aperture stop and the image sensor. For example, the antireflection structure may be formed on the lens surface facing the photographic image element, that is, on the lens surface opposite to the aperture stop.

  On the light receiving surface of the image sensor, a plurality of wavelength selection elements (color elements) that split light in a wavelength region corresponding to R, G, B, etc., that is, selectively transmit light in a predetermined wavelength region, are two-dimensionally arranged. When the color filter (filter array) is disposed on the light receiving surface of the image sensor, R, G, and B polka dot (dot pattern) ghosts are generated in the captured image depending on the imaging conditions. The anti-reflection structure of the present invention can reduce light in a predetermined wavelength range resulting from ghost generation in a low wavelength to a long wavelength by a laminated structure of an anti-reflection film that prevents ghost generation peculiar to such an imaging apparatus such as a digital camera. On the other hand, it is possible not to re-reflect to the image sensor side.

  The reflectance of the antireflection structure is determined in consideration of the wavelength band of the reflected diffracted light that enters the lens surface on which the antireflection structure is formed from the imaging element side. For example, when a color filter in which a plurality of wavelength selection elements that divide incident light into a predetermined transmission wavelength range is provided, the transmission wavelength range of each wavelength selection element is formed on the surface of the photographing lens on which the antireflection film is formed. It is preferable that the reflectance is determined so that the average value of the reflectance with respect to the representative wavelength becomes smaller than the average value of the average values of the lens surfaces of the photographing optical system. As the representative wavelength, for example, the wavelength having the largest transmission spectrum is determined. Alternatively, the reflectance is determined so that the average value of the reflectance with respect to the representative wavelength in the transmission wavelength range of each wavelength selection element is smaller than any average value on the other lens surfaces on which the antireflection film is not formed. Also good. For example, in the case of an image sensor provided with R, G, B color filters, the average value of the reflectance for a wavelength of 630 nm, the reflectance for a wavelength of 530 nm, and the reflectance for a wavelength of 420 nm is R on each lens surface of the photographing optical system. , G, and B are preferably set so that the average reflectance is smaller than the average value of the entire lens. Alternatively, it is preferable that the average value of the reflectance for the long 630 nm, the reflectance for the wavelength 530 nm, and the reflectance for the wavelength 420 nm is smaller than any of the average values of other lens surfaces on which no antireflection film is formed. .

  The imaging element has a wavelength selection element that separates incident light into a predetermined transmission wavelength range for each pixel, a representative wavelength in the spectral transmission wavelength range of the predetermined wavelength selection element, and a pixel corresponding to the predetermined wavelength selection element The reflectance is preferably adjusted according to the pixel pitch between the two. For example, the antireflection structure may be adjusted so that the reflectance on the long wavelength side is the lowest in the spectral transmission wavelength region of the wavelength selection element, compared to the central wavelength between the shortest transmission wavelength and the longest transmission wavelength. More preferred. More preferably, the reflectance is adjusted to be the lowest in the vicinity of the representative wavelength at which the diffraction angle, which is a calculated value obtained by dividing the representative wavelength by the pixel pitch, is the largest.

  In the antireflection structure, the reflectance is 0.3% or less when a light beam having a wavelength range of 400 nm or more and 700 nm or less is incident on the surface provided with the antireflection structure at an incident angle of 10 ° or less. It is preferable. In the antireflection structure, the reflectance is 0.2% or less when a light beam in a wavelength region of 600 nm or more and 700 nm or less is incident on the surface provided with the antireflection structure at an incident angle of 0 °. Is preferred. Further, in the antireflection structure, the reflectivity when a light beam having a wavelength range of 500 nm to 600 nm is incident on the surface provided with the antireflection structure at an incident angle of 0 ° is 0.1% or less. Preferably there is.

  The imaging element has a wavelength selection element that separates incident light into a plurality of types of transmission wavelength ranges for each pixel, and includes a representative wavelength in a spectral transmission wavelength range of a predetermined wavelength selection element and a predetermined wavelength selection element in the imaging element. It is preferable that the reflectance is adjusted according to a multiplication value of the number of corresponding pixels per unit area.

In the optical path of the reflected diffracted light, when the conjugate optical distance CJ, which is a distance optically conjugate with the reflection point of the photoelectric conversion element, and the photoelectric conversion element is within the following range, the lens surface It is preferable that the antireflection structure is provided on the surface.

-20mm <CJ <-10mm or 15mm <CJ <30mm

  An imaging optical system according to the present invention is an imaging optical system that includes a plurality of photographing lenses and forms an image of incident light from a subject on an imaging element having a plurality of photoelectric conversion elements. In the system, an antireflection structure is formed on at least one surface of a photographing lens arranged between the aperture stop and the image sensor, and the antireflection structure is formed of an antireflection film in which a plurality of layers are laminated. And the reflected diffracted light of the incident light on the surface of the photoelectric conversion element is prevented from entering the image pickup element again by reflection in the photographing optical system.

  ADVANTAGE OF THE INVENTION According to this invention, the imaging optical system and imaging optical apparatus which suppress effectively that the ghost produced by reflected diffracted light degrades an image are realizable.

It is a figure which shows reflection of the incident light which causes a general ghost. It is a figure which shows reflection of the incident light which causes the ghost by reflected diffracted light. It is sectional drawing which shows a part of imaging device in an imaging optical apparatus. It is a top view which shows a part of imaging device roughly. It is a top view which shows roughly the diffracted light which arises with an image pick-up element. 1 is a diagram schematically illustrating an imaging optical device according to an embodiment. It is sectional drawing of an antireflection film. It is a figure which shows the spectral reflection characteristic of an antireflection film. In the imaging optical device of the 1st modification, it is a figure showing the state where diffracted light re-enters an image sensor. It is a figure which illustrates the magnitude | size of the incident area in the light-receiving surface of the diffracted light which reentered the image pick-up element. In the imaging optical device of the 2nd modification, it is a figure showing the state where diffracted light re-enters an image sensor. It is a figure which shows the conjugate position of diffracted light. It is a figure which shows roughly each pixel of R, G, B in a special pixel arrangement | sequence.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram illustrating the reflection of incident light that causes a general ghost. FIG. 2 is a diagram illustrating reflection of incident light that causes a ghost due to reflected diffracted light.

  It is known that a ghost is generated in a subject image by incident light L incident on an optical system of a photographing apparatus (not shown) from the sun S. For example, as shown in FIG. 1, a ghost may occur when incident light L reflected inside the imaging optical system 30 enters the imaging element 40. This ghost is mainly a single circular shape or a polygonal shape due to the aperture shape of the aperture blade.

  On the other hand, when the incident light L is reflected by the image sensor 40, a plurality of diffracted lights DL traveling toward different angles are generated as shown in FIG. When the diffracted light DL is further reflected by the imaging optical system 32 and is incident again on the imaging device 40, the generated ghost (hereinafter referred to as reflection diffraction ghost) has a polka dot pattern in which a plurality of points are aligned.

  Such a reflection / diffraction ghost with a polka dot pattern is a very unnatural form, so that it is easily visible to an observer and tends to greatly reduce the evaluation of the subject image. Therefore, in the present embodiment, as described below, an antireflection structure for suppressing the reflection diffraction ghost is provided in the imaging optical device to prevent the image quality from being affected.

  FIG. 3 is a cross-sectional view showing a part of the image sensor in the imaging optical apparatus of the present embodiment. FIG. 4 is a plan view schematically showing a part of the image sensor of the present embodiment. FIG. 5 is a plan view schematically showing the diffracted light DL generated by the image sensor.

  The image sensor 10 of the present embodiment includes a photodiode 12 (photoelectric conversion element), a color filter 14 (wavelength selection element), and an array lens 16. The image sensor 10 includes a plurality of pixels, for example, a first pixel 101 and a second pixel 102. Incident light L from the subject passes through an imaging optical system (not shown) and enters an array lens 16 that covers the surface of the imaging element 10. The incident light L passes through the color filter 14 and reaches the light receiving surface 12S of the photodiode 12.

  A part of the incident light L may be reflected by the light receiving surface 12S to generate reflected light RL. Due to such reflection of the incident light L, diffracted light DL is generated between the pixels of the first and second pixels 101 and 102. In the present embodiment, as will be described later, the diffracted light DL is prevented from being reflected by the surface of the imaging optical system (not shown), and the deterioration of the image quality due to the diffracted light DL is prevented.

  The arrangement pattern of the pixels in the image sensor 10 of the present embodiment is a Bayer array as shown in FIG. That is, a large number of pixels are two-dimensionally arranged, and pixels (hereinafter referred to as G pixels) in which the green wavelength range is spectrally transmitted to the light receiving surface due to the spectral transmission characteristics of the green filter of the color filter 14 (see FIG. 3) are vertical. It is arranged every other pixel in both the direction and the horizontal direction. Then, between the two G pixels, a spectral transmission characteristic of a pixel (hereinafter referred to as an R pixel) in which a red wavelength region is spectrally transmitted to the light receiving surface by a spectral transmission characteristic of the red filter of the color filter 14 and a blue filter of the color filter 14. Thus, any one of the pixels (hereinafter referred to as B pixels) in which the blue wavelength region is spectrally transmitted to the light receiving surface is equally arranged by the same number.

  The distances RD, BD between the R pixels arranged closest to each other or the center points of the B pixels, that is, the inter-pixel distance (pixel pitch) is, for example, about 10 μm. In this case, the distance GD between the G pixels is about 7 μm. The pixel pitch indicated by the distances RD, BD, and GD is constant for each of the R, G, and B pixels. The diffraction angle of the diffracted light DL (see FIG. 3) is approximately calculated by the relational expression (1) of diffraction angle = representative wavelength / pixel pitch in the spectral transmission wavelength region of each color filter. In this specification, the direction difference between the diffracted lights of the next order such as the direction difference between the 0th order diffracted light and the 1st order diffracted light, the direction difference between the 1st order diffracted light and the 2nd order diffracted light, and the 1st order diffracted light and the 2nd order diffracted light. The difference in the traveling angle with the diffracted light of the next order, such as the difference in traveling angle, is called “diffraction angle”.

  Here, for example, the representative wavelength of the wavelength band that passes through the red filter of the color filter 14 is 630 nm, the representative wavelength of the wavelength band that passes through the green filter is 530 nm, and the representative wavelength of the wavelength band that passes through the blue filter is 420 nm. is there. Therefore, the diffraction angle of the diffracted light DL generated by the R pixel is 630 nm / 10 μm = 63 rad (see FIG. 5A), and the diffraction angle of the diffracted light DL generated by the G pixel is the largest at 530 nm / 7 μm = 76 rad. (See FIG. 5B), the diffraction angle of the diffracted light DL generated by the B pixel is the smallest at 420 nm / 10 μm = 42 rad (see FIG. 5C). As described above, the diffraction angles of the diffracted light DL generated for the R, G, and B pixels are different.

  In general, the reflection diffraction ghost generated by the diffracted light DL having a large diffraction angle is liable to adversely affect the image quality. This is because a plurality of polka dots are separated from each other, and a polka dot pattern is conspicuous in an image. Therefore, in the present embodiment, in this embodiment, the reflectance of the surface having the antireflection structure described later according to each value of the pixel pitches RD, BD, and GD for each type of spectral transmission wavelength characteristic of the color filter 14. Has been adjusted.

  In addition, as the representative wavelength for each type of the color filter 14, a wavelength corresponding to a transmittance peak due to each spectral transmission characteristic of the color filter 14, a center value of a wavelength range that transmits each color region of the color filter 14, or the like is adopted. Although these can be done, these are generally the values described above. In addition, the pixel in this specification refers not only to the photodiode 12 but also to the array lens 16 and the color filter 14 between both members, as illustrated in the first and second pixels 101 and 102 in FIG. And a cover glass that covers the image sensor 10 if necessary.

  Next, the imaging optical device of this embodiment will be described. FIG. 6 is a diagram schematically showing the imaging optical device of the present embodiment.

  The imaging optical device 20 includes an imaging optical system 36 and an aperture stop 42. In the imaging optical system 36, a plurality of photographing lenses through which incident light L from the subject Ob is transmitted are arranged. The amount of incident light L incident on the image sensor 10 is adjusted by the aperture stop 42. The image sensor 10 has a photoelectric conversion element inside.

  In the imaging optical device 20, an antireflection film 50 is provided as an antireflection structure for suppressing and preventing the above-described diffracted light DL from being reflected from the surface of the photographing lens in the imaging optical system 36. This is because when the diffracted light DL re-reflected on the surface of the photographing lens is incident on the image pickup device 10 again, a reflection diffraction ghost can be generated. The antireflection film 50 is formed on at least one surface of one of the photographing lenses included in the imaging optical system 36.

  Here, the antireflection film 50 is provided on the image sensor 10 side of the aperture stop 42, that is, between the aperture stop 42 and the image sensor 10. More specifically, the antireflection film 50 is formed on the surface of the imaging lens 361 included in the imaging optical system 36 on the imaging element 10 side.

  Thus, by disposing the antireflection film 50 between the aperture stop 42 and the image sensor 10, reflection of the diffracted light DL on the surface of the photographing lens can be effectively suppressed and prevented. This is because almost all of the diffracted light DL is incident on the photographing lens 361 closer to the image sensor 10 than the aperture stop 42, while the lens surface closer to the subject than the aperture stop 42 has a smaller diameter. The diffracted light DL is shielded by the diaphragm blades when the aperture is narrowed down (the strong ghost light is generally generated when the subject side is bright, and the aperture is likely to be reduced in diameter). This is because even if the light enters the lens surface on the side and is reflected again in the direction of the image sensor, it can be expected to be shielded by the diaphragm blades again. Therefore, by disposing the antireflection film 50 preferentially between the aperture stop 42 and the image sensor 10 to reduce the reflectance, the influence of an unpleasant ghost can be reduced efficiently.

  Next, the structure and characteristics of the antireflection film 50 will be described. FIG. 7 is a cross-sectional view of the antireflection film 50, and FIG. 8 is a diagram showing the spectral reflection characteristics of the antireflection film 50.

The antireflection film 50 has a multilayer structure in which a plurality of layers are stacked. That is, in the antireflection film 50, the first layer 51 on the photographing lens 361 side to the seventh layer 57 on the surface side are laminated. The first layer is made of alumina Al ₂ O ₃ , and the second layer 52, the fourth layer 54, and the sixth layer 56 are made of tantalum pentoxide Ta ₂ O ₅ , yttrium oxide Y ₂ O ₃ , praseodymium oxide Pr. ₆ O ₁₁ or the like. The third layer 53 and the fifth layer 55 are made of magnesium fluoride MgF ₂ , and the seventh layer 57 is a silica airgel porous layer. The thicknesses of the first to seventh layers 51 to 57 are all adjusted to about several tens of nm.

  By adjusting the material, thickness, and structure of the first to seventh layers 51 to 57 in this way, spectral reflection of light (see FIG. 6 and the like) incident on the antireflection film 50 at an incident angle of 0 °. The rate is as shown in FIG. That is, the reflectance with respect to the red diffracted light DL having a wavelength range of 600 nm to 700 nm is 0.2% or less, for example, 0.19%. Moreover, the reflectance with respect to the green light whose wavelength range is 500 nm or more and 600 nm or less is 0.1% or less, for example, 0.06%. The maximum reflectance (when the wavelength is 400 nm) of blue light having a wavelength range of 400 nm to 500 nm is about 0.29%, which is higher than the reflectance of red and green light. The reflectance of the antireflection film 50 is determined so as to prevent light in a wavelength band corresponding to R, G, and B from being re-reflected toward the image sensor. Specifically, the reflectance is set to be lower than the reflectance with respect to light in the wavelength region corresponding to the average R, G, and B of the photographing optical system. For example, when the lens surface of the photographic optical system 36 is constituted by the first to nth surfaces, the values A1 to AN of the surfaces obtained by averaging the reflectances for the R, G, and B representative wavelengths 630, 530, and 420 nm, respectively. Further, a value VA (= (ΣA1 + A2 +... + AN) / N) obtained by averaging them is used as a final average value, and the reflectance is set to be lower than that. Alternatively, the reflectance of the antireflection film 50 can be set to a value smaller than the reflectance of all other lens surfaces where the antireflection film 50 is not formed. For example, when the antireflection film 50 is formed on the lens surface of No. m (1 <m <n), the average reflectance AM of R, G, B on the lens surface is the average reflectance of other lens surfaces. .., AM-1, AM + 1,... AN is set smaller than any of the values.

  Here, as described above, the diffraction angle of the diffracted light DL generated in the G pixel is the largest, then the diffraction angle of the diffracted light DL generated in the R pixel is large, and the diffraction angle of the diffracted light DL generated in the B pixel is Smallest (see paragraph [0026], FIG. 5). As is clear from the above, in this embodiment, the wavelength at which the reflectance of the antireflection film 50 is lowest is adjusted to be in the vicinity of the wavelength having the largest diffraction angle.

  This is because the reflection diffraction ghost generated by the diffracted light DL having a large diffraction angle should be preferentially suppressed and prevented (see paragraph [0027]). This can be realized by providing the antireflection film 50 of the present embodiment and reliably preventing the diffracted light DL having a larger diffraction angle from re-entering the imaging element 10. Since the diffraction angle of the diffracted light DL = representative wavelength / pixel pitch in the spectral transmission wavelength region of each color filter (see paragraph [0025]) as shown in the relational expression (1) above, antireflection It can be said that the reflectance of the film 50 is adjusted so that the diffracted light DL having a larger value obtained by dividing the wavelength of the reflected light according to each wavelength region transmitted by the color filter by the pixel pitch is lower.

  In FIG. 8, the reflectivity when the incident angle with respect to the antireflection film 50 is 0 ° is shown. However, the incident angle of the diffracted light DL with respect to the antireflection film 50 is 10 ° or less, and the wavelength region is When the thickness is 400 nm or more and 700 nm or less, the reflectance of the antireflection film 50 is adjusted to be always 0.3% or less. For this reason, reflection of green light having a large diffraction angle is particularly preferentially prevented, and reflection of diffracted light DL of other colors can also be reliably prevented. Note that the incident angle here is a macroscopic incident angle and is different from an incident angle in consideration of a fine step or the like.

  Next, the relationship between the position where the antireflection film 50 is provided and the size of the reflection diffraction ghost will be described. FIG. 9 is a diagram illustrating a state in which the diffracted light DL is incident on the image sensor 10 again in the imaging optical device according to the first modification. FIG. 10 is a diagram illustrating the size of the incident area on the surface of the image sensor of the diffracted light DL that is incident on the image sensor 10 again. FIG. 11 is a diagram illustrating a state in which the diffracted light DL re-enters the imaging element 10 in the imaging optical device according to the second modification.

  Incident light L incident on the imaging optical apparatus 201 of the first modification is transmitted through the imaging optical system 36 and is collected on the surface of the imaging element 10 (strictly, the light receiving surface 12S of the photodiode 12). A part of the collected incident light L is reflected on the light receiving surface 12S, and diffracted light DL is generated. In the imaging optical device 201 of the first modified example, the antireflection film 50 is not provided, and the diffracted light DL is reflected by, for example, the surface 362S of the photographing lens 362 shown in the figure and reenters the imaging element 10.

  The diffracted light DL incident on the image sensor 10 is condensed on the image sensor 10 as illustrated. For this reason, the incident region of the re-incident diffracted light DL in the image sensor 10 is spot-like and small as illustrated in FIG. Thus, when the incident area is small (when the image is re-imaged on the imaging device 10), the appearance range of the reflection diffraction ghost polka dots generated on the image is narrow, and the size of each dot of the polka dots is small. It is relatively inconspicuous.

  On the other hand, in the imaging optical device 202 of the second modified example shown in FIG. 11, when the diffracted light DL is reflected by the surface 363S of the photographing lens 363 and re-enters the light receiving surface 12S, it is on the surface of the imaging device 10. The incident area becomes larger as illustrated in FIG. In this case, the polka dot pattern of the reflection diffraction ghost generated on the image is larger than that in the case of FIG. 10A, and further, the entire range is widened.

  On the other hand, when the incident region of the diffracted light DL that has re-entered the imaging device 10 further expands as illustrated in FIG. 10C, adverse effects on the image due to the reflected diffraction ghost can be suppressed. This is because the strength of each polka dot decreases, making it less noticeable, and many of the polka dots fall out of the effective area of the image sensor or become a single circle or polygon as a whole like a normal ghost. It is to become.

  As is clear from the above, the size (image size) of the incident region on the image sensor 10 formed by the diffracted light DL re-reflected on a certain surface on the imaging lens and re-entered on the image sensor 10. For example, when the diameter is between a predetermined lower limit value and an upper limit value, it can be said that it is particularly desirable to provide the antireflection film 50 on the surface thereof. This is in order to effectively prevent the reflection diffraction ghost that should be suppressed, particularly when the incident region is smaller than the lower limit (see FIG. 10A) or larger than the upper limit (see FIG. 10C). This is because the adverse effect on the image is not so great. In the first modified example (see FIG. 9), it is not necessary to preferentially provide the antireflection film 50 on the surface 362S, whereas in the second modified example (see FIG. 11), the surface 363S is not provided. Providing the antireflection film 50 can be important. In addition, as an example of the above-mentioned lower limit value and upper limit value, for example, in the image sensor 10 of 15.8 mm × 23.7 mm, the lower limit value is 2.5 mm and the upper limit value is 21 mm.

  Next, the relationship between the position where the antireflection film 50 is provided and the conjugate position of the diffracted light DL will be described. FIG. 12 is a diagram illustrating a conjugate position of the diffracted light DL.

  The position where the antireflection film 50 is provided can also be determined based on the distance from the conjugate position of the diffracted light DL to the image sensor 10 as follows. In this case, more accurate calculation is possible compared with the case where it determines based on the magnitude | size of the above-mentioned incident area. Here, as shown in the figure, a case where the diffracted light DL generated at the incident position A of the incident light L in the image sensor 10 is reflected by the surface 364S of the photographing lens 364 will be described as an example.

  The diffracted light DL incident on the photographing lens 364 is reflected, for example, at a point C on the surface 364S. Thus, the diffracted light DL reflected at a predetermined reflection angle determined by the refractive index n of the photographing lens 364 travels again toward the vicinity of the incident position A on the image sensor 10. A position where the diffracted light DL is condensed and a position where the diffracted light DL intersects the optical axis X passing through the incident position A and perpendicular to the imaging element 10 is defined as a conjugate position B.

  A distance between the incident position A and the conjugate position B, that is, a distance from the image sensor 10 to the conjugate position B is defined as a conjugate distance CJ. Here, for convenience of explanation, when the conjugate position B is closer to the photographing lens 364 than the image sensor 10, the conjugate distance CJ indicated by the solid line takes a negative value, and when the conjugate position B ′ is on the opposite side, Assume that the conjugate distance CJ indicated by the broken line takes a positive value.

  As shown in the figure, when the conjugate distance CJ is a relatively large negative value, it is not so important to provide an antireflection film (not shown here) on the surface 364S. This is because the incident area of the diffracted light DL re-entered on the light receiving surface 12S becomes sufficiently large as shown in FIG. 10C, for example. Similarly, when the conjugate position B is close to the incident position A and the conjugate distance CJ is close to 0, it is not necessary to preferentially provide the antireflection film on the surface 364S. This is because the above-described incident region is sufficiently small as illustrated in FIG.

In contrast, within the conjugate distance CJ (mm) is represented by _{R 1,} for example -20 mm <when within range of CJ <-10 mm, should be provided preferentially an anti-reflection film on the surface 364S . This is because the incident area of the diffracted light DL has the size illustrated in FIG. 10B, and a reflection diffraction ghost that greatly affects image quality can occur.

The same applies to the case where the value of the conjugate distance CJ is positive. When the value of the conjugate distance CJ is within the range of R ₂ such as 15 mm <CJ <30 mm, the surface 364S is preferentially given over other lens surfaces. An anti-reflective coating should be provided on top. As described above, when the conjugate distance CJ is between the predetermined lower limit value and the upper limit value in the state where the antireflection film is not provided, the antireflection film is provided on the surface of the photographing lens.

  As described above, according to this embodiment, the diffracted light DL having a larger diffraction angle provides the antireflection film 50 that preferentially suppresses and prevents reflection on the surface of the photographing lens, thereby degrading the image due to the reflection diffraction ghost. Can be effectively suppressed. In particular, even when the antireflection film 50 is provided on a limited number of antireflection films 50, for example, only one surface on the photographing lens, a large effect can be obtained by the efficient arrangement.

  The structure of each member included in the imaging optical device 20 is not limited to any embodiment. For example, the above-described embodiment may be applied to a monochrome image sensor. In this case, all the RGB pixels are arranged at the same position, and the pixel pitch can be regarded as common to the pixels of each color. Therefore, since the diffraction angle of the diffracted light DL is substantially proportional to the wavelength, the longer the wavelength, the larger the diffraction angle. For this reason, the reflectance is on the long wavelength side of the light receiving wavelength region of the color filter 14, for example, on the long wavelength side of the center wavelength between the shortest transmission wavelength and the longest transmission wavelength in the spectral transmission wavelength region in all color filters. It is preferable to set the minimum wavelength and use the antireflection film 50 corresponding to such wavelength characteristics.

  Further, an antireflection film 50 may be provided in an imaging optical device including an imaging device having an array other than the Bayer array. In this case, for example, in the special arrangement shown in FIG. 13, handling of R pixels and B pixels may be a problem. This is because the pixel pitch between the R pixels and the B pixels is not constant.

In such a case, the square root of the pixel density (or the ratio of the number of pixels of each color included in all pixels), which is the number of R pixels, B pixels, and G pixels per unit area in the image sensor, is calculated. Based on this value and the representative wavelength of each color filter, the reflectance of the antireflection film 50 is adjusted. This is because the square root of the pixel density is proportional to the average value of the distance between the pixels of each color, that is, the average value of the pixel pitch. In this case, instead of the relational expression (1) described above, that is, the diffraction angle = representative wavelength / pixel pitch relational expression within the spectral transmission wavelength range of each color filter, the diffraction wavelength is the representative wavelength within the spectral transmission wavelength range of each color filter. Based on the relational expression (2) of / (pixel density) ^1/2 , the reflectance is adjusted so that the diffracted light DL having a larger diffraction angle is preferentially lowered.

  In addition, although it is preferable to efficiently provide the antireflection film 50 on any one or a small number of photographing lens surfaces in the imaging optical system 36 by the above-described method, it may be provided on all photographing lens surfaces. . Further, when there is no significant difference in the value of the conjugate distance CJ between the lens surfaces included in the imaging optical system 36, the antireflection film 50 is formed on the lens surface closest to the aperture stop 42 among the lenses on the imaging element 10 side. Is preferably provided. For example, if a ghost always appears symmetrically with respect to a light source that can cause a ghost, such as the sun, it is likely to stand out compositionally.

10 Image sensor 12 Photodiode (photoelectric conversion element)
12S light receiving surface 14 color filter (wavelength selection element)
DESCRIPTION OF SYMBOLS 20 Imaging optical apparatus 36 Imaging optical system 361 Shooting lens 42 Aperture stop 50 Antireflection film 101, 102 Pixel L Incident light DL Diffracted light

Claims (2)

In an imaging optical apparatus comprising: an imaging optical system having an aperture stop; and an imaging element that has a plurality of photoelectric conversion elements and that receives incident light from a subject that has passed through the imaging optical system.
An antireflection structure is formed on at least one surface of the photographing lens disposed between the aperture stop and the imaging element in the imaging optical system,
The anti-reflective structure has a reflection preventing film formed by laminating a plurality of layers, the reflected diffracted light in the photoelectric conversion element surface of the incident light is incident on the imaging device again by reflections within the IMAGING optical system Suppresses the
The imaging element has a wavelength selection element for each pixel to split the incident light into a plurality of types of transmission wavelength ranges,
The antireflection film is formed according to a multiplication value of a representative wavelength in a spectral transmission wavelength region of the predetermined wavelength selection element and a number per unit area of pixels corresponding to the predetermined wavelength selection element in the imaging element. An imaging optical device characterized in that the reflectance on the surface of the taking lens is adjusted. An imaging optical system that includes a plurality of photographing lenses and forms an image of incident light from a subject on an imaging element having a plurality of photoelectric conversion elements,
Having an aperture stop,
An antireflection structure is formed on at least one surface of the photographing lens disposed between the aperture stop and the imaging element in the imaging optical system,
The anti-reflective structure has a reflection preventing film formed by laminating a plurality of layers, the reflected diffracted light in the photoelectric conversion element surface of the incident light is incident on the imaging device again by reflections within the IMAGING optical system Suppresses the
The imaging element has a wavelength selection element for each pixel to split the incident light into a plurality of types of transmission wavelength ranges,
The antireflection film is formed according to a multiplication value of a representative wavelength in a spectral transmission wavelength region of the predetermined wavelength selection element and a number per unit area of pixels corresponding to the predetermined wavelength selection element in the imaging element. iMAGING optical system to feature that reflectance is adjusted in the photographic lens surface that.

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