In late years, optical filters which sufficiently transmit light in a visible wavelength region and block light in a near-infrared region have been used for various purposes.
For example, a solid-state image sensing device (such as CCD or CMOS) is used in an imaging device, such as a digital still camera. In order to make the sensitivity of the solid-state image sensor close to the visibility of a human, an optical filter is disposed between an imaging lens and the solid-state image sensing device.
As optical filters for imaging devices, a near-infrared absorbing glass and a glass filter using the glass have been known, in which a CuO or the like is added to a fluorophosphate-based glass or a phosphate-based glass so as to enable selective absorption of light in the near-infrared wavelength region (hereinafter referred to as “near-infrared light”). However, a glass filter of light-absorbing type has insufficient performance of blocking the near-infrared light and an insufficient transmitting property in a wavelength band (630 to 700 nm) needed for imaging a dark part brighter. Moreover, it also has a constraint in formation of layers as to not inhibit the function of a solid-state image sensing device. Thus, in the current situation, an optical filter having a sufficient near-infrared cut filter function has not been obtained.
Accordingly, in order to solve the above-described problems, there have been developed, for example, a reflective-type interference filter in which an SiO2 layer and a TiO2 layer are stacked alternately on a s substrate so as to block the near-infrared light by reflection by interference of light, a film containing a dye which absorbs the near-infrared light in a transparent resin, and the like. Further, by combining the interference filter and the film, there has been developed an optical filter in which a resin layer containing a dye which absorbs near-infrared rays and a layer which reflects the near-infrared rays are stacked.
Among the above-described optical filters, a near-infrared cut filter in which a transparent resin layer containing a near-infrared absorbing dye and a near-infrared reflecting dielectric multilayer film are provided on a substrate constituted of the near-infrared absorbing glass is known to have a quite high near-infrared cut function. However, when a solid-state imaging device using this near-infrared cut filter is used to image a subject partially containing a very bright light source, an object which did not exist on the original subject may appear in a portion of the taken image, and the appearance of this object is a problem in the solid-state imaging device which is required to have higher precision of reproducibility of a subject image. This phenomenon is due to stray light which occurred by reflection or scattering in the optical system of the solid-state imaging device, and it is conceivable that all the stray lights incident at various angles affect the near-infrared cut filter.
However, in the design of the optical system of the conventional solid-state imaging device, only transmittance at up to about 30 degrees at most is considered as the incident angle of light to be incident on the near-infrared cut filter from a designed optical path. As the near-infrared cut filter having a similar structure, for example, there are only near-infrared cut filters in which, as the incident angle of light, only 0 degree and 30 degrees are considered (Patent Reference 1 (International Publication No. WO2014/030628)), or only 0 degree and 26 degrees are considered (Patent Reference 2 (International Publication No. WO2014/168189)). In other words, with respect to the stray light incident at various angles, although a relatively small range of incident angles is considered, there has been no attempt to effectively obtain imaging with high precision by considering a larger incident angle.
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