Patent ID: 11914180
Assignee: NITTO DENKO CORPORATION
Field: Optics (Instruments)
Classification: CPC G  H | IPC G

Claim 0:
1. An optical filter having back-scattering characteristics,
wherein the optical filter has a linear transmittance of 60% or higher for light having a wavelength in at least a part of a wavelength range not shorter than 760 nm and not longer than 2000 nm, and
wherein:
in a case where an incident light has a polar angle of 0° in a direction of incidence, a bidirectional reflectance distribution function value in a direction having an azimuth of 20° from a plane of incidence and having a polar angle of −60° is BRDF (0°; 20°, −60°),
in a case where the incident light has a polar angle of 30° in the direction of incidence, the bidirectional reflectance distribution function value in the direction having an azimuth of 20° from the plane of incidence and having a polar angle of −60° is BRDF (30°; 20°, −60°), and
in a case where the incident light has a polar angle of 60° in the direction of incidence, the bidirectional reflectance distribution function value in the direction having an azimuth of 20° from the plane of incidence and having a polar angle of −60° is BRDF (60°; 20°, −60°),
in a case where the incident light has a wavelength in at least a part of a visible light wavelength range, |BRDF (0°; 20°, −60°)−BRDF (30°; 20°, −60°)|/BRDF (0°; 20°, −60°) is 1.0 or smaller, and |BRDF (0°; 20°, −60°)−BRDF (60°; 20°, −60°)|/BRDF (0°; 20°, −60°) is 1.0 or smaller, and

wherein a transmittance curve of the filter in a visible light wavelength region includes a curved portion where the linear transmittance is decreased monotonously from a longer wavelength to a shorter wavelength side, and the curved portion is shifted toward the longer wavelength as an angle of incidence is increased.