An optical system is disclosed and includes an optical sensor, a plurality of photosensitive pixels disposed on the optical sensor, a wavelength-selective optical filter in optical communication with the photosensitive pixels, the wavelength-selective optical filter being disposed remotely from the optical sensor, an area disposed in the wavelength-selective optical filter, the area having a transmission spectrum different from a transmission spectrum of a portion of the wavelength-selective optical filter not in the area and a reflector, the wavelength-selective optical filter and a measurement subject each being disposed between the reflector and the optical sensor along an optical path.