“Measurement with Light” refers to the use of optical radiation as a tool for performing a measurement task. Reflectance and transmittance meters employ both light sources and a detection system. A spectrophotometer is a good example of this type of instrument.
Light is defined as optical radiation visible to the human eye over the wavelength range from 380 nm to 780 nm. The eye’s sensitivity to light varies over this spectral range, with the peak at 555 nm. The photometric action function V(λ) defined by the CIE applies to any application where light must be evaluated for visual purposes including reflection and transmission.
DIN 5036 part 3 and CIE 130-1998 recommend an integrating sphere with a minimum diameter of 50 cm for the measurement of photometric material properties such as light transmittance / reflectance, diffuse transmittance / reflectance and regular transmittance / reflectance.