This International Standard gives rules for the measurement of the spectral reflectance of plane surfaces and spectral transmittance of plane parallel elements using spectrophotometers over the spectral range 190 nm to 25 μm. The transmittance τ and the reflectance ρ of optical components are generally divided into two parts as follows: τ = τr + τd (1) ρ = ρr + ρd (2) where τr is the regular transmittance; τd is the diffuse transmittance; ρr is the regular reflectance; ρd is the diffuse reflectance. This International Standard applies only to measurements of the regular transmittance and the regular reflectance; it does not apply to those of the diffuse transmittance and the diffuse reflectance. This International Standard is applicable to test specimens which are coated or uncoated optical components without optical power.