Radiance coefficient q e, Bidirectional reflectance distribution function (BRDF) The measurement of optical properties of materials using integrating spheres is described in DIN 5036-3 and CIE 130-1998. The spectral composition of the radiation (CIE standard illuminants A, B, C, D65 and other illuminants D) The optical properties of materials are not a constant since they are dependent on many parameters such as: The quantities can apply to complex radiation or monochromatic radiation. Quantities such as reflectance and transmittance are used to describe the optical properties of materials. The absorptance α of a medium is defined by the ratio of absorbed radiant power to incident radiant power.īeing ratios of radiant power values, reflectance, transmittance and absorptance are dimensionless. Total transmittance is further subdivided in regular transmittance τ r and diffuse transmittance τ d, which are given by the ratios of regularly (or directly) transmitted radiant power and diffusely transmitted radiant power to incident radiant power. The transmittance τ of a medium is defined by the ratio of transmitted radiant power to incident radiant power. Total reflectance is further subdivided in regular reflectance ρ r and diffuse reflectance ρ d, which are given by the ratios of regularly (or specularly) reflected radiant power and diffusely reflected radiant power to incident radiant power. The (differential) reflected radiant power is given by the exitance M e multiplied with the size of the surface element: For a certain area element dA of the reflecting surface, the (differential) incident radiant power is given by the surface’s irradiance E e multiplied with the size of the surface element, thus The reflectance ρ is defined by the ratio of reflected radiant power to incident radiant power. In addition, reflectance, transmittance and absorptance might also depend on polarization and geometric distribution of the incident radiation, which therefore also have to be specified. For the latter, the spectral distribution of the incident radiation has to be specified. Thus, these three processes can either be quantified for monochromatic radiation (in this case, the adjective “spectral” is added to the respective quantity) or for a certain kind of polychromatic radiation. In general, reflection, transmission and absorption depend on the wavelength of the affected radiation.

Reflectance ρ, Transmittance τ and Absorptance α

The measurement of classical transmission and reflection with a handheld device requires measuring systems with intelligent correction methods.įig. 1: top: Direct, mixed and diffuse reflection bottom: direct, mixed and diffuse transmissionįig. 2: When directly reflected or directly transmitted, a unidirectional beam follows the laws of geometrical optics:ĭirect reflection (left): αi n = α out, direct transmission (right): n 1 × sin( α in) = n 2 × sin( α out) with n 1 and n 2 denoting the respective medium’s index of refraction.

For the measurement of scattering samples we would like to refer to a spectrophotometer specially designed for this purpose.

For the optical measurement of reflection, transmission, absorption, and photoluminescence integrating sphere measuring systems are suitable.

Exception: The Doppler Effect causes a change in frequency when the reflecting material or surface is in motion.Ībsorption is the transformation of radiant power to another type of energy, usually heat, by interaction with matter. Reflection, transmission and scattering leave the frequency of the radiation unchanged.

In this case, we speak about regular reflection (or s pecular reflection) and regular transmission (or direct transmission). When no diffusion occurs, reflection or transmission of a unidirectional beam results in a unidirectional beam according to the laws of geometrical optics (Fig. 2). In this case, we speak about diffuse reflection and diffuse transmission (Fig. 1). Both processes can be accompanied by diffusion (also called scattering), which is the process of deflecting a unidirectional beam into many directions.

Reflection is the process by which electromagnetic radiation is returned either at the boundary between two media (surface reflection) or at the interior of a medium (volume reflection), whereas transmission is the passage of electromagnetic radiation through a medium.

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