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Reflection and Transmission

Working Group 4.51

General information about the measurement of diffuse reflection

In contrast to regular, specular reflection, the incoming light in diffuse reflection is distributed over all directions in space. If the radiance L of the reflected radiation is independent of the direction, one speaks of a Lambertian reflector.

In reflectometry, both the spectral and the spatial distribution of the radiation diffusely reflected by solid materials is measured and characterized by the indication of characteristic numbers. In practice, reflection measurements on diffuse reflecting materials are carried out relative to a standard. For this purpose, the perfectly reflecting diffuser (PRD) which reflects the incoming radiation without losses, completely diffuse and with a Lambertian direction characteristic, has been established as a primary standard. The PRD is, however, a theoretical concept only, which cannot be realized experimentally or materially. Since there is no material with these characteristics, the realization of the primary standard is carried out with physical methods, i.e. by the measuring apparatus, within the scope of an absolute measurement and with the aid of a primary reference measurement procedure.

The interaction of optical radiation with materials is described quantitatively by material characteristics - in the case of the reflection e.g. by indication of the spectral radiance factor β(λ). Thereby, the characteristic numbers are no exclusively fixed material-specific quantities, but depend on a multiplicity of parameters such as, for example, the direction of irradiation and reflection, and on the respective cone angle. There are basically nine different measurement geometries for a reflection measurement. They consist of all kinds of possible combinations, in the case of which the incident and the reflected radiation is in each case hemispherical, conical or directional. Therefore, the material characteristic always also comprises an information regarding the measurement geometry used - e.g. the spectral radiance factor β(λ) in the geometry d:0° (diffuse irradiation from the half-space: Reflection measurement under 0°) or 45°:0° (irradiation under 45°: Reflection measurement under 0°), corresponding to β(λ)d/0 or β(λ)45/0. The hemispherical measurements are carried out with an integrating sphere. The directional geometries are realized with a gonioreflectometer.

All reflectometric measurements and calibrations carried out at PTB are absolute measurements - i.e. the value of the spectral radiance factor β(λ) of a sample is determined directly with the aid of physical principles. In contrast to this, a relative measurement can be distinguished, in the case of which a calibrated standard is used. The comparability and traceability of the scales of the spectral radiation factor at PTB are guaranteed by international comparison measurement with other national metrology institutes.

The Working Group 4.51 "Reflection and Transmission" is one of world-wide two standardizing laboratories of the ISO (International Organization for Standardization) in the field of reflection measurements for hemispherical/directional geometries.

In hemispherical/directed geometries (sphere geometries), diffuse reflection measurements are carried out in the spectral range from 250 nm to 2400 nm.

In directional/directional geometries (gonioreflectometry), the spectral range for diffuse reflection measurements extends from 250 nm to 1850 nm.