Directional/directional Geometries (Gonioreflectometry)
Gonioreflectometric methods, i.e. measurements in so-called directional/directional geometries, are the most fundamental physical methods for the determination of the reflection characteristics of materials. The characteristics usually depend on the direction of incidence on the sample and the detection direction of the reflected radiation, as well as on the respective cone angles of the two ray bundles. Standardized in the field of reflectometry are the directional reflection geometries 45°:0° and 0°:45°. This corresponds to an irradiation under 45° on the sample with measurement of the reflection under 0°, and an irradiation under 0° on the sample with measurement of the reflection under 45°. Due to its special construction, the robot-based gonioreflectometer of PTB allows, however, general angular distributions of the radiation reflected by the materials to be measured within the complete half-space above the sample.
Fig.1: Reference Gonioreflectometer
Measurement of the reflection indicatrix, determination of the BRDF
If - in the case of gonioreflectometric measurements, the sample is irradiated from an arbitrary, but then permanent direction, and if the radiation reflected into different directions is measured, or if the direction of incidence is varied and the detection direction maintained, the relative spatial distribution of this radiation is called "reflection indicatrix". It usually depends on the direction of incidence and on the detection direction. From this it follows that a complete determination of the diffuse reflection of a sample or a material is possible only when the reflection indicatrix is measured.
Fig 2: Geometric conditions of the irradiated (i) and reflected rays (r) on the gonioreflectometer
The angular section of the incident radiation can be varied in the range Φi = 0° to 360°, θi = 0° to 90°. The reflected radiation can be detected in the angular range Φr = 0° to 360°, with θr = 0° to 85°. The spectral range for calibrations with the robot-based gonioreflectometer extends from 250 nm to 1700 nm.
The maximum sample size amounts to approx. 500 mm x 500 mm, with a maximum weight of approx. 1.5 kg.