Hemispherical-directed Geometries (Sphere Geometries)
For measurements in hemispherical-directed geometries, i.e. diffuse irradiation from the half- space onto the sample to be measured with subsequent measurement of the directed reflection, an integrating sphere is used. The measurement procedure used for this purpose is the so-called Korte-Schmidt method which has been developed at PTB [1]. The reflection characteristic measured in this case is the spectral radiance factor β(λ).
In the case of the Korte-Schmidt method, the sample P to be measured is mounted at the centre of the integrating sphere (sphere diameter Ø=500 mm) in such a way that its surface lies in the equatorial plane of the sphere. An internal lamp L serves as the light source; it does not, however, irradiate the sample directly. Due to the special construction, first of all only the half-space II of the sphere is irradiated. By a multitude of diffuse wall reflections on the internal coating, the radiation is transmitted into the half-space I from which the sample P is then irradiated from all directions with a constant radiance L.
A perfectly diffusing reflector (PRD) at the sphere centre would have the same radiance as the wall of the half-space I which irradiates it. Consequently, the measurement of the radiance of this hemisphere is equivalent to the measurement of the radiance reflected by the PRD. The following figure shows the principle of measurement.
Fig.: Schematic drawing of the integrating sphere
This measurement facility allows reflection geometries to be adjusted in the range from d:0° to d:10°. The spectral range for calibrations with the integrating sphere extends from to 250 nm to 2400 nm.
Due to the mounting of the sample to be calibrated in the centre of the integrating sphere with the aid of a special support, the following maximum dimensions are obtained for the size of the sample:
i.) for round samples: Ø=76 mm, thickness: 15 mm
ii.) for square samples: edge length: 53 mm, thickness: 15 mm
[1] H. Korte und M. Schmidt, Über Messungen des Leuchtdichtefaktors an beliebig reflektierenden Proben, Lichttechnik 19, 135A-137A (1967)