Systemaufbau und Komponenten

The PSF to BSDF-method evaluates the distribution of light scattered from a plane sample (bidirectional scattering distribution function) from the 2-dimensional point -spread- function (PSF) usually measured with an imaging photometer.

The bidirectional scattering distribution function (BSDF) is a generalization of the concept of the BRDF (bidirectional reflectance distribution function) since it comprises scattering effects for both transmission and reflection.

Coordinate system for description of the geometry of light source, S, object to be measured (DUT) and imaging photometer, (receiver, usually an imaging photometer).

Measurement Procedure

Imaging photometer and light -source are both arranged within a plane perpendicular to the object of measurement (DUT) . The angle of inclination of both devices should be the same. The specular reflection component must be included in the " luminance image" of the photometer.
The distance of source and receiver to the sample determine  the range over which the direct ions vary across the sample area and the angular resolution of the measurement . With increasing distance (at constant DUT area) the angular range decreases and the angular resolution increases.

The imaging photometer must be focused on the light source. The focus is adjusted after placement of a mirror (e.g. polished black glass) at the location of the DUT. The entrance pupil of the optics of the imaging photometer has to be set to a minimum either by adjustment of the iris of the lens or by use of an additional aperture stop. After adjustment of the source intensity (must remain constant during the measurement) the distribution of the light reflected by the sample must be measured ( relative luminance).

Then, under identical geometrical conditions and at the same intensity of the light source the luminance of a known plane reflectance reference has to be measured for evaluation of the illuminance across the sample area. The illuminance is related to the luminance of the (diffuse) reflectance reference by a factor which has to be constant over the area.
Division of the distribution of the reflected luminance (of the sample) by the reflected luminance of the reflectance reference (this division is done for each single pixel of the intensity images) yields the reflectance for each area element of the sample. From this lateral distribution of reflectance the direct ional reflectance distribution (BRDF / BSDF) is calculated.

Advantages of the approach

• simple and compact apparatus without moving parts for direct ional scan,
• self-alignment about the specular direction,
• insensitive to uncontrolled parameter variations = excellent robustness,
• spectral analysis possible (spectrally tunable source or receiver) ,
• applicability to non- flat surfaces with arbitrary shape,
• simultaneous recording of all reflect ion components,
• small system signature for separation of the specular peak from the haze.

Limitations

• sample must be uniform over area included in the measurement,
• BRDF of the sample must be invariant to source inclination (over the range of direct ions included in one measurement),
• limited range of inclinations centered about specular (a range of ± 20° can be covered with a resolution o f < 0.1° FWHM) .

Applications

• electronic displays and their components (touchpanels, diffusers, anti-glare layers) ,
• paintwork (automotive, graphics),
• arbitrary scattering surfaces and films in
• transmission (windows) ,
• reflection (paper) .