
FraunhoferGesellschaft

Short description 
Numerical Goniophotometer
The numerical method enables planners to determine indicatrices of diffusion (BTDFs) of various façade systems in parallel
with actual measuring instruments (goniophotometers). The method described as "numerical goniophotometry" is intended to
determine the indicatix of scattering for a façade system and meets the following applications:
 
Consideration of the façade system's geometrical configuration. 
 
Consideration of the photometrical properties of the materials used. 
 
Parameterizability of type and position of the light source used for illuminating the test sample, developed by analogy with
actual measuring setups under recognized measurements protocols as in IEA Task 21
www.ieashc.org/task21/. 
 
Parameterizability of the sensors and, with this, determination of the resolution of the indicatrix of diffusion with regard to
postulated metrological boundary conditions, like observation of the photometrical limiting distance. 
To calculate the actual propagation of light, the method relies on a commercial
raytracing algorithm.
This algorithm is incorporated into a virtual measurement setup that allows to perform goniophotometriacl calculations.
A program featuring a graphic user interface facilitates the generation of virtual façade systems and the parametrization of
the virtual test station. The façade systems may be combined among each other in layer structures or with transparent glazing
structures.
Schematic representation of the numerical goniophotometry method. The diagram presents the lumious fluxes that were measured on the
sensor surfaces of the hemicube, which are going to be converted into luminance coefficients.The distributions apply for
lightredirecting slats, at a slat angle of 0° (top) and 40° (bottom) for 1 million rays traced, illuminated under a 60°
angle of incidence.
