Essentially, the programs’ strength lies in the ability to repeat photometric parameters for the assessment of a lighting installation. As opposed to the efficiency method, which merely facilitates the determination of an average illuminance on the working plane, illuminance is calculated for any defined measuring point in a room by summing up the various influences on said point. The examined room can be divided up into zones, e.g. work and surrounding areas, and their illuminance values can be assessed separately. Additionally, it is possible to examine vertical and tilted surfaces. The uniformity of illuminance can be determined for any defined surface.
The method works by considering the influences of all light sources for any given point in the defined measuring plane. This means that a portion of the lighting incident on the measuring point comes directly from the closest luminaire, additional portions stem from more distant luminaires, and other portions are reflected by walls and the ceiling.
When using the photometric data in ULD or ROLF format (see section ) the effects of extended or subdivided or composite light-emitting surfaces in luminaires should be considered. Other usable luminous intensity distribution curve data formats (IES, LDT, T14) in comparison only consider one light source each as the point of origin of the luminous intensity distribution.
Besides the illuminance distribution, photometric design also facilitates the determination of luminance and associated glare effects in the room and at the luminaires. Significant optimisation can be gained from smart arrangements of suitable luminaires.
Calculation results can be displayed either as tables, diagrams or two-/three-dimensional false colour images. Furthermore, the programs facilitate visualising (e.g. through ray tracing, see figure) the lighting situation in order to get an impression of its visual effects during the design stage (see following section).
The evaluation of the results and also the displayed visualisation require a high degree of experience. It is the task of the designer to create a sensible arrangement of suitable luminaires which generates an optimum lighting situation.
The various lighting calculation programs mostly vary in terms of handling. Regarding the generated calculation results, they hardly differ at all. The results are readily repeatable. Deviations from measurements incorporated in implemented installations often amount to 10% or sometimes more. Besides possible tolerances in technical light source values, deviations in reflective properties of real room surfaces compared to those used in planning as well as shadowing by various objects in the room are decisive causes. Particularly with mostly indirect distribution lighting, great diligence is required in the design stage.
The programs are continuously developed further and adapt to current standards and market requirements.