Very often, lighting designers and engineers want to know exactly how much light a fitting emits in a given direction. Unless a luminaire is perfectly diffusing, its luminous intensity will vary depending on the view point of an observer. To measure this distribution, the direction can be described in terms of angles of azimuth and elevation, similar to the virtual grid that covers our Earth, allowing us to pin-point the position of any place in terms of its geographical latitude and longitude.

The result is pairs of azimuth and elevation angles. The reference angle (0, 0) points downwards, although different coordinate systems are possible. To each pair of azimuth and elevation, a luminous intensity is assigned as measured on a goniophotometer. Usually, the intensity will be given in cd per 1000lx and has to be multiplied with the total light output of the lamp.

Intensity tables are available in electronic form for most recent luminaires. They are mainly used as input for computer-based lighting and visualisation programs. To make the tables more meaningful for the lighting designer, they can be visualised in polar curves, isocandela or isolux diagrams.

Fitting and its corresponding luminous intensity distribution
(Images courtesy of Marlin Lighting)

Goniophotometer
(Image courtesy of TechoTeam)