A luminaire property that indicates the values of the luminous intensities radiated in all relevant directions by the luminaire. Usually the luminous intensities are given as intensities per 1,000 lamp lumen when the lamp or lamps are operated under standard test conditions: I/1,000 lm.
Luminaire Performance Characteristic
The light distribution of a luminaire represents the most important performance characteristic of a luminaire. It is the basis for the determination of photometric characteristics of luminaires such as upward and downward light output ratio, utilization factors for defined areas or zones, beam angles, and glare figures. Luminaire manufacturers produce for their luminaires photometric datasheets which show these photometric characteristics together with a graphical representation of the light distribution itself.
Strictly speaking, the light distribution can only be measured for a point source. In practice the measurement inaccuracy will be negligible provided the optical path length of the measuring setup is at least ten times the length of the light emitting surface of the luminaire. For narrow beam projector type of luminaires, greater optical path lengths are required. The instrument used for measuring light distributions is called a goniophotometer. It exists of a mounting support for the luminaire, a photocell, and a, usually complex, system that rotates the combination of photocell and luminaire such that the intensities from all different light directions can be measured. Mirrors are used to keep the actual dimensions as small as possible while keeping the required optical path length. It is important that the luminaire rotates in such a way that its normal operating position is at all times maintained. The measurements should be carried out under standard test conditions. These conditions refer to ambient temperature, air movement, mounting position, and lamp ballast type. The coordinating systems used for indicating the directions are either the so-called B-beta (usually used for projector type of luminaires) or the C-gamma system. The B angles of the B-beta system can be understood as the angles the pages of a book make when the book axis is held horizontally. The beta angles are then angles in each page measured from the middle of the book axis. The C-gamma system in the same analogy can be understood as a book with its axis held vertically. C angles are the angles the pages make, and gamma angles are angles in the pages from the middle of the book axis.
All lighting calculations of lighting installations are based on the light distribution of the luminaires used. For this purpose the light distribution is given as a two-dimensional table, the so-called I-table. There are some different formats for these I-tables such as CIE, IESNA, Eulumdat, CIBSE, and some luminaire manufacturer’s specific formats. Eulumdat is the most popular format in Europe, whereas the IESNA format is more popular in the USA. The CIE format is an attempt to define a “recommended file format for electronic transfer of luminaire photometric data” on a global level, in order to overcome the regionally different formats currently in use. Many light calculation programs can use some different formats as input. Moreover, free software for conversion from one format to the other is available as well. The layout of the I-table is different for different type of luminaires. So have road lighting luminaire I-tables more values around the direction of the horizon than for other directions because these values very much determine glare, general lighting interior luminaire I-tables have a somewhat more regular layout because of the more diffuse character of these luminaires and flood and accent lighting luminaires (projector type of luminaires) have I-tables with extra values around the direction of the maximum beam intensity.