Measurement of photometric properties

When it comes to measuring photometric luminaire properties, there are two fundamental methods of observation. One relates to the entirety of the light emitted from the luminaire; the other relates to its spatial distribution.

The appropriate measurement setup for examining light in its entirety for properties is the Ulbricht sphere. The inner barium sulphate coating of the sphere produces a uniform distribution of light through its diffuse, spectrally neutral reflection, thereby facilitating measurement at an opening anywhere in the outer wall. The Ulbricht sphere is therefore suitable for spectral measurements as well as relative measurements regarding available luminous flux.

Figure 2.24: Luminous flux measurement in the Ulbricht sphere

The spatial distribution of light emitted from a luminaire is generally visualised using a luminous intensity distribution curve (or a luminous intensity distribution model, see figure). The luminous intensity distribution measurement can be carried out as a distant-field measurement using a goniophotometer with rotating mirror. This requires a great distance between luminaire and sensor in relation to the luminaire’s dimensions. For linear luminaires with a length of ca. 1.5 m, this means a minimum length of 25 m for the laboratory space.

Modern goniophotometers today are much more compact and allow for near-field measurements. For this purpose, the sensor setup is moved past the luminaire in small increments at a small distance in relation to the luminaire’s dimensions. At each measuring point, the luminous intensity distribution coming from said measuring point is measured. The results from this measurement, which are much more detailed in comparison to those from a rotating-mirror goniophotometer, currently cannot be processed using commercially available lighting design programs. To date, no uniform data format for near-field visualisation of luminaires has been defined either. The data gathered using near-field measurement are thus converted to distant-field data by the goniophotometer’s measuring software in order to make them available for lighting design programs.

Figure 2.25: Luminous intensity distribution measurement using a near-field goniophotometer