Measurements of the output from optical radiation sources are essential for a wide range of applications, including Earth Observation measurements; safety and effectiveness of phototherapy treatments; low energy lighting solutions; film, television and displays; signalling for transportation, etc. Measurements may be made spectrally (termed 'spectroradiometry') or according to the response of the human eye ('photometry').

To provide a consistent measurement base for photometry, two standard observer functions have been adopted internationally: V(λ) - for photopic vision (daytime lighting levels); and V'(λ) - for scotopic vision (low lighting levels). At intermediate lighting levels (termed the 'mesopic region') the response varies gradually between these two functions; the appropriate weighting function at any given level is defined by the CIE system for mesopic photometry.

NPL has extensive facilities for characterising all kinds of light source, including a new goniometer facility which can characterise both the spectral and spatial distribution of the output from a source.

Recent enhancements to NPL's capability include the evaluation of LED-based sources, in particular testing to LM79 and hazard assessment to EN60825.