The 50lx mantra: origin and application
Museum lighting with 50lx
Where does the 50lx reference value come from?
An illuminance of 50lx has become a standard with exhibition organisers for the illumination of light-sensitive exhibits in galleries and museums. The 50lx however cannot be justified due to reasons of conservation, but correspond to a lighting level enabling a young person with adapted eyes to clearly identify an object with bright colour tones, low details and simple structures. The value therefore specifies a threshold with visual perception. Darker and higher-detail exhibits must be more brightly lit to achieve a comparative impression. The age of the observer is also a factor. Older people sometimes need two to three times as much illuminance for the same visual experience.
Is illuminance a suitable measure?
Designing with illuminance values represents a practical simplification in many cases. Illuminance in lx is a measure of the radiation incident on a surface, weighted with the spectral sensitivity of the human eye. This however does not correspond to the spectral sensitivity of materials typically shown in museums. Museums should really individually measure the spectral sensitivity of their exhibits to determine correct exposures on sensitive objects. They seldom have the technology or budget for this purpose though. Illuminance levels on the other hand are easy to measure and calculate. They therefore represent a practical simplification for spectra with low relative damage factors.
