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.