LED light research. Spectral characterisation facility being used to measure a beam of light from an LED. This test determines the type of light being emitted in order to check the optical safety of the LED. Other LED safety tests include looking at the brightness of the LED and the diameter of the light beam when it projects onto the retina (back) of the eye, a measurement known as the 'apparent source size'. Photographed at the National Physical Laboratory in Teddington, UK.