LED light research. Researcher measuring a beam of light from an LED (light emitting diode) using an apparent source size measurement facility. The LED's 'apparent source size' is not the size of the LED device itself but the diameter of the light beam projected on the retina (back of the eye) after it has passed through the focusing parts of the eye. The lenses here are representing the focusing parts of the eye in order to check the optical safety of the LED. Other LED safety tests include looking at the radiance (brightness) and the spectral characteristics (the type of light emitted). Photographed at the National Physical Laboratory in Teddington, UK.