Visual display unit research. Physicist using a photon counter to analyse the light levels of a visual display unit (VDU, also known as a computer monitor). Photons are packets of electromagnetic energy that are emitted from an energy source. They take the form of visible light, ultraviolet light, infrared light, microwaves, radio waves, gamma rays and X-rays. Here, the researcher is counting the number of visible light photons emitted from the VDU, so that he can quantitatively measure the strength of light produced by the screen. This will enable the monitor to be calibrated so that it is safe and efficient to use. Photographed at the National Physical Laboratory in Teddington, UK.