A Tactical Engagement Simulation (TES) is a training system for using weapons. Laser transmitters are used instead of bullets, larger rounds, or shorter-range guided weapons such as anti-tank missiles. A laser transmitter is mounted on the weapon and aligned with the weapon's barrel. Gallium arsenide (GaAs) is often used as the stimulated medium and this produces a wavelength of 904 nanometres, in the near infrared band outside the sensitivity of the human eye which is from about 400 to 700 nm (0.4 to 0.7 micrometres).