Abstract: A gas detector for detecting the presence of a light absorbing gas within an atmosphere (10) containing the gas includes a light source (12), a detector (14) arranged to detect a presence of incident radiation from the light source (12) and to provide an output signal (46) representative of the level of incident radiation from the light source (12). Electronic circuitry (130) is arranged to process the output signal (46) to provide an indication of the incident radiation. A power supply (40, 36) for cyclically energizing the light source (12) is also provided. The power supply is arranged to deliver substantially constant power at the or each level of energization of the light source (12).