Abstract: A two-wavelength I.R. pyrometer for monitoring the rapid heating of materials by C W lasers or electron beams. The I.R. radiated from the material is separated into two closely adjacent wavelengths and signal values representative of those wavelengths are obtained. These signal values are then processed through an electronic circuit where the signal value of one of the signals is held substantially constant by a feedback control while the other signal value is influenced by approximately the same feedback control applied to the first signal value and wherein the resultant signal values are then processed through a circuit determining a ratio of one signal value divided by the other. This is then a measure of the temperature of the material. Also disclosed is a chopper wheel for a two-wavelength I.R. pyrometer. The wheel has a plurality of circumferentially extending I.R. filters of alternating wavelengths (.lambda..sub.1, .lambda..sub.2).