Thermographic instruments using single-element line scanning usually have very low line frequencies in comparison with television. To meet the requirements for higher frame rates in thermography and infra-red viewing a line scanner with an indium antimonide detector and a rotating germanium prism has been designed. It produces heat images with 100 lines per frame on a television-type display tube at a flicker-free frame rate. The sweep efficiency of the scanning system is almost 100 per cent. The optical aberrations introduced by the scanning prism are analysed and found to be at a minimum when the refractive index lies between 3 and 4. Examples of heat pictures taken with different modes of operation are given.