In his early experiments with television, Baird had great difficulty in reducing the intensity of the light used to illuminate his subjects without impairing the results achieved by his apparatus. The photocells which were available in 1926 were small cells which had a high ambient noise level. This, together with the parasitics introduced by the then dull-emitter valves which were used in the amplifiers, caused the received picture to have a very poor signal to noise ratio. Baird accordingly set out to select cells whose colour response matched the luminosity-wavelength characteristic of his floodlights and this led him to experiment with various coloured lamps and filters. According to J.D. Percy, it was during these tests that the lamps were masked, as an experiment, with wafer thin ebonite sheet so that all visible light was cut off and only infra-red radiation played on the subject. 'Much to Baird's surprise, the picture was not only visible at the receiver, but the signal/noise ratio, since he was using red sensitive cells, was surprisingly good.' Baird had earlier tried ultra-violet rays but these had proved to be very objectionable for the subject and had the added disadvantage that the lenses used attenuated the radiation very considerably. Infra-red rays, on the other hand, did not cause any discomfort and therefore 'noctovision', as Baird named his latest discovery, came into being and occupied much of his attention for the next five years.

Inspec keywords: television; history

Other keywords: wafer thin ebonite sheet; ultraviolet rays; noctovision; luminosity-wavelength characteristic; dull-emitter valves; John Logie Baird; ambient noise level; infrared radiation; television pioneer; illumination; light intensity reduction; phonovision

Subjects: History of science