The photometric characteristics of a signal, for this signal to be perceived with a certain degrees of likelihood, depend on the type of background against which it stands out and the viewer's psychic condition. The tests described in this report were carried in a laboratory in order to proceed in the light of definite data, but the conditions under which they took place were as close as possible to those of real road-driving. They will be checked by some outdoor measurements. The observer having taken place in the laboratory's mock car-cabin had to make coincide a mobile vertical bar which was moving according to a pre-established programme. He was to report his seeing the signal by taking his foot off the gas and his no longer seeing it by stepping on it. The first measurements as made possible by 75 observers of red lights against a white background with a luminance of 1 440 cd/m2 have proven the importance of luminance at low intensities. The tests are still carried on presently in order to ascertain the relative importance of each of the various parameters that play a part
Abstract