This paper begins by describing the operational requirements for the luminous range of colour light signals for German Railways (DB AG) using the combination (Ks) and main-distant (MD, German abbreviation: HV) signalling systems as an example. A further section discusses the theoretical principles. What relationship exists between the luminous range and intensity of a colour light signal? How does one evaluate light phantoms? How does one measure and evaluate the effects of ambient light? In the light of the theoretical principles, the current state of the art is then examined as exemplified by the optical system of the Siemens V 136 Colour Light Signal. The question arises as to how the evaluations need to be modified for the new LED signals. Suggestions are presented for the photometric design and evaluation for the demonstration of technical safety (safety case). Finally, in the conclusion, positive prospects concerning the possibilities of using LED signals are presented.