Patent No. 9448, classification B-61L.
Optical system for different coloured signals that remain permanently lighted - especially for use in railway signalling systems.
 

Description: The  invention relates to an optical system for a signalling device which has signals in varying colours that remain permanently lighted, of application particularly in train signals.
As is known, before the advent of dichroic filters, signal emission devices that were permanently lighted used an electro-optical-mechanical system composed of a spherical mirror at whose centre of curvature was placed a light source composed of a light bulb, and an aspherical lens focusing at the centre of the light source. The different colours of the signal were obtained by using several flat filters, of coloured glass, arranged in succession, perpendicularly to the axis of the conical beam of the rays, at a point between the light source and the aspherical lens. Selection and control of the colour filter mechanism was achieved by means of a relay. The result was a complex, costly system which was both inefficient and of high energy consumption.
With the invention of dichroic filters and their application in railway signals with permanently lighted signals, it has been possible to eliminate the movable elements of the system; (...)
Nevertheless, some significant unsolved problems still remained in these solutions, which had  negative effects on brightness, bulk and the optical perfection of the optical systems used. (...)
The invention presented here has the purpose of eliminating the inconveniences listed above relating to bulk of mechanical parts, brightness and the stigmatism of a signalling device using dichroic filters for creating different coloured signals. (...)