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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Headlight design comparing 'faked' and true parabola systems, and the principle of the 'Depressed Beam'.

Identifier  ExFiles\Box 61a\1\  scan0390
Date  20th March 1929 guessed
  
- 2 -

A small parabola naturally gives more trouble in adjustment of the bulb-focus when changing bulbs, and convenient focussing means must therefore be provided.

(2) The lamp bulb should be mounted .070 above the axis of the parabola. The filaments being .140 apart, this brings the lower filament at the true focus and the upper .140 above.

(3) There has been considerable discussion about the best treatment of the light after it leaves the bulb. There are two schools, one favoring a "faked" parabola and the other a true parabola and special lens. The faked parabola consists of an "offset" upper half etc. and must lose efficiency when on the main filament. We favor the true parabola and special lens of which the best example is the "Depressed Beam". The principle of this is shown in the lower half of SK-199 herewith. A is the bright light from the middle "slice" of the reflector which is thrown straight forward (spread sideways of course) B & C are the dimmer rays from the top and bottom of the reflector which are thrown downward to illuminate the foreground.

Very truly yours,

Maurice Olley

r/
SK-199
  
  


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