||The original Tru-Flector lamps invented by Sylvania marked a dramatic step forward in projector lamp technology. The incorporation of a large reflector inside the bulb allowed unparalleled optical performance to be reached. However despite their increased efficiency, their design introduced a new limitation which left room for improvement.
The diameter of the internal reflector should be as large as possible to increase the light collection efficiency. This firstly necessitates a bulb of larger diameter than normal, and secondly requires that the reflector of course must be disposed on the central axis of the lamp (where the width available inside the glass tube is greatest). This means that the filament must also be brought relatively close to the centre of the bulb, and owing to the bulb's large diameter the distance from the filament to the projector's optical system is greater than normal. Increased spacing to the filament results in lower optical system efficiency.
The problem was solved in a particularly elegant fashion by General Electric's invention of this bulb having a conical cross-section. This permitted the internal reflector, and of course the filament as well, to be brought closer to the wall of the bulb and hence to the projector optics. The result was a significant increase in screen lumens, and this type DFN (A1/24) was the first to be introduced - to replace the conventional type DFC (A1/222).
Incidentally this lamp, designed for use horizontally in 8mm movie projectors, features a secondary tungsten ballast filament for increased efficacy. The reflector is made from stamped aluminium sheet having a polished surface.