Class A1 Projector with Offset Mirror - A1/24 DFN

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.
Manufacturer: Thorn Lighting Ltd  
Lamp Power: 150 Watts  
Lamp Voltage: 120 Volts  
Lamp Current: 1.25 Amperes  
Cap Type: G17q-a23 Brass with Nilo K Pins
Bulb Type: TC39x42.5 TC12x13 in eighths/inch
Bulb Finish: Clear Soda-lime Aluminium Mirror
Filament Type: CC-8 Axial Coiled Coil
Atmosphere: Argon, Nitrogen
Luminous Flux: 3150 lm (filament lumens)
Luminous Efficacy: 21.0 lm/W (filament efficacy)
Peak Intensity: 15,000 cd  
Focal Distance: 57.15 mm 2¼"
Colour Temperature & CRI: CCT: 3150K CRI: Ra 100
Chromaticity Co-ordinates: CCx: CCy:
Rated Lifetime: 25 hours to 50% survival
Burning Position: Base Down to Horiz.  
Max. Overall Length: 81.0 mm 3 3/16"
Light Centre Length: 39.7 mm 1 9/16"
Factory: Leicester England
Date of Manufacture: March 1967 Date Code C7
Original / Present Value: GB £1 17s 3d (1969) GB £22.08 (2007)
References: 1) Thorn Lighting Comprehensive Catalogue, UK, 1969/70, p.CK5