Early Pre-Focussed Projection Lamp 500T12/P
GE was one of the pioneers of electric lamps for image projection apapratus, having first introduced special high brightness carbon filament 'stereopticon' lamps for magic lantern slide projectors in the early 1900s. Within a year of the introduction of gas-filled tungsten lamps, projection versions having a compact bunch-type filament in spherical bulbs had been introduced, and a major development was achieved prior to 1920 when the planar grid-type filament was introduced. This made a tremendous improvement in the homogeneity of light projected across the image.

However for many years, one of the troublesome aspects of projection lamps was the requirement for the lamp filament to be precisely aligned at the focus of the optical system to ensure maximum screen brightness and uniformity. No two lamps had their filaments in the same position, due to the variations inherent in mounting the filament on a glass stem assembly, sealing this into a glass bulb, and then applying the traditional Edison screw cap. The problem was most elegantly solved by Robert S. Burnap of GE's Edison Lamp Works at Harrison NJ in 1929, and the lamp on this page is one of the first made according to his invention.

The cap is made in two parts. First a metal liner is cemented to the glass bulb with all the usual inherent dimensional variations. The outer cap is then placed over this, and the two components aligned in an optical projection microscope to precisely orientate the filament with respect to the outer cap, before fixing the position permanently by a soldering operation. The outer cap bears two lateral fins around its upper rim, and these form the reference point to which the filament is aligned.
Manufacturer: General Electric U.S.A.
Lamp Power: 500 Watts
Lamp Voltage: 120 Volts
Lamp Current: 4.17 Amperes
Cap Type: P28s Brass + vitrite
Bulb Type: T-38 T-12 in eighths/inch
Bulb Finish: Clear GE772 leaded borosilicate
Filament Type: C-13 Planar Grid Single Coil
Atmosphere: Argon-Nitrogen Red phosphorus getter
Luminous Flux: Approx. 12,000 Lumens
Luminous Efficacy: Approx. 24 Lumens per Watt
Beam Intensity: N/A
Beam Distribution: N/A
Colour Temperature & CRI: CCT: 3200 Kelvin CRI: Ra 100
Chromaticity Co-ordinates: CCx: 0.423 CCy: 0.399
Rated Lifetime: 50 hours?
Burning Position: Vertical base down ± 25°
Overall Length: 146 mm 53/4 inches
Light Centre Length: 55.5 mm 23/16 inches
Factory: Ohio Lamp Plant, Warren U.S.A.
Date of Manufacture: January 1932 Date Code: 97
Original Value: Unknown
References: 1) Lamps for a Brighter America, Paul W. Keating, publ. McGraw-Hill, 1954, pp.128, 153.
2) A Century of Light, James A. Cox, publ. The Benjamin Company, 1979, pp.82.
3) Biography of Robert S. Burnap, James Hooker, Lamptech Website, July 2019.
4) Picture Projection with Mazda Lamps, C.E. Egeler & R.E. Farnham, Bulletin 33-C of National Electric Lamp Works of GE, 1-Jan-1929, pp.40, 47-48.
5) Mazda Lamp Guide for Motion Picture & Stereopticon Projectors, F.E. Carlson, GE Lamp Division, NELA Park Engineering Department, July 1940.