Westinghouse DRS A1/58 Bi-Plane Filament
Following the introduction of projection lamps having a planar grid filament, the next major innovation was the development, by Westinghouse, of the Bi-Plane filament as featured in this lamp. The monoplane grid filament lamps had to be employed with a spherical oondensing mirror placed behind the lamp, and slightly offset. This produced an image of the rear side of the filament aligned so as to fill the gaps between the adjacent coiled sections, and present a solid luminous surface. However the intensity of the reflected image was about 40% reduced, owing to reflection and absorption losses of the mirror and passing the light twice through the lamp envelope.

As early as 1915, Westinghouse patented the concept of a lamp having two planes of filament coils, one behind the other and offset such that the rear coils filled the spaces between the front coils. The result was a major increase in screen brightness, since a 1000W bi-plane filament could effectively be fitted into the area of a monoplane 500W filament. Moreover, the rear mirror in the projector could be dispensed with, along with the necessity for precision re-alignment with each lamp change.

Far from being a simple mechanical re-arrangement of the filament, the bi-plane innovation required substantial improvements in lamp design, and was not commercially introduced until 1933. One of the key difficulties was to ensure a sufficiently stable filament and avoid arcing between the individual segments. Modern lamps often use tungsten wire having a lower potassium concentration so as to reduce the thermionic effect, and gas fillings containing a small trace of hydrogen to increase the ionisation potential.
Manufacturer: Westinghouse
Lamp Power: 1000 Watts
Lamp Voltage: 120 Volts
Lamp Current: 8.33 Amperes
Cap Type: P28s/25 Brass & Vitrite
Bulb Type: T-63 T-20 in eighths/inch
Bulb Finish: Clear Borosilicate
Filament Type: C-13D Bi-plane Grid
Atmosphere: Argon-Nitrogen Red phosphorus getter
Luminous Flux: 28,500 Lumens
Luminous Efficacy: 28.5 Lumens per Watt
Beam Intensity: N/A
Beam Distribution: N/A
Colour Temperature & CRI: CCT: 3325 Kelvin CRI: Ra 100
Chromaticity Co-ordinates: CCx: CCy:
Rated Lifetime: 25 hours
Burning Position: Vertical base down
Overall Length: 146 mm 53/4 inches inches
Light Centre Length: 55.5 mm 23/16 inches inches
Factory: U.S.A.
Date of Manufacture: Approx. 1940 Date Code: 85H 120
Original Value:
References: 1) ANSI DRS lamp specifications
2) Lamps for a Brighter America, Paul W. Keating, publ. The McGraw-Hill Company, 1954, pp.158.
3) A Century of Light, James A. Cox, publ. The Benjamin Company, 1979, p.82-84
4) US Patent 1,313,857, Incandescent Lamp having Bi-Plane Grid Filament, A.R.Dennington, Westinghouse Lamp Co., Filed 16-Mar-1915.