National Mazda Grid Filament Stereopticon

One of the principal developments in the field of projection lamps was the invention of the so-called Grid Filament, as featured in this very early example.

Within one year of the 1913 introduction of coiled tungsten filament lamps for general service lamps, this new technology was applied to stereopticon projection lamps. At first the filament was bent into a series of parallel sections, arranged in a wreath shape similar to general lighting lamps but rather more compact so as to attain a higher brightness light source. However the depth of the filament assembly prevented it being brought into the focal plane of the projection apparatus. Moreover the light from some filament segments was partially blocked by other parts of the filament. This limitation was overcome by arranging the parallel coiled filament sections into the shape of a planar grid, having approximately the same square shape and aspect ratio as the images to be projected. The result was an immediate improvement in the brightness of projected images for a given lamp power. Around the same time, the design of the bulb envelope was also reduced from the former G-30 spherical bulbs to the new T-20 tubular bulb featured here. This allowed the filament to be brought closer to the optical system, thereby realising another increase in screen brightness.

It is believed that this development took place between approximately 1915 and 1921 - GE literature of 1915 still refers only to the bunch-filament G-30 design of stereopticon lamps, and this lamp features its exhaustion tip on the bulb crown - thereby dating it to prior to 1921 when the tip was relocated to the inner stem area.
Manufacturer: National Lamp Works of GE
Lamp Power: 400 Watts
Lamp Voltage: 115 Volts
Lamp Current: 3.48 Amperes
Cap Type: E26s Brass & Vitrite
Bulb Type: T-63 T-20 in eighths/inch
Bulb Finish: Clear Borosilicate
Filament Type: C-13 Monoplane Grid
Atmosphere: Argon-Nitrogen Red phosphorus getter
Luminous Flux: 10,000 Lumens
Luminous Efficacy: 25 Lumens per Watt
Beam Intensity: N/A
Beam Distribution: N/A
Colour Temperature & CRI: CCT: 3000 Kelvin CRI: Ra 100
Chromaticity Co-ordinates: CCx: 0.452 CCy: 0.409
Rated Lifetime: 50 hours
Burning Position: Vertical base down
Overall Length: 140 mm 51/2 inches
Light Centre Length: 76.2 mm 3 inches
Factory: U.S.A.
Date of Manufacture: Approx. 1920 Date Code: p4 A5
Original Value: