Inside Frosted 'Pearl' A-Line Incandescent

The apparently simple development of an incandescent lamp having a diffuse bulb combined with high light transmission took a surprisingly long time to solve - nearly five decades! The problem was eventually overcome in 1925 when Marvin Pipkin of GE developed the inside-frosted lamp shown here.

Earlier frosting techniques had to be applied on the outside, which is in compression and much stronger than the inner surface which is always in tension. But the rough external surface quickly attracted dust and reduced light output. Pipkin made inside frosting feasible by first etching the glass with an ammonium bifluoride and hydrofluoric acid mixture, then re-strengthening it with a second treatment of dilute acid to smooth the roughened surface. The resulting 'pearl' finish achieved excellent diffusion, less than 1% light absorption, and does not become dirty over time. The now ubiquitous pear-shaped or 'A-line' bulb envelopes were introduced in parallel with the new finish, another major step which standardised lamp designs and reduce costs.

Pipkin's patent was not upheld because a similar technique had already been applied outside the lamp industry, and this his method quickly became the preferred finish among all lamp manufacturers globally. Despite the development of later still improved techniques, it was only displaced in the 1990s when changing environmental regulations frowned upon the excessive quantities of waste hydrofluoric acid being generated by the lamp industry.

This lamp does not bear any date mark, but the Edison brand, packaging artwork and lack of a coiled-coil filament suggest it was made prior to 1936. Curiously, the inner stem is made from pale blue glass, for an unknown reason.
Manufacturer: Edison Lamp Works of GE
Lamp Power: 50 Watts
Lamp Voltage: 120 Volts
Lamp Current: 0.42 Amperes
Cap Type: E26s/25 Brass & vitrite
Bulb Type: A-67 A-21 in eighths/inch
Bulb Finish: Inside Frosted Soda-lime glass
Filament Type: C-9/3s Single Coil Wreath
Atmosphere: Argon-Nitrogen Red phosphorus getter
Luminous Flux: Approx. 600 Lumens
Luminous Efficacy: Approx 12 Lumens per Watt
Beam Intensity: N/A
Beam Distribution: N/A
Colour Temperature & CRI: CCT: 2700 Kelvin CRI: Ra 100
Chromaticity Co-ordinates: CCx: 0.460 CCy: 0.411
Rated Lifetime: 1000 hours
Burning Position: Universal
Overall Length: 125 mm 415/16 inches
Light Centre Length: 86 mm 33/8 inches
Factory: Harrison N.J. ? U.S.A.
Date of Manufacture: Approx. 1925-1936
Original Value: US $0.15 (1937)
References: 1) GE Large Lamp Catalog, 1936, USA.
2) GE Large Lamp Catalog, 1936, USA.
3) The History of the Incandescent Lamp, John W. Howell & Henry Schroeder, publ. The Macqua Company, 1927, pp.176-179.
4) The Electric Lamp Industry, Arthur A. Bright, publ. The Macmillan Company, 1949, pp.326-328
5) Lamps for a Brighter America, Paul W. Keating, publ. The McGraw-Hill Company, 1954, pp.119, 126-127, 226.
6) A Century of Light, James A. Cox, publ. The Benjamin Company, 1979, p.64, 71, 136.
7) New and Better Lamps (NELA Booklet G3), GE Lamp Division, NELA Park Engineering Department, 20-Feb-1926.
8) Biography of Marvin Pipkin, Edward Covington & James Hooker, Lamptech Website, July 2019.