GE Purple-X Incandescent Blacklight
Blacklight became popular after the World War 1 invention of Wood's Glass, by Professor Robert Williams Wood of Carnegie University. His material consists of a barium-sodium-silicate glass doped with 9% nickel oxide, the latter causing strong absorption of visible wavelengths while transmitting both Infrared as well as UV-A wavelengths between about 300 and 450nm, peaking at 365nm. It was developed to allow secret communications by pulsed light signalling, with an ultra-violet detector being used to receive the signals during the daytime and an infra-red detector for night operation. The filtered light is hardly visible to the human eye and therefore difficult for an enemy to intercept.

One of the peacetime uses of Wood's Glass was to filter the visible light of electric lamps, transmitting the UV to allow fluorescent objects to be studied without their radiation being obscured by visible radiation. At first it was necessary to use fully enclosed luminaires to black all radiation, except the UV which could exit via a small window of this glass. Already by 1939, GE simplified this with its Purple-X mercury lamp having a bulb of Wood's Glass. In 1941 the company extended that concept to its Blacklight-Blue fluorescent tubes, also made with an envelope of the same glass.

This incandescent Purple-X was introduced prior to 1948, being listed in the catalogue of that year and that date also appearing on its instruction leaflet. It is based on a highly loaded Photoflood type lamp whose filament is driven at about 3200K to deliver increased actinic radiation in applications only requiring a small source. It operates at furiously high temperatures, and operation should be restricted to about 5 minutes to reduce the risk of breakage.
Manufacturer: General Electric U.S.A. Type 250A21/60
Lamp Power: 250 Watts
Lamp Voltage: 115-125 Volts Rated @ 115V
Lamp Current: 2.17 Amperes @ 115V
Cap Type: E26s/25 Brass & Vitrite
Bulb Type: A-67 A-21 in eighths/inch
Bulb Finish: Blacklight Wood's Glass
Filament Type: C-9/3s Single Coil Wreath
Atmosphere: Argon-Nitrogen Red P getter
Luminous Flux: 5 Lumens
Luminous Efficacy: 0.02 Lumens per Watt
UV-A Flux: 0.675 Wrad 320-400nm
UV-A Irradiance: 25 mW/m² @ 1 metre
Wavelength: Peak: 385nm Dominant: 420nm
Chromaticity Co-ordinates: CCx: 0.200 CCy: 0.185
Lifetime: 50 hours @ 115V Max. 5' running
Burning Position: Universal
Overall Length: 109.5 mm 45/16 inches
Light Centre Length: 85.7 mm 33/8 inches
Factory: Cleveland, OH U.S.A.
Date of Manufacture: 1953 September Date Code: 69 •
Original Value: US $1.25 (1948)
References: 1) GE USA Large Lamp Catalog, 1948, p.30.
2) GE USA Large Lamp Catalog, 1954, p.62.
3) GE Blacklight Lamps Brochure, 1970, USA.
4) Lamps for a Brighter America, P.Keating, McGraw-Hill Book Company Inc, 1st Edition (1954) p.205.
5) Photoelectric measurement.