GE Purple-X Mercury Blacklight H100-BL4
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 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.

Prior to 1939 GE introduced this Purple-X lamp, employing an outer bulb made from Wood's glass and requiring no extra filter. The rest of its design is based on the general lighting A-H4 Mercury lamp, the outer bulb being enlarged from the usual T-10 to this larger T-16 size due to the great heat absorption by the black glass. The glass contains the usual iron oxide impurity found in most soft glasses, which prevents the escape of harmful UV below about 320nm. These lamps do not emit dangerous shortwave UV and are completely safe to observe when lit. Vision may appear blurred but this is not harmful, it is simply due to fluorescence of the eye which can make focussing difficult.
Manufacturer: General Electric Company, U.S.A.
Lamp Power: 100 Watts
Lamp Current: 0.9 Amps
Lamp Voltage: 130 Volts
Cap Type: E29s/32 Brass + vitrite
Bulb Type: T-51 T-16 in eighths/inch
Bulb Finish: Blacklight Wood's Glass
Electrodes: Tungsten Coil Thorium strip emitter
Atmosphere: Hg | Ar Outer: Nitrogen
Luminous Flux: 5 lm
Luminous Efficacy: 0.05 lm/W
UV-A Flux: 1.25 Wrad 320-400 nm
UV-A Irradiance: 300 mW/m² @ 1m distance
Chromaticity Co-ordinates: CCx: 0.175 CCy: 0.030
Rated Lifetime: 6,000 hours to 50% survival
Warm-up & Re-strike Time: 3 minutes 3 minutes
Burning Position: Universal
Overall Length: 140 mm 51/2 inches
Light Centre Length: 87 mm 37/16 inches
Factory: Cuyahoga, NELA Park Cleveland OH, U.S.A.
Date of Manufacture: 1971 May Date Code: 89
Original Value: US $25.95 (1973)
References: 1) GE Lamp Catalogue, USA, 1972, p.47
2) Lamps for a Brighter America, Keating, P., McGraw-Hill Book Company Inc, 1st Edition (1954) p.205