BTH-Mazda Incandescent Blacklight Military
The term 'blacklight' originated following the World War I invention of Wood's Glass, by Professor R.W. Wood of Carnegie University. It is a black glass consists of a barium-sodium-silicate matrix doped with 9% nickel oxide, the latter causing strong absorption of visible wavelengths while still transmitting both Infrared as well as UV-A wavelengths between about 300 and 450nm, peaking at 365nm.

Wood's glass was developed to allow secret communications by pulsed light signalling, with a UV receiver being used during the daytime, and an IR detector provided easier reception at night. Later it was applied for the production of UV-A transmitting lamps for fluorescence effects, which became especially popular during World War II. Blacklight lamps were developed primarily for aircraft cockpits and ship's bridges having control panels printed with fluorescent inks, which allowed ambient light levels to be reduced so as to preserve night vision. Experiments were also made with automotive headlights and fluorescent roadsigns for driving during air raid conditions, but never widely adopted.

This lamp is believed to be an experimental British model from the early part of WW2. Being based on a tungsten filament the UV output is extremely low, and by the early 1940s blacklight applications had already migrated to more powerful discharge lamps. It is marked with the crow's foot broad arrow indicating property of the UK War Office, but is not numbered or listed in the UK Defence Standards for electric lamps. Lamps for aircraft use would normally be rated 28V, but many British wartime aircraft operated on 12V systems, so it is not known if this lamp was intended for aircraft, marine or road use - or for signalling applications.
Manufacturer: The British Thomson-Houston Co. Ltd.
Lamp Power: 60 Watts
Lamp Voltage: 12 Volts
Lamp Current: 5 Amperes
Cap Type: B15d/17 Brass & Vitrite
Bulb Type: P-50 P-16 in eighths/inch
Bulb Finish: Blacklight Wood's Glass
Filament Type: C-2V Single Coil Vee
Atmosphere: Argon-Nitrogen Red P getter
Luminous Flux: ~2 lumens
Luminous Efficacy: ~0.03 Lumens per Watt
UV-A Flux: ~0.2 Wrad 320-400nm
UV-A Irradiance: ~8 mW/m² @ 1 metre
Wavelength: Peak: 385nm Dominant: 420nm
Chromaticity Co-ordinates: CCx: 0.200 CCy: 0.185
Lifetime: 100h @ 13.5V?
Burning Position: Universal
Overall Length: 63 mm 21/2 inches
Light Centre Length: 28.5 mm 11/8 inches
Factory: Rugby England
Date of Manufacture: 1943 September Date Code: 21
Original Value: Unknown
References: 1) Photoelectric measurement
2) UK Ministry of Defence Standard for H.M. Government Miniature Lamps, DefStan 62-10 (1994) - Data for equivalent clear lamp #6240-99-995-2507
3) UK Ministry of Defence List of Electric Lamps for Government Departments (1953) - Data for equivalent clear lamp #6240-99-995-2507
4) Black Light Aids Night Fighters, Popular Science, November 1942, pp.62-63
5) Ultra Violet and White Lighting for Aircraft Instruments, Chittenden P., Aircraft Engineering and Aerospace Technology V.30 No.1 1958 p9–10