ЛУФ-4 (LUF-4) DC Blacklight Glow
The ЛУФ-4 (LUF-4) is intended to provide a small source of UV-A energy for the illumination of aircraft cockpits, whose instruments & panels were often labelled with fluorescent paints. Its purpose is to reduce light levels within the cockpit so as to maintain improved night vision of the pilots, while also making military aircraft less visible to the enemy. These lamps produce a small amount of visible light in addition to the UV energy at 365nm, and to remove that it was customary to use them behind a black Wood's glass filter.

This Soviet development appears to be an improvement of the American F5000, the first lamp that had applied a UV-A phosphor to convert some of the useless UV-C mercury radiation to more valuable blacklight wavelenghts. The F5000 was for many years the standard aircraft cockpit lamp employed all around the world, and is believed to have been equivalent to the Russian УФО-4 (UFO-4). The original lamps were rather complex to operate because the filament required manual heating before ignition, and sometimes this was accidentally left running and caused premature failure. The ЛУФ-4 solves this by connecting a bi-metal strip between one end of the filament and the anode ring, to automatically break the filament circuit and strike the discharge as soon as the filament has been preheated.

Cap connections are same as the F5000, but one of the end contacts is not connected since the bi-metal switch takes over its function. It is intended for use on 24-28V DC circuits across a fixed 30Ω and variable 95Ω resistor, the latter being adjusted with each new lamp change to achieve the rated current of 350mA because there was quite some tolerance in resistance of the cathodes between individual lamps.
Manufacturer: СПО «Светотехника» SPO "Svetotekhnika"
Lamp Power: 4 Watts  
Lamp Voltage: 11 Volts  
Lamp Current: 0.35 Ampere  
Cap Type: BAY15d Brass + Vitrite
Bulb Type: RP-37 RP-12 in eighths/inch
Bulb Finish: 360BL phospor Soda-lime glass
Electrodes: Tungsten Coil cathode Metal ring anode
Electrode Gap: 10mm 0.394 inches
Atmosphere: Hg | NeAr
Luminous Flux: 10 lumens
Luminous Efficacy: 2.5 lm/W
UV-A Irradiance: 20 mW/m² @ 1m from bulb crown
Colour Temperature & CRI: CCT: 15,000K N/A CRI: Ra10
Chromaticity Co-ordinates: CCx: 0.245 CCy: 0.290
Rated Lifetime: 300 hours
Warm-up & Re-strike Time: 1 minute Approx 15 seconds
Burning Position: Universal
Overall Length: 63 mm 21/2 inches
Factory: Saransk Russia
Date of Manufacture: 1989 Q2 Date Code: II-89
Original Value: Unknown
References: 1) Instruction and wiring sheet supplied with lamp
2) Similar lamp at the Museum of Old Lighting, Russia
3) Application photograph in a Soviet Ilyushin Il-18V aircraft cockpit, at Airliners.net website
4) Application photograph in a Soviet Ilyushin Il-18V aircraft cockpit,showing two ЛУФ-4 spotlamps overhead centre, at Russia Beyond website
5) Black Light Aids Night Fighters, Popular Science, November 1942, pp.62-63
6) Photoelectric and Spectroradiometric Measurements @ 350mA.