GE-Mazda Miniature Oval-Section Blue Fluorescent
Soon after GE's launch of the first fluorescent lamps in 1938, the range was extended to include 'miniature' T5 16mm diameter types. First was the 9" 6W lamp in 1940, joined by the 12" 8W and 6" 4W the following year. The performance of small discharge lamps is limited by so-called 'end losses', an electrical phenomenon in which the volt drop (and hence power dissipation) in front of the electrodes is much greater than along the positive column discharge. The magnitude of this loss is roughly constant irrespective of lamp length, and therefore proportionally more severe for short lamps, which limits their performance.

In an effort to overcome the problem, this lamp was made with flattened glass. This caused the volt drop to increase from 32V to 38V - thereby reducing the relative magnitude of the end losses. It was later discovered that the main reason for the improvement was the reduced ambipolar diffusion length of mercury ions to the wall, which is countered by increased electron mobility and electron temperature. That leads to reduced electron density, which it is now known is required for a high lamp efficacy.

This lamp is made with calcium tungstate phosphor, whose blue emission extends partly into the UV-A region. Its purpose was to excite fluorescent dials of aircraft cockpits, and in this application by orienting the major diameter of the oval section towards the dials, it provided 2.5 times more flux than a circular tube. However the oval glass was weak and prone to implode. It was replaced by a standard round-section tube as soon as the '360BL' phosphor had been developed, whose increased UV-A efficacy matched the output of this oval lamp in standard round tubing.
Manufacturer: General Electric Co. of U.S.A.
Lamp Power: 4 Watts
Lamp Current: 0.12 Amps
Lamp Voltage: 38 Volts
Cap Type: S14s Brass
Bulb Type: T-8x19 T-2½x6 in eighths/inch
Bulb Finish: Blue CaWO4
Electrodes: Coiled-Coil (Ba,Sr)CO3 emitter
Discharge Length:
Atmosphere: Hg | Ar
Luminous Flux: ~40 lm (@ 100h)
Luminous Efficacy: ~10 lm/W (@ 100h)
Chromaticity Co-ordinates: CCx: 0.187 CCy: 0.196
Dominant Wavelength & Saturation: 477 nm 63%
Rated Lifetime: 500 hours
Warm-up & Re-strike Time: 2 minutes Instant
Burning Position: Universal
Nominal Length: 150 mm 6 inches
Factory: NELA Park, Cleveland OH U.S.A.
Date of Manufacture: 1941
Original Value: Unknown
 
References: 1) Fluorescent and Other Gaseous Discharge Lamps, W.E.Forsythe & E.Q.Adams, publ. Murray Hill 1948, pp. 96, 119-120, 186, 191-192, 197
2) Fluorescent Lamps, C.L.Amick, publ. McGraw-Hill 1942, p. 18
3) The Electric Lamp Industry, A.A.Bright, publ. Macmillan 1949, pp. 412, 501
4) GE Large Lamp Catalogue, USA, 1948, p.21
5) US Patent 2,482,421, Flat Tube Electrical Device, A.E.Lemmers, Filed 21-Oct-1943