Westinghouse SAH-250B Short Arc DC Mercury
Owing to the very high luminance of the high pressure mercury arc, these light sources soon became very popular for projection applications where colour rendering was not a great requirement. The intrinstic brilliancy and stability of the arc could further be enhanced by DC operation and Westinghouse manufactured this popular SAH-250B for that purpose.

Its primary application was in American comparators manufactured by Jones & Lamson, being suitable for the models FC14, TC10, Epic 30 and Epic 220. Projection microscopy and ordinary image projection were catered for by the SAH-250A which was an AC lamp that was entirely suitable for these less demanding applications.

Constructionally both lamps are virtually identical - the only difference lies in the electrodes. The AC lamp employs electrodes substantially the same as for ordinary high pressure mercury lamps, whereas on the DC model shown here, one electrode has been fused to form a bead at its tip. This allowed it to handle the greater heat that is always present on the anode electrode. A platinum coating behind this electrode serves to conserve heat. Note also the great thickness of the quartz arc tube in this lamp. Its operating pressure is many times higher than ordinary mercury lamps, this serving to improve the brilliancy of the arc and also to improve the spectral output of the lamp by line broadening.

The arc tube is sealed into a T12 hard glass outer bulb and is capped with a standard medium pre-focus base, so that no optical alignment is required when replacing lamps. An ST101 getter serves to remove hydrogen from the outer.
Manufacturer: Westinghouse Electric Company
Lamp Power: 250 Watts
Lamp Current: 5.95 Amps DC only
Lamp Voltage: 42 Volts DC only
Cap: P28s/25 Brass with vitrite insulator
Bulb Finish: Clear
Bulb Type: T-38 T-12 in eighths / inch
Overall Length: 152 mm 6.0 inches
Light Centre Length: 71 mm 2.8 inches
Electrodes: Backwound thoriated tungsten, anode beaded over
Atmosphere: Inner : Argon Outer : Nitrogen
Luminous Flux: 10,000 lm @ 100 hours
Luminous Efficacy: 40 lm/W @ 100 hours
Colour Temperature: Not Published
Chromaticity Co-ordinates: Not Published
Rated Life: Not Published
Factory: Bath, New Hampshire, U.S.A.
Date of Manufacture: Circa 1975
Original / Present Value: Unknown
 
References: 1) Philips USA Lamp Catalogue, 2002.