1978 Philips CSX 75W/2

The 75 W CSX is the smallest xenon short-arc lamp ever produced by Philips in the Netherlands. Such small low-wattage light source is used in various optical and technical applications, such as microscopy and absorption spectroscopy, due to its broad near-continuous spectrum that ranges from the UV to the IR domains, and its small ultra-bright arc which allows an effective optical control of the emitted light. Because it is designed for an operation on DC current, the lamp shown here is built with a pair of asymmetric electrodes separated by a 0.6-mm gap where an arc no wider than half a millimeter at half length is formed. The resulting source luminance is 40 kcd/cm² in average, which is several orders of magnitude higher than that of any incandescent light sources, including of the arc type. It is for this reason, and because of the wider and better balanced emission spectrum, that such small xenon arc lamps replaced special incandescent lamps like the Pointolite in their applications during the 1970s.

The suffix in the CSX 75W/2's reference indicate that this is a lamp intended for systems provided with a series-connected high-voltage ignitor. There existed an earlier /1 type built with a side auxiliary electrode for ignition using a high-voltage source (e.g. a Tesla coil) external to the lamp's circuit. Improvement in circuit design and the issue of shadowing from the side electrode led to the quick demise of that type, with the lamp configuration shown here becoming de facto a standard in small scientific and technical optical devices employing this kind of light source. The DC current operation of the CSX 75W/2 results in a much higher power loss at the positive electrode due to the collection of energetic electrons from the plasma. Consequently this is the larger of the two electrodes and in order to limit its temperature some of its side surface has been roughened by way of sand blasting in order to enhance radiative cooling. The considerably smaller cathode has a heat choke to ensure an optimum tip temperature for a long service life and an effective emission of electrons (a thoriated tungsten material is used). The shape of those electrodes was optimized to limit shadowing and to ensure that most of the light emitted by the tiny arc is available to light-collection optics.