Deuterium arc lamp

Used in photospectroscopic appliances as a source of UV-radiation, this device has a continuous spectrum from about 400 nanometres (which is still visible dark violet) down to 190 nm. At this wavelength, the air gets totally opaque, because this short radiation is absorbed by breaking the oxygen molecules to form ozone. The Deuterium isotope is preferred to "simple" Hydrogen (in older lamps) because of its more intense UV-radiation and even-levelled spectrum. For best transparency the bulb is made of Quartz or synthetic UV-transparent glass (or at least with such a window). These lamps are built as simple direct-heated diodes with an oxide-coated Tungsten filament located at one side of the bulb. Filament ratings are usually 3 to 12 Volts at 1 to 3 Amperes. Lamps must be preheated, hovever in more powerful types external heating can sometimes be switched off after ignition (at about 300 - 500 Volts) as the arc provides enough heat to sustain emission. Arc voltage usually is about 100 to 200 Volts at 100 to 500 mA, depending upon lamp power. To reach the anode, the arc must pass a narrow hole (which can be seen in the photo) which focuses it to a very bright spot - just like an optical lens - and this spot is the true light source. There are also (higher power) types with a vertical slot-like "lens". The anode (a simple small piece of tin) is usually closed in and shielded by this electric lens (which usually has no connection to anode or cathode). The rectangular "external" window defines the limits of the emitted UV-beam. These lamps provide a very concentrated, narrow-angled UV beam. In operation, the arc emits a blueish light, similar to Mercury rectifiers, since there are also a lot of emission lines in the visible range (the spectrum gets continuous from about 400 nm downward). Needless to say one should not look directly at one of these things! Also these lamps get very hot very quickly - the high volatility of the Hydrogen is a perfect carrier for the generated heat. Even medium-power lamps often require forced air cooling within the lamp housing. High quality lamps have a special coating on the outer bulb surface to inhibit the formation of excessive amounts of ozone, which gets fully activated after some hours of burn-in time.

Thanks to Uli from Saarbruecken, Southwest Germany for providing this information.