Osram HBO 50/3

Small super high pressure lamps were especially popular as point sources for medical and scientific applications. Some of the applications were microscope lighting and endoscope lighting. They were also popular for other small projection applications and applications were a UV source is needed.

The lamp consist of thick walled quartz discharge tube, with connection on each side. It is filled with a mixture of mercury and a starting gas. The two electrode differ in size as this lamp is designed to operate on DC. The anode (+) is bigger in order to absorb the energy of the electrons which bombard it. The Cathode (-) is small and particularly pointy in order to have best emissive characteristics. To facilitate first ionization at ignition the electrodes are made of thoriated tungsten. The gap between the two electrodes is less than a mm. This short discharge lengths and the high current density in the arc due to its narrowness give this lamp one of the biggest arc power per unit of volume of all discharge lamps, just surpassed by reflector versions of Osram HBO lamps.
As mercury is liquid at room temperature and the noble gas filling is at rather low pressure these lamps don't need extremely high voltages when a cold lamp is lighted. Pulses of several kV are enough in difference to the several 10kV found with Xenon short arc lamps. However when a hot restrike is desired for this lamp pulses of several 10kV are also needed.