||This curved mercury capillary lamp was developed by the Advanced Radiation Corporation, manufacturer of special arc lamps in the USA. Fundamentally it operates on similar principles to other mercury capillary lamps and owing to high pressure operation, at around 100 atmospheres, it has a broad emission through the visible and UV spectrum.
Curved mercury capillary lamps find an application in Projection Mask Alignment equipment, commonly employed in projection photo lithography where their high concentration of rich UV radiation is especially valuable. In this particular lamp the sides of the quartz arc tube only are externally coated with a dichroic mirror which is transparent to the infra-red and visible wavelengths, but a good reflector of UV radiation. This serves to increase the UV energy from the top of the bend by approximately 30%. The reflection of some blue wavelengths accounts for the yellowish tint noticeable when the lamp is observed from the sides.
Mercury capillary lamps traditionally require a high degree of forced water cooling to prevent their immediate destruction, but this would be rather inconvenient to arrange for a curved lamp of this design. This lamp is made suitable for air-blast cooling by increasing the arc length slightly to reduce the loading on the quartz wall.
Typical lamp life is around 100 hours but may be longer if used continuously. It is recommended that a lamp older than 100 hours is not re-ignited because the epoxy in the hex cap is likely to have failed as it cooled. Thus the lamp will move out of alignment and may overheat to the point of destruction.