1969 Thorn MB/U 400W

The introduction of medium-wattage high-pressure quartz mercury lamps in the early 1950s led to the demise of the original atmospheric-pressure hard-glass variant that was first introduced in 1932. The transition to fused silica enabled a shorter arc and an operation at higher temperature and pressure, which results in much improved performances. This technology also benefited from the introduction of the magnesium fluorogermanate phosphor in 1950, which provided the first really effective mean to correct the red-deficient color of mercury arcs. However, because of a large remaining installed base of luminaires designed for the first generation of mercury lamps with glass arc tube, a separate type of upgraded mercury lamps were designed specifically to be geometrically and optically compatible with their predecessor.

The Thorn MB/U 400W shown here falls in that category and features a wide and elongated 330 x 51 mm tubular jacket that can be supported at its extremity, and a quartz burner whose center is located at 205 mm from the end cap extremity, dimensions that match those of the original 400 W hard-glass mercury lamp. The arc operation at 4.7 bar raises the light output by nearly 18 % to 18,800 lm (initial, later raised to 20 klm), but also results in a shorter and narrower light source which can affect the beam pattern projected from optical systems designed for the broader atmospheric mercury arc. Thorn addressed this particular issue by sand blasting the outer surface of the quartz burner so as to increase the apparent size of the luminous arc by way of light scattering. Another interesting feature of Thorn’s medium-wattage MB/U is the use of small electrodes made of braided tungsten wires, a structure which retains the electron-emissive material necessary for an efficient electrode operation. Such design limits the rate of burner blackening over time and thus ensures a good lumen maintenance through life.

Clear tubular mercury lamps were eventually superseded in most of their applications during the 1970s as the more efficient high-pressure metal halide and sodium lamps took its place in floodlighting applications. At the same time, the cheaper color-corrected mercury lamp became more popular in general illuminations, causing the MB/U to finally fade into obscurity in the early 1980s in those applications. However, these clear tubular lamps remained used in specialized applications such as in the graphic arts and in general photochemical processes, including the ageing tests of materials. This area was finally taken over during the 1990s by specialized metal halide sources.