Classic American 100W BT25 mercury lamp in operation

Shown here is a true American classic, a 1979 Sylvania H38JA-100/DX mercury lamp with a BT25 bulb having a so-called "clear band" phosphor coating pattern. This characteristic feature can be seen on the bulb side opposite to the end cap. The phosphor coating was removed where the frame springs touch the bulb so as to prevent the release of the fluorescent material and its deposition on the burner, where it would cause blackening due to thermal decomposition. Improvement in phosphor coating in the early 1980s eventually led to the disappearance of such feature. On the subject of the fluorescent coating, it is also interesting to note that for this lamp Sylvania employed a relatively low fraction of diffusing material in the fluorescent powder, resulting in a rather thin coating which makes the bright mercury arc easily visible. Such feature aimed at increasing the source luminance so as to improve the optical control of the emitted light.