This is undoubtedly an episcopic lamp for Fresnel projectors used in TV/movie/photographic studios, as also in theaters and almost in any kind of scenic uses. It has an internal evaporated reflecting layer in the shape of a spherical cap that directs the light emmited by the grid-shaped filament to the front of the globe towards the stepped lens. The lamp itself when fixed to the lampholder, can be moved forward and backward respect to the Fresnel lens to make diverge, collimate or converge the output beam.
Most common power ranges of such lamps are 500, 1000, and 2000 w.
When I was a photography student I often used these Fresnels to get some special effects (side lighting, backlight, concentrated or spot lighting) in portrait, mainly when taking pictures in B/W.
Today, Fresnel projectors are fitted with halogen lamps, which as a drawback, needs of an external mirror to concentrate the light against the lens. Also, some Fresnels uses HID lamps too, especially when a high colour temperature (?K) value is needed and a high efficiency is desirable.
Regards
