1941 Philips 6131 (300 W / 130 V)

The introduction of 9.5 mm film by Pathé (France) in 1922 and of 16 mm film by Kodak (USA) the following year made film-making and -projection affordable to the masses, thus taking these activities outside the professional realm. Home motion picture became really popular following Kodak's release of the more economical 8 mm format in 1932. From this point forward photo and film equipment manufacturers developed and released a wide variety of cameras and film projectors to address the growing demand in the consumer market. Lamp manufacturers were involved in that process in order to design and develop the specialized light sources that were needed for the projection of narrow gauge films in economical projectors. Requirements for that application included a compact and bright source of light for an effective control of the emitted light, and a mains-voltage operation whenever possible to avoid the use of costly and bulky transformers. This resulted in a whole new category of incandescent light sources which emerged during the 1930s: the home cinema lamps.

Manufactured in 1941, the Dutch-made Philips 6131 shown here is representative of those early special lamps. The source of light consists of a compact 300 W mains-voltage radiator structure built of two coiled coil filaments of 65 V each. Those emitters are supported by an intricate wire frame structure designed to prevent arcing upon failure by ensuring that the maximum voltage difference between the filaments does not exceed 65 V. In order to maximize the source brightness the filaments are pushed to the edge of their operating limits, raising the light output to 7400 lm and resulting in an impressive lumen efficacy of nearly 25 lm/W while bringing the light color temperature above 3000 K. However, the high operating temperature caps the average lamp life to 25 hours only, the shortest practical source lifespan for the intended application. Philips made two variants of the lamp, one for low-voltage 100–130 V mains (featured here) and one for high-voltage 200–250 V mains. The latter type was built with a different filament having a curved monoplane configuration, which is less efficacious (23 lm/W) on account of the thinner tungsten wire used and the longer coiled filament, resulting in increased thermal losses.

The 6131 falls in the standard range category of home cinema lamps as its design made it suitable for a use in various projectors across brands. The lamp is built with a standard P28s prefocus end cap that allows a precise positioning of the light source in the projector's optics. In order to maximize screen brightness, a spherical mirror was usually placed behind the lamp, aligned with the projector optics and with the filaments positioned in the system's focal plane. This optical configuration results in such a high thermal load on the lamp that its bulb is made of a hard aluminosilicate glass that can withstand high operating temperatures up to about 600 °C.

The lamp is designed for a base down operation and relies on natural convection for its cooling. In order to keep the bulb area around the filament clear through life, the glass jacket is designed with an elongated top section where the evaporated tungsten condensates. The evaporated filament material is transported by the convection currents flowing in the argon-nitrogen gas fill of the bulb. The top part of the lamp is coated with black iron silicate paint, which reduces the amount of stray light towards the projector's top vents and lowers the bulb temperature at its extremity by increasing the infrared emissivity there. In the later design of the lamp that coated extremity was replaced by a conforming aluminum cap which played a different thermal management role. Instead of promoting radiative cooling of the bulb, aluminum spread out the heat flow thanks to the metal's high thermal conductivity.