1970 Philips 6002E (100 W / 16-17 A)

The Philips 6002E is a reference lamp with a small transversal tungsten ribbon filament suitable for the calibration of measuring instruments in terms of surface luminance, color temperature, and spectral power distribution. In order to limit the effects of parasitic light reflections on spectral or photometric measurements, the incandescent filament is offset and placed close to one side of the tubular glass bulb. The latter is made of a hard borosilicate glass so as to withstand the high local temperature resulting from the proximity with the thermal radiator. Moreover, this particular glass material provides the additional benefit of having a higher optical transmittance in the short-wave end of the spectrum than more conventional soft glasses. To ensure a good maintenance of the lamp’s optical characteristics, it is filled with a gaseous atmosphere which limits the evaporation rate of the filament, and is specified for a cap down operation. The latter ensures that convection currents carry the evaporated tungsten to the top of the bulb, where it condenses. This way, the section of bulb wall next to the incandescent filament does not darken rapidly.

The lamp’s function as a luminance reference requires also a homogeneous filament temperature, which is often difficult to achieve in practical light sources, especially in the presence of a gaseous atmosphere. The filament placement relative to the gas convection flow is critical to the proper management of the latter’s effect on the radiator’s heat losses. The filament position in the 6002E and its mode of operation result in the ideal condition to keep the effects of convective heat losses on the filament to a minimum. Moreover, the configuration of the filament, which is a band rather than a wire, allows a very effective temperature distribution by way of thermal conduction in the bulk of the tungsten material. Finally, the effect of end losses resulting from heat conduction in the thick lead wires are limited by the side filament sections which act as thermal buffers.

Introduced in 1970, the model 6002E is a modern variant of Philips’s older W2 source. The lamp needs a stabilized DC current source for its operation and requires a primary standard for its calibration, which was usually done at Philips after proper ageing and stabilization. This source proved highly useful and popular in its intended applications and the Dutch produced it for two and half decades until the mid-1990s, when Philips sold part of its special lighting manufacturing operation to other, more specialized lamp manufacturers like Dr.Fischer or Cathodeon.