1969 Toshiba TA4487C

Incandescent light sources are particularly useful for the calibration of photometric- and spectral-measuring instruments because their optical output (flux, luminance, spectrum) can be stable within less than 0.1 % when operated under well controlled conditions and for short periods of time (i.e., up to several tens of hours). Special filament lamps are thus designed to serve as references in photometric parameters such as the luminous flux, luminous intensity, color temperature, monochromatic luminance temperature, spectral distribution and total source optical output. Their structure depends primarily on their function and these special lamps can have very intricate constructions whose purpose is to ensure the most reliable, stable and well-defined optical output possible.

Flux standards of the type shown here have the simplest design of all reference lamps and are intended for the calibration of Ullbricht spheres. Its construction is very similar to that of a standard GLS lamp, except for its coiled filament which is welded to the lead-in and support wires so as to provide maximum mechanical stability and to ensure a perfect control the radiator's heat conduction losses and contact resistances. The filament is usually made of a tungsten wire of high homogeneity, designed for the most stable electrical properties over time. All lamps of this kind are usually aged for around a hundred hours before actual use in order to reach an optimal material, electrical and optical stability as required for the application.

The TA4487C presented here was made with utmost care by Toshiba of Japan. A very interesting feature of this particular model is its coiled radiator which is segmented so the welds are all on straight sections of the filament. The lamp is designed to operate on the standard Japanese mains voltage of 100 V, which meets the requirement of a filament voltage below 120 V so as to ensure a short radiator length needed to guarantee an excellent stability in optical output. Typical of reference lamps, this TA4487C was delivered with a certificate providing the source's key characteristics. Note that the date indicated in this document (25.VII.44) is relative to the Japanese traditional calendar and corresponds to the 44th year of the Shōwa era, i.e., 1969. Evidence for that is the presence of Toshiba's "humbrella" logo, which was introduced in 1950.