Decreasing dependence of the calibration of the tungsten strip lamp on the temperature of the lamp base with increasing filament length

Abstract

Calibrations of tungsten strip lamps are basically in terms of the temperature at a small central part of the filament. We calculate this temperature Tm, which is dependent on the lamp current I, the temperature Te at the ends of the filament and the dimensions of the filament including the length L, by solving a heat balance equation. For a given filament with specific L and I, a slightly different Te results in a slightly different Tm. From the results the change of Tm relative to Te, dTm/dTe is computed. Further results by varying L show that dTm/dTe at a fixed Tm always decreases with increasing L. For 62 mm ⩽ L ⩽ 104 mm and Te = 400 K, dTm/dTe at Tm = 1337 K decreases by a factor of approximately 2.5 for every 12 mm increase in L. Due to the diminishing amount of decrease there is little gain by increasing L beyond 104 mm. Since Te is linked to the temperature Tb of the lamp base, we expect dTm/dTb to decrease similarly with increasing L. We measure dTλ/dTb, where Tλ is the spectral radiance temperature at a wavelength λ of 650 nm and from which dTm/dTb is deduced, of three tungsten strip lamps of the high-stability type for a filament of 62 mm length and two similar lamps of 74 mm filament length at a Tb close to room temperature and 900 K ⩽ Tm ⩽ 1600 K. Results show that dTm/dTb of the longer filament is less than that of the shorter filament. It is beneficial to reduce dTm/dTb by reducing dTm/dTe so that Tm, and therefore the lamp calibration, is less dependent on the temperature of the lamp base. Our theoretical and experimental results have indicated that the reduction is possible by increasing the filament length. This finding may be applicable to the improvement of the performance of the tungsten strip lamps to match the ever-improving performance of the photoelectric radiation thermometers.