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

摘要

Calibrations of tungsten strip lamps are basically in terms of the temperature al a small central part of the filament. We calculate this temperature T_m, which is dependent on the lamp current I, the temperature T_e 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 T_e results in a slightly different T_m. From the results the change of T_m relative to T_e, dT_m/dT_e is computed. Further results by varying L show that dT_m/dT_e. at a fixed T_m always decreases with increasing L. For 62mm < = L < = 104mm and T_e = 400 K, dT_m/dT_e at T_m = 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 104mm. Since T_e is linked to the temperature T_b, of the lamp base, we expect dT_m/dT_b to decrease similarly with increasing L. We measure dTt_(lambda)/dT_b,, where lambda is the spectral radiance temperature at a wavelength A. of 650 nm and from which dT_m/dT_b 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 T_b, close to room temperature and 900 K < = T_m < = 1600K. Results show that dTt_m/dT_b of the longer filament is less than that of the shorter filament. It is beneficial to reduce dTt_m/dT_b by reducing dT_m/dT_e so that T_m, 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.