Approximation function of spectral irradiance of standard lamps

Band filter radiometry has recently become popular. Its application for detector-based spectral-irradiance measurements requires improved characterization of standard lamps. This paper presents an approximation function for the spectral irradiance of NPL/GEC and FEL lamp data, reported in the 1990 international comparison measurements of the Consultative Committee for Photometry and Radiometry (CCPR). Characterization with a minimum number of parameters, applicable within the visible spectral range, is attempted. Since the spectral emissivity of tungsten is nearly linear in the spectral range from 400 nm to 900 nm, the irradiance of the standard lamps was approximated with three parameters, representing a linear polynomial multiplied by the Planck function. The same linear polynomial was used for each lamp of the same type, and was fitted only for temperature and intensity. The average difference between the measured and the approximated data was found to be less than 0.085% for the NPL/GEC lamps and less than 0.095% for the FEL lamps. This means that the relative spectral emissivity of each lamp of the same type is practically the same. The temperature fitting was characterized by the average difference between the measured and approximated data in the ultraviolet and infrared spectral ranges.