SPECTRAL NONLINEARITY CHARACTERISTICS OF LOW-NOISE SILICON DETECTORS AND THEIR APPLICATION TO ACCURATE MEASUREMENTS OF RADIANT-FLUX RATIOS

Accurate measurements of high radiant flux ratios require detailed knowledge about detector nonlinearity. The double aperture method employed yields a quantity NL, that is twice the linearity error of the detection system when a flux ratio of two is measured. An arbitrary flux ratio can be corrected for nonlinearity by a straightforward analytical procedure if NL is a unique, smooth and reproducibly measureable function of photocurrent and wavelength. NL also must be known up to the highest photocurrent involved in the flux ratio measurement. Spectral nonlinearity characteristics of low noise silicon detectors have been measured at photocurrents between 3 × 10-10 A and 10-7 A and wavelengths between 600 nm and 1100 nm. The analytical correction procedure has been verified by transmission measurements involving a flux ratio of about 200 with a residual linearity error below 1.6 × 10-4 at constant preamplifier sensitivity. The dynamic range covered reached about 2000. Therefore flux ratios of about 2000 can be measured with such accuracy by one step if the preamplifier sensitivity is changed by an accurately known factor above ten. Consequently in radiation thermometry with silicon detectors the temperature error due to detector nonlinearity can be kept below 10 mK between gold point and zinc point.