Influence of vignetting errors on the relative spectral responsivity of trap detectors

Abstract

Originally developed for application with laser beams, trap detectors are widely used as transfer standards in modern monochromator radiometry. However, a non-optimized beam focus applied to trap detectors can result in significant spectral errors. This paper shows that a defocused and improperly aligned beam entering a Si reflection trap detector can cause an apparent modification of the measured relative spectral responsivity. This is true especially in the ultraviolet (UV) region of the spectrum, where the measured relative differences are of the order of several percent due to the significant influence of the direct band-band transitions of Si on the reflectance of the Si photodiode. The spectral range from 248 nm to 600 nm was investigated in detail. Calculated curves, simulated by a computerized geometrical model, were fitted to the measured data. The geometrical model uses experimental spectral reflectance data.