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

Originally developed for application with laser beams, trap detectors are w idely used as transfer standards in modern monochromator radiometry. Howeve r, a non-optimized beam focus applied to trap detectors can result in signi ficant spectral errors. This paper shows that a defocused and improperly al igned beam entering a Si reflection trap detector can cause an apparent mod ification of the measured relative spectral responsivity. This is true espe cially in the ultraviolet (UV) region of the spectrum, where the measured r elative differences are of the order of several percent due to the signific ant 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 investig ated in detail. Calculated curves, simulated by a computerized geometrical model, were fitted to the measured data. The geometrical model uses experim ental spectral reflectance data.