Abstract
Based on simple principles, spectrophotometry nevertheless demands a lot of precautions to avoid errors. The following properties of spectrophotometers will be discussed together with methods to test them: Spectral properties-wavelength accuracy, bandwidth, stray light; photometric linearity; interactions between sample and instrument-multiple reflections, polarization, divergence, sample wedge, sample tilt, optical path length (refractive index), interferences. Calibration of master instruments is feasible only by complicated procedures. With such a master instrument standards may be calibrated which greatly simplify performance checks of instruments used for practical work. For testing high quality spectrophotometers the use of emission lines and nearly neutral absorbing solid filters as standards seems to be superior, for some kinds of routine instruments the use of absorption bands and liquid filters may be necessary.