. Based o~ simple principles, s l~ectrop hotoI?ctry nevertheless demands a lot of precautIOns to aVOi d errors. The followmg properties of spectrophotometers will be discussed together with methods to test them:~pectra~ properties-wavelength accuracy, bandwidth, stray light; photometri c linearity; mteractlOns between sample a~d instrument-multiple reflections, polarization, divergence, sample wedge, sample tIlt, optical path length (refr active index), interferences .Calibratio~ of master instruments is feasiJ:>le only by complicated procedures. With such a master mstrument standards may be calIbrated which greatly simplify performance checks of mst:u:nen~s used for practical work. For testing high quality spcctrophotometcrs the u~e of emiSSIOn lmes and nearly neutral absorbing solid filters as standards seems to be supenor, for some kinds of routine instruments the use of absorption bands and liquid filters may be necessary.

It is shown that the linearity of the photometric scale of spectrophotometers is not self-evident. The widely used Lambert-Beer test does not give a conclusive proof of linearity. Light addition methods do not show this disadvantage. In the form of the supplementary light method, as used here, they are suited for testing commercial instruments. The arrangement for testing the Zeiss DMR 21 recording spectrophotometer is shown, and the results do not reveal a nonlinearity exceeding 0.1% T. The accuracy of the method is limited only by the precision of the instrument under test.

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A ray-tracing program designed for grating couplers has been used for an extensive analysis of production tolerances for a focusing grating coupler. In addition the program permits the calculation of coupler behavior in a wave-optical approach including the effects of transverse and axial intensity variations across the grating. The effects of the lateral and longitudinal source point position, wavelength, and waveguide effective index have been studied. The results indicate clearly that in many applications the laser wavelength is the most critical parameter.