Wavelength related interference artifacts may appear superimposed on the output data curve of a scanning spectrophotometer utilizing a photodiode array as photometric detector. These may be suppressed by interposing a transparent optical element in the optical beam adjacent to its incidence on the array. Such an element should have no optical power along an axis parallel to the wavelength axis of the array and should provide varying phase displacement of the rays of the beam transverse to the wavelength axis. Several forms such an optical element may take are disclosed.