The effect of spectral background due to impurities, solvent or radiation @S05487@ may be reduced if the difference in the absorbances of a sample measured at two selected wavelengths is obtained. This is often achieved by repetitively switching from one @W06659@ to the other. Double-@W06659@ @S05848@ does this automatically by allowing two beams of radiation of different wavelengths to pass through the cell. One beam is fixed at a longer @W06659@ and the other measures @A00028@ while being scanned over a limited @W06659@ range at shorter wavelengths.