A spectrophotometer for the direct determination of sample extinction by simultaneous measurement at three wavelengths and application of Allen's correction has been developed. White light is passed, first through a blue heat filter, and then through the sample and finally through a trichromator which selects the required three wavelengths. These wavelengths illuminate the base region of three FPT 100 type photo diodes which operate as linear transducers converting the incident light intensity to a current which is converted to a voltage in head amplifiers. A computation sub-unit together with a logarithmic amplifier generate the function required for the Allen correction, and the corrected extinction value is displayed digitally on neon display tubes. The spectrophotometer was designed for application to the measurement of estrogens by the Kober color reaction which requires that the three wavelength settings be 480 nm, 520 nm and 560 nm. The results obtained with pure standards and extracts of pregnancy urine were directly comparable with those obtained more laboriously using a conventional spectrophotometer and reading the results manually at the three different wavelengths (r = 0.998 n = 128). The principles are applicable to a more general spectrophotometer or densitometer where extinctions are required at a number of wavelengths enabling several components to be measured simultaneously or correction for impurities to be made.