Double beam (dual wavelength) spectrophotometers generally employ alternating, rather than simultaneous, illumination of a single sample with wavelengths &lgr;1and &lgr;2to secure large aperture viewing without optical cross‐talk. Although this technique allows the use of phase sensitive detection, which greatly reduces the effect of noise entering the system between light switch and demodulator, it also impairs the system's immunity to lamp noise, as compared to a true differential system, in which &lgr;1and &lgr;2simultaneously illuminate the sample. To improve the noise rejection, we have devised a method of compensation which uses a second photomultiplier to monitor the time shared light beam before it enters the sample. The resulting signal is normalized and subtracted from the measure signal in a differential amplifier over a bandwidth including frequencies considerably above the switching frequencyfc. Details of design are presented together with experimental data showing that greatly improved performance can be obtained with 450 W xenon and AH‐6 type 1000 W mercury arc lamps. In the latter case a 20‐fold decrease of low frequency noise is obtained at 280 nm.