The inconvenience of the photographic technique led Dobson to design and build his photoelectric spectrophotometer which enabled the relative intensity at two wavelengths to be measured directly. It was completed in 1927 or 1928, the design being remarkably advanced for its day. It is probably the first spectrophotometer to use the principle of synchronous detection of a weak signal. It used a photocell with a sodium cathode made by T. C. Keeley, who had also made the photocells used in the microphotometer which Dobson built for measuring the photographic plates. This instrument, like the Fabry spectrographs, was built by Dobson himself in his own workshop.