Spectrophotometers are used to measure the intensity of electromagnetic radiation. Usually the measurements are confined by filters to a very narrow spectral range and the instrument is used to detect the change in brightness after the light radiation has either passed through a sample or been reflected off it. Early devices used the naked eye to determine the differences in intensity between two beams.
Arthur Hardy (1895-1977), a physicist at the Massachusetts Institute of Technology, decided to replace the eye with the new cesium photocells, and thus detect intensities electronically. The plan was to produce a Spectrophotometer that automatically scanned through the visible spectrum and produced a pen-drawn spectrum showing how the light intensity varied with wavelength. Beam splitters and rotating polarizers were used and the two beams were compared by blinking quickly from one to another using a flicker photometer technique. Working in collaboration with the firm General Electric, the first operational production machine was ready by 1935. Soon, the National Bureau of Standards was using Hardy's spectrophotometers to test pigments and dyes and to set paint color standards. The fully automated machine was extremely expensive, and soon cheaper versions were being produced that required manual fixed-point scanning of the spectrum.
Spectrophotometers found their way into industrial and scientific laboratories. Similar instruments have been used widely to monitor the seasonal and latitudinal variation of ozone, and a spectrophotometer led to the discovery of the ozone hole over Antarctica.
