For absorption spectrophotometry extending into the far ultraviolet, a diffraction grating presents advantages over a quartz prism because of its uniform dispersion. A wide range photoelectric spectrophotometer, which employs a plane diffraction grating (5700 lines/cm) as dispersing element, a single concave mirror for collimating and focusing, and narrow fixed slits (70μ) is described. A commercial hydrogen lamp and a small tungsten filament lamp serve as light sources for the ultraviolet and visible regions. The sensitive detector consists of a vacuum photocell followed by an electrometer valve and galvanometer in a bridge circuit. By carefully designed baffles, stray light is reduced to an amount which, at wavelengths above 2100A, is negligible compared with the monochromatic output. The residual stray light, entirely caused by scatter from the grating and mirror surfaces, can be determined by adjusting the grating angle to a nominal wavelength setting below 1800A. It is shown that on this basis a reliable correction can be applied to the absorption measurements below 2100A which extends the useful range of the instrument down to at least 1900A. The present long wave limit of the instrument, determined by the response of the photocell, is 6000A. Tests with a line source have shown that the wavelength calibration, based on the grating formula, is correct to within 5A, and that the spectral band width is about 8A throughout the whole range. The use of the instrument for absorption measurements of alkali halide crystals, of which an example is given, has demonstrated its accuracy and reliability.
© 1951 Optical Society of AmericaFull Article | PDF Article