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Chapter 10:
Grating Spectrometers
Grating spectrometers make use of the diffraction of light from a regularly spaced, ruled surface. They disperse the light by a combination of diffraction and interference rather than the refractive index variation with wavelength. Joseph Fraunhofer was probably the first to rule and use diffraction gratings (in 1823). Henry Rowland later built extremely precise ruling engines that could make relatively large—about 10-inch—gratings of high resolving power. Rowland also invented the concave grating. Albert Michelson, America's first Nobel laureate, developed interferometric techniques for ruling gratings that were used and improved by John Strong and George Harrison. It was R. W. Wood who introduced the blaze, and Strong who ruled on aluminum films that had been coated on glass (for greater stability and precision than speculum). Although Michelson indicated in 1927 the possibility of generating, not ruling, gratings interferometrically, it was not until the advent of lasers that holographic gratings were developed. There are three types of gratings: ruled gratings, holographic gratings, and replica gratings made from molds of gratings that have been ruled. The latter two are used in most commercial instruments today because they are relatively cheap and reproducible, and reflective gratings do not depend on the transparency of a medium, as does a prism.
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