immersion grating

An immersion grating is a type of grating in which the light enters transparent material through a flat surface, then from the inside the light reflects off an edge of the material that forms a stair-step grating (making it an echelle grating), then passes back through the transparent material and out through a flat surface, perhaps where it entered. Like other gratings, it serves as a kind of disperser for spectrographs. It differs from a (typical) grism in that the grating is reflective rather than transmissive (though some may consider an immersion grating to be one particular type of grism, i.e., a type of "grating prism"). An advantage of the immersion grating is that it requires less volume to incorporate into a spectrograph, thus of interest when the space requirements are tight, which can be the case for space telescopes as well as on the ground for instruments that otherwise might be too large, e.g., if cryogenic cooling is desired. Volume can also be significantly reduced given a spectrograph's target spectral resolution if it can be achieved with a series with fewer dispersers: refraction by the immersion grating's transparent material significantly magnifies its overall dispersion beyond what is achieved by the grating, the higher the refractive index, the greater the effect. A germanium immersion grating (GIG) is of interest for infrared spectroscopy: germanium is transparent in the infrared, with a high refractive index.