Gratings 　　Diffraction gratings are optical elements that are similar to lenses or mirrors.  Diffraction gratings are superimposed with a precise pattern of microscopic periodic structures. Usually these are in a pattern of corrugated surface grooves (a surface-relief grating), though some gratings are formed by the periodic variation of the refractive index inside the grating itself (a volume grating). Gratings used to disperse ultraviolet (UV) and visible light usually contain between 300 and 3000 grooves per millimeter, so the distance between adjacent grooves is on the order of one micron.  Diffraction gratings may be either ruled or holographic, although there is a wider range of groupings within each style 　　Holographic diffraction gratings are formed when a series of interference fringes, corresponding to the grooves of the desired grating, are recorded on a photosensitive layer, and the subsequent chemical treatment forms a modulated profile on the surface of the blank by selective dissolution.  This particular type of diffraction gratings includes many configurations such as planar, curved (for example concave and toroidal), aberration-corrected, with uniform and non-uniform groove spacing. Non-uniform spacing can provide superior focusing characteristics. Holographic gratings are the type of choice for the visible and UV. They typically exhibit less stray light and "ghost" spectra than do classically ruled gratings, because they have fewer random and systematic imperfections. Grating Specification Sheet Grooves Wavelength Blank Size Ruled Area 20 10.0 80×100×20 70×90 25 0.63 55×55×10 50×50 50 1.0~1.5 70×70×12 60×64 50 12.8 110×70×20 40×40 50 15~25 50×60×15 60×63 60 10.6 55×55×10 100×65 75 2.0 55×55×10 48×48 100 0.43 60×60×15 55×55 100 10.6 110×110×20 100×100 100 9.8 25×25×8 20×20 100 0.63 55×55×15 50×50 100 2.5~15 70×70×15 64×64 100 16.0 148×80×20 130×70 131 1.5~2.5 190×180×25 180×155 150 0.31~0.43 55×55×15 50×50 150 0.5 70×70×15 64×64 150 5.3 30×30×8 25×25 150 10.6 55×55×15 50×50 200 0.54~0.7 70×70×15 64×64 200 5.0 60×60×10 55×55 210 0.64 90×110×20 87×90 250 0.8 70×70×12 64×64 300 0.6 110×110×20 100×100 300 2.5 45×45×10 40×40 300 3.0 55×55×15 50×50 300 0.43 70×70×12 64×64 300 0.64 90×70×15 64×64 300 1.0 70×70×12 64×64 300 2.0 70×70×12 64×64 400 0.54 55×55×15 50×48 400 2.0 70×70×15 64×64 400 symmetry 70×70×1525 25×25 450 0.43 55×55×15 48×40 600 1.6 70×70×15 64×64 600 0.36 55×55×15 50×50 600 3.0 70×70×15 60×35 600 10.6 70×50×20 65×40 600 0.5~1.0 90×90×20 40×35 600 0.63 55×55×15 44×17 1200 0.64 70×60×15 60×50 1200 0.25 45×45×10 40×40 1200 0.31 30×30×8 25×25 1200 0.5 45×45×10 40×40 1200 0.31 25×25×5 20×20 1200 1.06 110×110×20 100×105 1200 0.25~0.31 55×55×15 50×50 1200 0.25 70×70×15 64×64 1200 0.21 70×70×15 64×64 1200 0.25 70×70×12 64×64 1200 0.64 110×110×20 100×102 1800 0.5 55×55×15 50×50 2400 0.25 45×45×12 40×40 79 0.5 260×135×45 250×125 150 1.9~2.1 85×76×16 85×76 75 1.25~2.5 58×58×10 58×58 1200 holographic 58×58×10 58×58 3600 0.2 30×30×8 30×30 1200 0.25~0.8 50×40×10 48×38 100 transmission 60×45×2 55×40 300 transmission 60×45×2 55×40 600 transmission 60×45×2 55×40 4320 0.22 60×60×10 58×58