Editing Diffraction grating (section)

=== VPH (Volume Phase Holography) gratings ===
Another method for manufacturing diffraction gratings uses a [[photosensitive]] gel sandwiched between two substrates. A holographic interference pattern exposes the gel, which is later developed. These gratings, called ''volume phase holography diffraction gratings'' (or VPH diffraction gratings) have no physical grooves, but instead a periodic modulation of the [[refractive index]] within the gel. This removes much of the surface [[scattering]] effects typically seen in other types of gratings. These gratings also tend to have higher efficiencies, and allow for the inclusion of complicated patterns into a single grating. A VPH diffraction grating is typically a transmission grating, through which incident light passes and is diffracted, but a VPH reflection grating can also be made by tilting the direction of a refractive index modulation with respect to the grating surface.<ref>{{Cite web|date=June 1998|title=Volume Phase Holographic Gratings|url=https://www.noao.edu/noao/noaonews/jun98/node4.html|url-status=live|website=National Optical Astronomy Observatory|archive-url=https://web.archive.org/web/19991112030703/http://www.noao.edu:80/noao/noaonews/jun98/node4.html |archive-date=12 November 1999 }}</ref> In older versions of such gratings, environmental susceptibility was a trade-off, as the gel had to be contained at low temperature and humidity. Typically, the photosensitive substances are sealed between two substrates that make them resistant to humidity, and thermal and mechanical stresses. VPH diffraction gratings are not destroyed by accidental touches and are more scratch resistant than typical relief gratings.