Editing Autochrome Lumière (section)

== Structure and use ==

Autochrome is an [[additive color]]<ref name="auto"/> "mosaic screen plate" process. The medium consists of a glass plate coated on one side with a random mosaic of microscopic grains of [[potato starch]]<ref>{{cite web|url=http://americanhistory.si.edu/collections/object.cfm?key=35&objkey=262 |title=Collections &#124; National Museum of American History |date=7 February 2012 |publisher=Americanhistory.si.edu |access-date=2013-01-29}}</ref> dyed red-orange, green, and blue-violet (a variant of the standard red, green, and blue additive colors); the grains of starch act as color filters. [[Carbon black|Lampblack]] fills the spaces between grains, and a black-and-white [[panchromatic]] [[silver halide]] [[emulsion]] is coated on top of the filter layer.{{Citation needed|date=February 2021}}

Unlike ordinary black-and-white plates, the Autochrome was loaded into the camera with the bare glass side facing the lens so that the light passed through the mosaic filter layer before reaching the emulsion. The use of an additional special orange-yellow filter in the camera was required to block ultraviolet light and restrain the effects of violet and blue light, parts of the [[Visible light#Electromagnetic spectrum and visible light|spectrum]] to which the emulsion was overly sensitive. Because of the light loss due to all the filtering, Autochrome plates required much longer exposures than black-and-white plates and films, which meant that a tripod or other stand had to be used and that it was not practical to photograph moving subjects.<ref>{{cite journal | journal = The Lancet-Clinic | volume = LXXXXIX | issue = 26 | title = The New Lumiere Process of Color Photography | author = M. L. Heidingsfeld | date = June 27, 1908 | url = https://books.google.com/books?id=8g8CAAAAYAAJ&q=Autochrome+Lumi%C3%A8re+orange+green+violet&pg=PA785 | publisher = Lancet-Clinic Pub. Co. }}</ref> The plate was reversal-processed into a positive [[Transparency (photography)|transparency]] {{Ndash}}that is, the plate was first developed into a negative image but not "fixed", then the silver forming the negative image was chemically removed, then the remaining silver halide was exposed to light and developed, producing a positive image.{{Citation needed|date=February 2021}}

The luminance filter (silver halide layer) and the mosaic chrominance filter (the colored potato starch grain layer) remained precisely aligned and were distributed together, so that light was filtered in situ. Each starch grain remained in alignment with the corresponding microscopic area of silver halide emulsion coated over it. When the finished image was viewed by transmitted light, each bit of the silver image acted as a micro-filter, allowing more or less light to pass through the corresponding colored starch grain, recreating the original proportions of the three colors. At normal viewing distances, the light coming through the individual grains blended together in the eye, reconstructing the color of the light photographed through the filter grains.{{Citation needed|date=February 2021}}[[File:Microphoto of Autochrome plate.jpg|thumb|The colored starch grains in an Autochrome plate, greatly enlarged.]]