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NOTES_ON_OPTICAL_PRINTER_TECHNIQUE.md

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+
+## IMAGE SUPERPOSITION
+
+In an overall combination of two images, the two can infuse each other as lightness or as darkness, or they can be slapped onto each other. 
+There are three basic types of image superposition, named according to how they are made.
+
+Pictures A & B combined by...
+
+1. _Double exposure from positives._ The print film is eared twice, once from A's positive, once from B's positive.
+2. _Double exposure from negatives._ The print film is exposed twice, once from A's negative, once from B's negative.
+3. _Bipack._ Two films, either A's and B's positives, or else A's and B's negatives, are inserted together in the printer gate. The print film is exposed once, from this pair.
+
+The print film is unspecified.
+It is in the final positive print that the three types of combination are compared, and they look very different.
+For B&W the differences can be described by how tones combine.
+
+With (1), lightness dominates.
+Where one tone combines with another tone the result is nearly the lighter of the two tones.
+
+With (2), darkness dominates. 
+The result is nearly the darker of the two tones.
+
+With (3), there is contrastification which complicates the tone combination.
+If a bipack is examined ray (unprinted) wherever both images are clear the bipack is clear.
+Wherever either image is black the bipack is at least that black. 
+Wherever both images are black the bipack is doubly black.
+The bipack, which appears dark, has a tonal range doubling that of the
+single images.
+The bipack is unprintable in toto.
+
+To abstract a picture from the unprintable bipack printing exposure is typically increased 1-4 stops.
+With 4 stops increase, where clear and black coincide prints as a dark grey would—-not a clear domination of either lightness or darkness.
+With 2 stops increase there is darkness domination.
+
+## GAMMA & BIPACK
+
+If the bipack is printed onto gamma $ material, to reduce the contrast to normal, it is a true tonal blender, without dominance, of the two images. 
+As the graphs below show, the gamma 1/2 bipack is the mean between the type (1) and type (2) double exposures.
+
+## INCIDENTALLY
+
+A type (3) of a type (1) and a type (2) is just a type (3) again.
+
+Four idealized graphs summarize the three basic types of superposition and the gamma 1/2 bipack.
+
+Example: 
+
+> 4 has density .75 and B has density 1.75 in one place. From the first graph, the double exposure from positives has density about 1.0 in that place.
+
+**![Graphics depicting DOUBLE EXPOSURE FROM POS'S, BIPACK (1.0 COMPENS) DOUBLE EXPOSURE FROM NEGS and GAMMA 1/2 BIPACK](#)**
+
+For a double exposure it doesn't matter which exposure is first, or what time separates the two.
+In some multi-head optical printers, using a beam-splitter, the two exposures are simultaneous.
+Either way, the two films can be independently adjusted for exposure, filtered, etc.
+
+For a bipack it doesn't matter which film is in front.
+Also the two may be optically instead of mechanically bipacked.
+In some multi-head optical printers the films are in separate gates, ones projection becoming the other's illumination.
+In a simple printer one film may be in the gate and the other in the camera, in front of the print film.
+Any way, the two films ehare one exposure adjustment and filtration.
+
+When films will be physically bipacked they should first be wiped with a lubricating film cleaner.
+This is good practice for all optical printing when delicate originals receive heavy handling.