Shadow printing

The theoretical resolution capability of shadow printing (using a conventional photoresist) with a mask consisting of equal lines and spaces of width b is given by (2)... 

The shadow effect is believed to result from a blending of a darkened portion of the lens periphery of particular dimensions, shape, and patterns with the darkened portion of the wearer s eyelid attributable to the wearer s own lashes or the portion of the eyelid to which the lashes are or are ordinarily affixed or to a shadow, which may be attributable to the wearer s own lashes or the upper eyelid margin to which the lashes are affixed. A shadowed printed lens is shown in Figure 7.9. 

The original techniques employed for exposure of resists used simple shadow printing techniques where the resist-coated substrate was flood-exposed with ultra-violet light through a patterned mask which was close to, or in intimate contact with, the resist surface. These systems are still used for production of small numbers of devices where their simplicity and cheapness are a great advantage. They also make best use of the whole spectral output of the irradiation source which is normally limited only by the absorption of the mask substrate material. 

Ma.ke-Rea.dy, Because pressure is needed for ink wetting and transfer and because the image elements in letterpress vary in size, the same amount of printing pressure or squeeze exerts more pressure on highlight dots than on shadow dots. This necessitates considerable make-ready to even the impression so that the highlights print correctly and do not puncture the paper. Precision plates and premake-ready systems have helped reduce make-ready time, but it is still appreciable for quaUty printing and is a reason letterpress has been largely replaced by other processes. 

Fig 1. Electron micrograph of a platinum/carbon replica prepared by the fast-freeze, deep-etch, rotary-shadow replica technique printed in reverse contrast. Cell walls of onion parenchyma have an elaborate structure with many thin fibres bridging between thicker cellulosic microfibrils. Scale bar represents 200nm. 

The next process is to tone it, that is, to substitute gold or oxide of gold, for the silver, which forms the shadows of the picture. This is done by immersing the print for two or three minutes in the following solution ... 

In lithography, particles can damage the printed pattern either by imbedding themselves in the resist or by casting shadows from the mask... 

Changes and variations in image tone may not always be as glaringly apparent. Often the difference is subtle, so much so that someone viewing a print might not even be aware that it is neutral, cool, or warm, unless the fact is drawn to their attention, or they see an example of each, side-by-side. The simplest test to determine whether or not a paper is cold-, warm-, or neutral-toned, is to develop it for two minutes in a neutral tone developer such as D-72 and compare it to other papers developed similarly. If a paper is warm- or cold-toned the color can be seen in the shadows. 

As with two-tray development, the time in each developer can be varied, though the minimum time in the amidol is about 1 minute. However, too long in the water bath could result in streaking. A good starting point would be to develop the print for 90 seconds in the amidol, and 30 seconds in the water bath. Keep an eye on the shadow areas of the print. When they appear to be about 80% complete, move the print to the water bath. 

Keep making prints while changing exposure time until you have the highlights perfect. Note that I have not mentioned the shadow areas. They are to be ignored until the exposure time is properly obtained. 

I learned this simple trick from Ruth Bernhard while printing her negative, Rockport Nude.The figure in Ruth s negative has wonderful mid-tone and highlight contrast but is very flat in the shadows. It is difficult to retain the contours of the model s exquisitely muscular physique as they recede into shadow. 

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