Color Separation
The first step in reproducing full-color continuous-tone copy is to make color separations. As the name suggests, color separation is the breaking down of the original copy into the four process colors: yellow, magenta, cyan, and black. There are two ways to do this, photographically and electronically.
Photographic Separation. Photographic separations can be made by process camera, enlarger, or by contact printing. To make color separations, the original copy is photographed four times through special filters. This results in four separation negatives, each carrying a record of the amount and distribution of one of the process colors found in the original copy.
Because process printing is halftone printing, the separations must be screened before the printing plates can be made. To do this, each separation is photographed through a screen that breaks the image down into thousands of tiny dots. To ensure that these dots will fall in correct relation to one another (to avoid a moiré), each screen is set at a different angle.
Separating and screening can take place as a single operation, in the direct method, or as two independent operations, in the indirect method.
Direct Method. The copy is separated, screened, and scaled in the same operation, resulting in four screened separation negatives. This method is fast and efficient, but it does have one disadvantage: if there is any change in the size of the reproduction after the separations have been made, the original copy has to be reseparated and rescreened. This is because reducing or enlarging screened negatives produces an undesirable effect on the dot size.
Indirect Method. The copy is only separated, resulting in four continuous-tone (unscreened) separation negatives. One of the major advantages of this method is that because the separation negatives are unscreened, they ran be enlarged or reduced to make any number of reproductions without having to go back to the original copy. Another advantage is that color can be corrected by masking or retouching the film. After scaling and color corrections, the negatives are screened.
Electronic Separation. Although some color separation is still being done photographically, most is now being done by electronic scanners. The electronic scanner "reads" the colors in the original copy and produces screened or unscreened positive or negative separations. Some scanners not only make separations, but can also strip in background tints at the same time. One of the major advantages of the electronic scanner is that it allows for methods of color correcting to be built right into the system, eliminating much of the handwork involved in conventional color correcting.
The highest resolution devices available are drum scanners. The drum scanner has one restriction, however: it can only separate copy that is flexible enough to be wrapped around a scanner drum. This limits acceptable copy to those materials that can be bent. Flatbed scanners and photographic separations, on the other hand, can be made from either transparent copy or reflection copy (copy viewed by reflected light, such as paintings and photographs).
Color Correcting Separations. The best time to adjust color is before the separations have been made. By telling the printer in advance what you want, he can make adjustments while separating; he can modify individual colors, change the overall tone from warm to cool, or increase or decrease contrast.
Once the copy has been separated, it is still possible to do a certain amount of color correcting on the unscreened separations. This is done either on the positives or the negatives, usually by masking or retouching. After the separations have been screened all that can be done to correct color is dot-etching, which chemically reduces the size of the halftone dots.