Notes from the session
The colour libraries may be accessed when using the colour picker and allow the user to choose colour in accordance to the reference system.
However, this process does not retain the reference number, key to the printing process.
The solution lies in the channels palette;
Spot channels may be added from the channel menu.
The colour of the spot channel may be selected using the colour libraries, changing the name of the channel appropriately.
Thus, painting within the spot channel will apply a simulation of over-printing.
Selecting the spot channel options allows for solidity adjustments. 100% will make the ink completely opaque and reject the effects of over-printing.
Using tones of grey will then produce tints of the selected colour.
When saving images with spot channels, is is important to ensure the 'Spot Colours' check box is ticked in the save dialogue window. The format must either be .tiff or .psd.
In Illustrator, one may inspect the overprint imagery using the separations preview window.
Separations may be accessed in the print options.
Separations are executed using halftone, being the process in which gridded dots create the illusion of tone. Lpi refers to the lines of dots per inch.
Image resolution should be twice the size of the lpi, thus we generally use 300 dpi on-screen as most printers use an lpi of around 150.
Layered patterns of dots are rotated to created what are referred to as Moire patterns, where the layers create an optical interference.
A correctly prepared document will reveal it's spot colours in illustrator.
Rotations for screen printing:
15, 75, 105, 155
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