Thank you, that is very helpful. In this context, is "tone" = lightness/darkness? Also, how and where are these curves generated, and where do they reside?
I'll try...
Yes, "tone" basically means lightness/darkness. But in a special way.
When you create an image in, for example, PhotoShop (raster image), or Illustrator (vector image), or graphics in InDesign (raster/vector) what you're actually doing is specifying tones that range from 0% ink (light) to 100% ink (dark) and all the tones in between. So, for example, when you specify a "Dark Blue" that color may, behind the scenes, be made up of a 50% tone of Magenta ink, and a 75% tone of Cyan ink, and a 15% tone of Black ink.
A printing press, like an offset press, cannot print (continuous) tones. It can only print 100% ink or 0% ink - it's a binary printing device. So the tones are translated into "halftone screens" that simulate the look of the various requested tones. The process of converting from continuous tones to halftones is accomplished in the printshop's "RIP" (Raster Image Processor").
Due to the mechanics of printing the tones you request are not reproduced as specified. E.g. when you ask for a 50% patch in your file you may end up with a 68% on the press sheet.
There are industry standards for the press sheet tones you should expect on press for the tones you have requested in your file. If your printshop is printing to industry standards then the shop will create tone compensation curves that change the tones you request into the tones needed on the plate that will result in the correct tone, according to the industry standard, on the press sheet.
Those tone compensation curves are created by the prepress dept in the print shop based on some tests that they do when setting up their production system. They are embedded in the RIP and are applied when your job is processed for printing.
You might find my blog of interest:
http://the-print-guide.blogspot.ca/
