Disappointingly moiré does not result from the work of Swiss photographer, Ernst Moiré, eponymous hero of this book. More prosaically the word derives from the word for watered silk or other materials.  For those of you who weren’t around in Victorian times, moiré silk is a weave which when viewed aslant against the light shows a shifting wavy, watery pattern. It’s also called moire, and given that it appears to be an English word in origin (Chambers relates it to mohair, which was apparently also woven in this manner) the accent is probably a late addition, to make the word seem more exotic. Having spent some time involved in the woolen trades, I find this sort of up-classing of terminology entirely convincing. We were always calling things names which sounded classy and might one hoped justify an up-charge.

In the graphic arts moiré is altogether undesirable. The Wikipedia article is rather good I think in spite of the qualifications which someone has added at the top calling for documentation and criticizing the way it’s written.  We come across moiré patterning usually when we submit as original artwork a photo cut from a newspaper or book. An already printed photo will carry its original screen, and when it gets rescreened for your publication there’s often (usually/always?) a conflict between the two screen angles which sets up a pattern of darker and lighter areas regularly spread across the picture.  The book cover illustrated above incorporates a moiré pattern which you can see well in the sky.  This illustration from the Wikipedia article shows the effect in action.

This angle difference is what makes your screen finder work. When you lay its radiating lines over a halftone the pattern of interference will give you the screen value of the halftone.