Like
black and white photographs or images, colour images published
in books appear to have a continuous range of tones. However this
is an illusion which is created by thousands of dots of various
colours.
Detail
of a course half tone screen image. In the printing industry photographs
are scanned with a half tone screen, which breaks the image up
into a series of dots that can pick the ink up and pressed onto
the paper. In the black areas the dots are closer – in the
white areas they are further apart.