From Discworld & Terry Pratchett Wiki
An iconograph is a wonderful device that allows you to make "instantaneous paintings". In fact, an imp with brushes, pencils and a good eye for colours is put in a box, and when you push the button, you open a little window on the box and the imp draws really fast what it sees through the opening. Salamanders are used when more light is necessary for the imp to paint a good picture. All but the cheapest of today's iconographs can paint in colour.
Imps have no imagination whatsoever, and as a result, paint very accurate pictures. They do whatever they are told so long as it is within the limits of their training, such as being able to "zoom" in and paint in very small detail, or even to paint the picture of a cart and its number if it exceeds the speed limit.
First introduced in Ankh-Morpork by the Agatean tourist Twoflower (in The Colour of Magic), they are now very common, and are used in the Watch (Feet of Clay and The Fifth Elephant) or in newspapers. Very small imps, called nano-imps, are used in very small iconographs that can be kept in a pocket, a favorite tool of spying diplomats (see The Fifth Elephant).
A multi-imp variation had been used in the moving pictures industry (see Moving Pictures). Another recent adaption is using three imps with different coloured goggles to achieve coloured printing. A vampire slightly well-known in Ankh-Morpork was experimenting with the use of dark light from Überwaldean land eels, but no progress has been reported (see The Truth).
Theoretically the lifespan of the imp is endless, but the imps of the cheaper iconographs seem to disappear rather quickly. Independently of this, the painting colours used by the imp have to be refilled as they are used up, and the imps themselves require regular feeding, though they seem to be able to survive without any form of sustenance for several weeks.
Normally, the user of the iconograph is called "iconographer", but another term could well be "Photographer".
The principle of splitting the light unto three strips of monochromatic film with different filtering is how Technicolor(tm) actually works. It also needs a lot of money to work.