Emulsion
The light-sensitive coating on film or paper, consisting of silver halide crystals suspended in gelatin.
Measured in
µm
In depth
The size and distribution of the crystals determine almost everything about the film's behaviour: larger crystals catch light more readily, giving higher speed but coarser grain. Colour films layer several emulsions, each sensitised to a different part of the spectrum, with filter layers between them.
Examples in practice
A fast 3200-speed film has visibly larger emulsion crystals than a fine-grained 50-speed film.