(Science: equipment) A colour–filter disk to be placed, as a dark field stop would be placed, below the substage condenser.
The central circular area, that is filled with one of the two or three colour filters, should safely subtend the objective aperture. The annular quadrants around this are normally contrasting in colour. This is/are the colour shown by the organisms or other specimen detail against the coloured field. The effect is one kind of optical staining.