The dichroic filter is a glass substratum on which an alternate succession of dielectric materials is lain. The number and the thickness of the layers distinguish the variety of colours of filters, for example a red filter from a yellow one or a blue from a green one. The different thickness determines a different transmission curve.

An example: a blue filter allows only to the radiations below 500 [nm] to pass through, the rest of the light is reflected. That causes that the transmitted light is blue and that that reflected one is yellow. Therefore the dichroic filters can be used also like coloured mirrors.

A filter that allows the transmission of radiations with wave length below 510 [nm] (about 10 nm more than the blue one) determines a transmitted light that will be still blue, but now a clearer blue.

These differences derive from little variations in the layers’ thickness. If a coating’s total thickness  is between the 500 and the 1000 [nm], that is at maximum a micrometer, (that is a thousandth of millimetre), we can understand how much precision is requested to lay same substratum to get an equal thickness and therefore equal colour.

Not only the thickness varies the transmission curve of a filter, other very complex parameters are linked to the physical and optics characteristics of the materials lain.

fig 7