Dichroic Filter

Dichroic filters use the principle of thin film interference and produce the same color as an oil film on the water surface. When light hits the oil film at an angle, some light is reflected from the top surface of the oil and some light is reflected from the bottom surface that is in contact with the water. Since the light reflected from the bottom has a slightly longer travel path, some wavelengths are enhanced by this delay, while others tend to be canceled out, resulting in visible color.

In dichroic mirrors or filters, instead of using an oil film to create interference, alternating layers of optical coatings with different refractive indices are built on a glass substrate. Interfaces between layers with different indices of refraction create phase reflections that selectively enhance certain wavelengths of light and interfere with others. These layers are usually added by vacuum deposition. By controlling the thickness and number of layers, the passband of frequencies can be adjusted to the width of the filter and made wider or narrower as needed. Because unwanted wavelengths are reflected rather than absorbed, dichroic filters do not absorb this unwanted energy during operation and therefore do not become more all energy in the passband).

Similar dichroic prisms are used where white light is intentionally divided into various color bands (for example, in color video projectors or color TV cameras). However, it is now more common for cameras to have arrays of absorption filters to filter individual pixels on a single CCD array.