Optical filters

Optical filters are components that typically do not have an optical power and can be added to an optical system to transmit light selectively:

by allowing only one or more wavelengths to pass
by blocking only one or more wavelengths
by decreasing the intensity of light
by changing the polarization of light

They are particularly useful when

you cannot illuminate the sample with the desired light
you need to isolate a single wavelength reflected or transmitted by the sample
you want to reduce unwanted effects like reflections or straylight.

Filtering and coloured samples: concept scheme and monochromatic result. On the left, Red light is reflected off the red background, but is absorbed by the blue circle. On the right, Blue light is reflected off the blue circle, but is absorbed by the red background.

There are different types of optical filters that can be divided according to the technology used to produce them and their purpose.

The main ones are:

Absorption filters, glass or plastic products, with the addition of materials that absorb only a few wavelengths and allow the others to pass.
Dichroic (or interference) filters, typically made of glass, covered with a coating that reflects some wavelengths and allows others to pass. They are the most commonly used ones in the imaging field to create very precise and selective band-pass filters.

Color camera

Mono camera.

Red filter.

Green filter.

Blue filter.

Common examples are:

the infrared cut-off filters used in the chambers to filter the infrared straylight to which the sensors are sensitive,
the color filters provided by Opto Engineering for telecentric and fixed focal lenses

Neutral density filters (ND), which evenly reduce the intensity of all wavelengths, absorbing or reflecting light. They are typically used to extend exposure times without having to close the lens or decrease the intensity of the light used to illuminate the sample.
Polarizing filters, typically plastic (Polaroid), they block or transmit light depending on its polarization. The most commonly used ones are linear polarizers, which produce linearly polarized light starting from a non-polarized incident beam of light, and circular polarizers (quarter-wave lamina) that produce circularly polarized light starting from a linearly polarized incident beam of light.