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.