To understand how an optical filter works, we basically must understand what light is and how it behaves from a purely physical point of view. Visible light is a "portion" of the electromagnetic spectrum of a certain wavelength. Similarly, ultraviolet light, infrared light or any of its segments are ranges within the complete electromagnetic spectrum.
Putting a filter in front of the optics, we can select which wavelength reaches the photoreceptors of the sensor, capturing only those that are more favorable for the objective of the project and rejecting the others, which are absorbed, reflected, or deflected by the filter. Likewise, filters can be used to act on the incident light without necessarily having to cut one part and let another pass through.
Bandpass filters allow the selective passage of wavelengths within a certain range. They are widely used in many applications, such as artificial (machine) vision, microscopy, spectral imaging (multispectral, hyperspectral), etc...
The wavelength range of the passband is typically several tens to hundreds of nanometers. There are narrow bandpass filters that allow only one or two tens.
There are also filters that allow transmitting several non-adjacent bands, they arre the multiband filters, especially useful in microscopy.
For demanding applications, there are ultra-narrow band pass filters that only allow the passage of a few nanometers or even with a pass band of less than a nanometer.
Long pass filters are wavelength cut filters. These filters allow the passage of wavelengths above a certain value while those below are cruelly rejected. It should be noted that the name refers to the domain of wavelengths (lambda). In this way, “high pass 700” passes wavelengths above 700 nm (or hertz (Hz) frequencies below their frequency domain equivalent) while blocking lambdas below 700 nm.
Short pass filters are like long pass filters but in reverse. Thus, the range of wavelengths allowed will be below the reference lambda.
If what you need is the opposite of a bandpass filter, a filter that passes almost the entire spectrum and rejects only a small band, then we have notch filters.
Among the different technologies and qualities of optical filters, absorption filters are the most common. These filters absorb the rejected wavelengths. On the contrary, the interferential filters reflect the rejected wavelengths and thanks to the multilayers deposited on the substrate, the interference generated is destructive, canceling the reflection.
A special type of filter is the dichroic filter (or dichroic mirror). With it, the selected band is transmitted, and the rest is reflected at a specific angle, typically 0 or 45 degrees.
Of course, filters are not perfect and for this reason, their walls are not vertical. For this reason, the transition between what is blocked and what is allowed to pass is not abrupt (as would normally be desirable) but rather the walls of the filter have a certain slope. The higher the slope (steeper), the more accurate the filter, and the more expensive it is.
There are also filters that attenuate all the light components, to avoid saturating or damaging certain components such as the sensor itself. We then have neutral density filters, characterized by the amount of light they attenuate: their optical density (OD).
The use of filters is usually recommended, even without a filtration function, in order to protect the main optics from dirt or damage.
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