A thin-film optical filter is produced by placing thin layers of substances that contain specialized optical properties in an alternating fashion on top of a membrane; glass that made specifically for optical purposes, for example. As light passes through an optical filter, its wavelengths change directions as they pass through each layer of the filter. The thin-film coating alters the refractive indices, which results in internal interference, a process that helps to minimize interference from internal reflections. The wavelengths of light can pass through, absorbed, or reflect off of the filter. The kind of optical filter and the wavelength will determine how the light reacts to the filter.
There are different types of optical filters. Some can transmit light, while others can reflect it, and still others can block it completely. All types can process any wavelength, from the UV range to the IR range. Generally, optical filters are categorized into five key groups according to the spectral shape of the filter.
1. Bandpass filters. These optical filters transmit a variety of wavelengths while also blocking out the neighboring light.
2. Notch filters. Notch optical filters block out a range of wavelengths while transferring the light on both sides.
3. Shortpass edge filters. Optical filters that belong to this category transmit short wavelengths of light and block out longer ones.
4. Longpass edge filters. Short wavelengths of light are blocked by longpass edge filters, while longer wavelengths are transmitted through them.
5. Dichroic filters. Certain kinds of wavelengths are reflected by dichroic filters, while other ranges pass through them.
Typically, edge, bandpass, and notch optical filters work at small angles of incidence (AOI), such as 0 degrees. Dichroic filters, however, are designed to work at an AOI of 45 degrees or larger and are designed in edge, bandpass, or notch arrangements.
Multiband configurations of optical filters can also be made. Multiband optical filters are bandpass that allow multiple passband. These filters contain several blocking regions and they diffuse all neighboring wavelengths of light. Polychroic filters, for example, are dichroic filters that contain several notches or bands.
While the majority of optical filters are grouped into the above mentioned categories, customized filters can be made. Custom filters can have any type of spectral shape you can think of; for instance, light waves from a xenon lamp can be made to resemble the spectrum of light the sun produces when passed through a customized filter. Other types of specially made optical filters can correspond with random spectral shapes.
Since optical filters are so versatile, they can be used in a variety of ways, including:
- Remote sensing
- Solar imaging
- Fluorescence microscopy