Spectral filters are used to selectively transmit light according to its wavelength. Spectral filters are not to be mistaken with color filters, which can be differentiated by an associated schott glass number. There are several types of spectral filters. Examples include light filters and dichroic filters. A light filter a transparent filter that reduces light (or some wavelengths of light) passing through it. A dichroic filter is an accurate color filter used to selectively pass light of a small range of colors while reflecting other colors. Other spectral filters are commonly available.
There are many ways in which spectral filters function. Dichroic filters operate on the principle of interference. With dichroic filters, alternating layers of optical coatings are built up upon a glass substrate, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. By controlling the thickness and number of layers, the frequency of the passband of the filter can be made as wide or narrow as desired. Because unwanted wavelengths are reflected, dichroic filters don't absorb much energy during operation and therefore don't become as hot as the equivalent conventional filter. A light filter can be mounted in threaded (1 1/4-24 thread) black anodized rings. Light filters are often utilized in microscopes. The light filters are strategically oriented within a specialized block that enables the illumination to enter from one side and pass to and from a specimen in defined directions along the microscope optical axis. A light filter may be repeatedly interchanged on any SED, SEL, and SHD detector. A spectral filter is generally used to either select or eliminate information from an image based on the wavelength of the information. Spectral filtering is usually effected by passing light through a glass or plastic window that has been specially treated to transmit or absorb/reflect some wavelengths. Since the light entering a sensor will have a spectral distribution that depends on the illumination source's spectral characteristics and the reflectance of the illuminated scene, using a filter to select image regions with known spectral properties can help extract information. Spectral filters are designed and manufactured to meet most industry specifications.
Spectral filters are used in many applications including display color generation, medical diagnostics, microscopy, remote sensing, and spectroscopy. These spectral filter applications use polarized optical systems. Spectral filters should adhere to standards established by the International Standards Organization (ISO).