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Classifications of Filters

Although there are many types of filters, they all have the same basic function: transmit light of a specific wavelength or color range while preventing the remaining wavelengths from passing through.


1. Two main types of optical filters?


Optical filters are divided into two categories: absorption filters and dichroic filters (also called interference filters).


Absorbent filter: often made of glass added with selected organic or inorganic compounds. These added compounds are used to absorb (essentially block) certain wavelengths while transmitting others.


Dichroic filters are almost the exact opposite of absorptive filters: they reflect selected wavelengths instead of absorbing them, and then transmit the remaining wavelengths. Both types of optical filters have many functions, but dichroic filters can obtain most of the characteristics from the optical coating: based on the thickness and characteristics of the filter coating, it can reflect or transmit precise wavelengths. This makes dichroic filters particularly effective in the scientific field. The camera also uses dichroic filters to separate light wavelengths according to film specifications.


2. In addition to absorptive and dichroic filters, there are several other filter types


Monochromatic filter: The monochromatic filter transmits only one color (narrow wavelength).


Infrared filter: This type of endothermic filter can transmit visible light, but will block the mid-infrared spectrum. Incandescent bulbs depend on infrared filters.


UV filter

The ultraviolet filter effectively blocks the ultraviolet range of the spectrum, and it transmits visible light like an infrared filter. This type of filter is often used in cameras because film is sensitive to ultraviolet light, which the human eye is not. In order to capture an image on film under the perception of the eye, all ultraviolet wavelengths must be filtered out.


Neutral density filter

The neutral density filter reduces the intensity of light of all wavelengths by filtering out only a part of each wavelength. The optical density of the filter combined with certain logarithms can accurately determine the amount of each wavelength to be blocked. This type of filter is useful in photography because they allow users to accurately capture images even in bright light. The neutral density filter can be absorptive or reflective.


Long pass filter

This type of filter is usually made of colored glass and can transmit longer wavelength spectra, such as infrared, ultraviolet, and visible light. Long-pass filters can block shorter wavelengths, and are often used as emission filters in fluorescence microscopes, and are a key component of dichroic (unidirectional) mirrors.


Short Pass Filter

Short-pass filters have the opposite function of long-pass filters: they transmit short wavelengths.


Bandpass filter

When the long-pass filter and the short-pass filter are combined, the resulting device is a band-pass filter. Although band-pass filters transmit less light than short-pass and long-pass filters, the number of layers in the filter can be controlled to transmit a wide or narrow range of light.


Polarizing filter

Polarization filters block or transmit light according to the polarization of the filter. Reflections (such as water or shiny surfaces) are highly polar-polarizing filters (such as those used in sunglasses and some camera lenses) can block certain spectra that cause reflections, making them more visible. Polarizing filters can darken bright components in color photography (such as the sky) because of the high intensity of ultraviolet light in the sky. They can also help manage reflections by minimizing the appearance in the final image.

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