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What are the categories of filters?

Optical filters are commonly used optical filters, which are devices that selectively transmit light of different wavelengths, usually flat glass or plastic devices in the optical path, which are dyed or have interference coatings. According to spectral characteristics, it is divided into pass-band filter and cut-off filter; in spectral analysis, it is divided into absorption filter and interference filter.

1. Barrier filter is made by mixing special dyes in resin or glass materials. According to the ability to absorb light of different wavelengths, it can play a filtering effect. Colored glass filters are widely popular in the market, and their advantages are stability, uniformity, good beam quality, and low manufacturing costs, but they have the disadvantage of a relatively large passband, usually less than 30nm. of.

2. Bandpass interference filters
It adopts the method of vacuum coating, and coats a layer of optical film with a specific thickness on the surface of the glass. Usually, a piece of glass is made by superimposing multiple layers of films, and the interference principle is used to allow light waves in a specific spectral range to pass through. There are many types of interference filters, and their application fields are also different. Among them, the most widely used interference filters are bandpass filters, cutoff filters, and dichroic filters.
(1) Bandpass Filters can only transmit light of a specific wavelength or narrow band, and light outside the passband cannot pass through. The main optical indicators of the bandpass filter are: center wavelength (CWL) and half bandwidth (FWHM). According to the size of the bandwidth, it is divided into: a narrowband filter with a bandwidth <30nm; a broadband filter with a bandwidth>60nm.
(2) Cut-off filter can divide the spectrum into two regions, the light in one region cannot pass through this region is called the cut-off region, and the light in the other region can fully pass through is called the passband region, Typical cut-off filters are long-pass filters and short-pass filters. Long-wavepass filter of laser light: It means that in a specific wavelength range, the long-wave direction is transmitted, and the short-wave direction is cut off, which plays the role of isolating short-wave. Short wave pass filter: A short wave pass filter refers to a specific wavelength range, the short wave direction is transmitted, and the long wave direction is cut off, which plays the role of isolating the long wave.

3. Dichroic filter
Dichroic filter use the principle of interference. Their layers form a continuous series of reflective cavities that resonate with the desired wavelength. When peaks and troughs overlap, other wavelengths are destructively eliminated or reflected. Dichroic filters (also known as “reflective” or “thin film” or “interference” filters) can be fabricated by coating a glass substrate with a series of optical coatings. Dichroic filters generally reflect unwanted portions of light and transmit the rest.
The color range of dichroic filters can be controlled by the thickness and order of the coatings. They are generally much more expensive and more delicate than absorption filters. They can be used in devices such as dichroic prisms in cameras to separate light beams into components of different colors.

Post time: Sep-29-2022