Filter
A filter is an optical device used to select the desired radiation wavelength band. A common characteristic of filters is that no filter can make an object appear brighter in imaging because all filters absorb certain wavelengths, thereby making objects appear darker.
Types
Color Filter
These are various colored flat glass or gelatin films with transmission bands spanning hundreds of angstroms. They are often used in broadband photometry or installed in stellar spectrometers to isolate overlapping spectral orders. Their main feature is that they can be made quite large in size.
Thin Film Filter
Generally, they transmit longer wavelengths and are mostly used as infrared filters. The latter is deposited on a certain substrate.
Principle
Filters are made of plastic or glass plates with added special dyes. For example, a red filter only allows red light to pass through, and so on. The original transmittance of glass plates is similar to that of air, allowing all colored light to pass through, making it transparent. However, after adding the dye, the molecular structure changes, and the refractive index also changes, leading to changes in the passage of certain colored light. For example, when white light passes through a blue filter, it emits a blue light, with very little green and red light, most of which is absorbed by the filter.
Function
Filters play a significant role in machine vision. They are suitable for use in life sciences, imaging, industrial or defense industries, and applications such as fluorescence microscopy, spectroscopy, clinical chemistry, or machine vision detection. They are also widely used in photography.
Characteristics
Their main feature is that they can be made quite large in size. Using the vacuum coating method alternately forms metal-dielectric-metal films with a certain thickness of high refractive index or low refractive index, or all-dielectric films, constituting a low-order, multi-stage, series-connected solid-state Fabry-Perot interferometer. The selection of the material, thickness, and serial connection method of the film layer is determined by the required center wavelength and transmission bandwidth λ.
Wavelength
Various interference filters with center wavelengths ranging from ultraviolet to infrared and wavelengths λ ranging from 1 to 500 angstroms can be achieved. The peak transmittance of metal-dielectric film filters is not as high as that of all-dielectric film filters, but the latter has more serious sub-peak and sideband issues. In thin film interference filters, there is also a circular or rectangular variable interference filter suitable for space astronomical measurements. In addition, there is a dichroic filter that is placed at a 45° angle to the incident beam, which can decompose the beam into two different colors of light with high and uniform reflectance and transmittance, suitable for multi-channel multi-color photometry. Interference filters generally require perpendicular incidence, and as the incidence angle increases, they move towards shorter wavelengths.
This characteristic can be used to adjust the center wavelength within a certain range. Since λ and peak transmittance change significantly with temperature and time, extreme care must be taken when using narrowband filters. Due to the difficulty in obtaining large-sized uniform film layers, the diameter of interference filters is generally less than 50 millimeters. Some people have used splicing methods to obtain interference filters as large as 38 square centimeters, mounted on a 1.2-meter Schmidt telescope in the UK for photographing monochrome images of large nebulae.
Classification
Filter products are mainly classified according to spectral bands, spectral characteristics, film materials, and application characteristics.
Spectral Bands: Ultraviolet filters, visible light filters, infrared filters;
Spectral Characteristics: Bandpass filters, cutoff filters, beam-splitting filters, neutral density filters, reflection filters;
Film Materials: Soft film filters, hard film filters;
Hard film filters not only refer to film hardness but more importantly, their laser damage threshold. Therefore, they are widely used in laser systems, while soft film filters are mainly used in biochemical analyzers.
Bandpass Type: Only light within the selected band passes through, and light outside the passband is cut off. Its optical indicators mainly include center wavelength (CWL) and half-bandwidth (FWHM). It is divided into narrowband and broadband. For example, narrowband 808 filters NBF-808.
Short-Wave Pass (Low-Pass): Light shorter than the selected wavelength passes through, and light longer than that wavelength is cut off. For example, infrared cutoff filters, IBG-650.
Long-Wave Pass (High-Pass): Light longer than the selected wavelength passes through, and light shorter than that wavelength is cut off. For example, infrared transmitting filters, IPG-800.
FALenses Technology specializes in providing machine vision core hardware. You can go to the official website of FALenses Technology at https://www.falenses.com/ for more information.