What is a filter

What is a filter?

A filter is an optical device used to select the desired wavelength of radiation. One commonality of filters is that no filter can make an image of a celestial object brighter, because all filters absorb certain wavelengths, thus making the object darker.

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Types of Filters

In principle, the filters can be divided into several types. The following is an introduction to these different types of filters.

Absorption filter ( Barrier filter)

It is made of resin or glass material mixed with special dyes. According to the ability of absorbing light of different wavelengths, it can play a filtering effect. The glass filter with color is the most popular in the market, its advantages are stable, uniform, good beam quality, and low manufacturing cost, but it has the disadvantage of large passband, usually less than 30nm Yes.

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Interference filter (Bandpass interference filters)

It adopts the method of vacuum coating, and a layer of optical film with a specific thickness is coated on the surface of glass. Usually, a glass is made of multi-layer films. The interference principle is used to let the light wave in the specific spectral range pass through. There are many kinds of interference filters and their application fields are also different. Among them, bandpass filter, cut-off filter and dichroic filter are widely used.

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(1) Band pass filter (Bandpass Filters)

Only light of a certain wavelength or narrow band can be transmitted, and light beyond the passband can not be transmitted.

The main optical index of band-pass filter is the central wavelength (CWL), half bandwidth (FWHM) 。

According to the bandwidth, it can be divided into: bandwidth < 30nm It is a narrow band filter;

Bandwidth > 60nm The above are broadband filters.

(2) Cut off filter( Cut-off filter )

The spectrum can be divided into two regions, one region of light can not pass through this region is called the cut-off region, while the light in the other region can fully pass through the pass band region. The typical cut-off filters are long wave pass filter and short wave pass filter.

Long wave pass filter : It refers to the specific wavelength range, the long wave direction is through, while the short wave direction is cut-off, which plays the role of isolating short wave.

Short wave pass filter :   Short wave pass filter refers to the specific wavelength range, the short wave direction is through, while the long wave direction is cut-off, which plays the role of isolating long wave.

(3) Dichroic filter ( Dichroic filter)

You can select a small range of colors that you want to pass through the light as needed, and reflect other colors.

There are other types of filters:

Neutral density filter (Neutral Density Filters)

Also known as attenuator, it is used to prevent the sensor or optical elements of the camera from being damaged by the strong light source. It can absorb or reflect the part of the light that has not been transmitted, so as to reduce the transmittance of a part of the spectrum evenly.

Fluorescence Filters (Fluorescence Filters)

Its main function is to separate and select the characteristic band spectrum of excitation light and emission fluorescence in biomedical fluorescence test and analysis system. It is a key component applied to biomedical and life science instruments.

Astronomical filter (Astronomy Filters)

A filter used to reduce the effect of light pollution on the quality of astronomical photographs.

Key indicators of filters

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Passband: The range of wavelengths through which light can pass is called passband.

bandwidth (FWHM) :   Bandwidth is a wavelength range used to represent the frequency spectrum passing through a specific part of the filter through the incident energy. It is expressed by the width at half of the maximum transmittance, also known as half width nm 。 For example, the peak transmittance of a filter is eighty %, so 1/2 namely forty %, forty %The corresponding left and right wavelengths are 700nm and 750nm That’s half the bandwidth 50nm 。 Half width less than 20nm Is called a narrow band filter 20nm Is called band-pass filter or broadband pass filter.

Central wavelength( CWL ) : Band pass or Narrow band filter The peak transmission wavelength of, or Band stop filter The peak reflection wavelength at the peak transmittance of 1/2 The midpoint between the wavelengths, that is, the midpoint of the bandwidth, is called the central wavelength.

Transmittance (T):   It refers to the passing capacity of the target band, expressed as a percentage, for example, the peak transmittance of the filter (Tp)> eighty %Refers to the maximum value that light can pass through after attenuation through the filter eighty %The higher the transmittance, the better the transmittance.

Cut off range: Is the wavelength interval used to represent the energy spectral region lost through the filter, that is, the wavelength range outside the passband,

Cut off rate (Block): The transmittance corresponding to the wavelength in the cut-off range, also known as the cutoff depth, is used to describe the cut-off degree of the filter, and the transmittance is expected to reach zero It is impossible to make the transmittance of the filter close to zero, in order to better cut off the unwanted spectrum. The cut-off rate can be measured in terms of transmittance or optical density (OD)It is related to the transmittance (T)The conversion relationship is as follows: od = log10 (1/T)

Width of transition zone :According to the different filter cut-off depth, the filter cutoff depth and transmittance peak are specified 1/2 The maximum allowable spectral width between positions.

Edge steepness : Namely [ (λ T80- λ T10)/ λ T10] *100%  

High level counter revolutionaries (HR) :   Most of the light is reflected through the filter.

High permeability (HT) :   With high transmittance, the energy loss of light passing through the filter is very small.

Incident angle: the angle between the incident light and the normal of the filter surface is called the incident angle. When the light is vertically incident, the incident angle is zero °。

Effective aperture:The physical area that can be effectively utilized in optical devices is called the effective aperture, which is usually similar to the external size of the filter, concentric, and slightly smaller in size.

Starting wavelength: The starting wavelength is defined as Long wave pass filter Medium transmittance increased to peak value 1/2 In band-pass filter, the corresponding wavelength can also be defined as five %Or ten %The wavelength corresponding to the peak transmittance of.

Cut off wavelength: The cut-off wavelength is defined as Short wave pass filter Medium transmittance reduced to peak value 1/2 In band-pass filter, the corresponding wavelength can also be defined as five %Or ten %The wavelength corresponding to the peak transmittance of.

Surface specifications and size parameters of filters

Surface quality

The surface quality of the filter is mainly due to some defects such as scratches and pits on the surface. The most commonly used specifications for surface quality are as follows MIL-PRF-13830B Description of scratches and pitting specifications, the pit name is obtained by dividing the pit diameter in microns ten Generally, the scratch pit size is in the eighty to fifty Within the scope will be called standard quality; stay sixty to forty Within the range, it is regarded as accurate mass; And in the twenty to ten Range will be considered high precision quality.

Surface smoothness

Surface flatness is a kind of surface accuracy measurement, which is used to measure reflector , window piece Prism The deviation of smoothness is usually based on the ripple value( λ) They are made up of multiple wavelength test sources, and one stripe corresponds to the other 1/2 And the smoothness is 0 one λ, It represents the general quality level; The smoothness is λ /4 , represents the precise quality level; The smoothness is λ /20 , represents the quality level of high precision.

Tolerance: the tolerance of the filter is mainly on the central wavelength and half band, so the indication of the tolerance range of the filter product.

Diameter tolerance

In general, the influence of the filter diameter tolerance is not very big in the use process, but the diameter tolerance must be considered when the optical device is installed on the holder. In general, the diameter tolerance is within (±0.1 mm)It’s called the general mass (±0.05 mm)It’s called precision mass (±0.01 mm)It’s called high quality.

Center thickness tolerance

The center thickness is the thickness of the central part of the filter. In general, the tolerance of center thickness is within (±0.2mm)It’s called the general mass (±0.05mm)It’s called precision mass (±0.01mm)It’s called high quality.

Principle of Filters

Filters are made of plastic or glass with special dyes, and red filters allow only red light to pass through, and so on. The transmittance of glass is about the same as air, all colored light can pass through, so it is transparent, but after dyeing, the molecular structure changes, the refractive index also changes, the passage of certain colors of light will have changed. For example, if a beam of white light passes through a blue filter, a beam of blue light is emitted, while green light and red light are very little, and most of them are absorbed by the filter.

The role of the filter

Filters are very useful. Widely used in the photographic world. Some master photographers shoot the landscape, why the main scene is always so prominent, how to do? This uses the filter. For example, you want to use the camera to shoot a yellow flower, the background is blue sky, green leaves, if the usual shot, it can not highlight the “yellow flower” the theme, because the image of the yellow flower is not prominent enough. However, if you put a yellow filter in front of the lens, blocking part of the green leaves scattered green light, blue sky scattered blue light, and let the yellow flowers scattered yellow light through a large number, so that the yellow flowers appear very obvious, highlighting the “yellow flowers” theme.

The characteristics of filter

The main feature is that the size can be made quite large. Thin film filters, which generally transmit longer wavelengths, are mostly used as infrared filters. The latter is in a certain base, using vacuum coating method alternately formed with a certain thickness of high refractive index or low refractive index metal-dielectric-metal film, or full dielectric film, constituting a low-level, multi-stage tandem Fabry-Perot interferometer. The choice of material, thickness and tandem method of the film layer is determined by the desired central wavelength and transmission bandwidth λ.


Various interferometric filters are available for any wavelength from UV to IR, with λ ranging from 1 to 500 angstroms. The peak transmittance of metal-dielectric film filters is not as high as that of full dielectric films, but the secondary peaks and sidebands of the latter are more serious problems. There is also a circular or long variable interference filter in thin film interference filters, which is suitable for space astronomy measurement. In addition, there is a two-color filter, which is placed at an angle of 45° with the incident beam, and can decompose the beam into two different colors of light with high and uniform reflection and transmission, which is suitable for multi-channel multi-color photometry. Interference filters generally require vertical incidence, when the angle of incidence increases, moving in the direction of the short wave.
This feature can be used to align the center wavelength within a certain range. Since both λ and peak transmittance vary significantly with temperature and time, great care must be taken when using narrow-band filters. The diameter of interference filters is generally less than 50 mm because large uniform film layers are difficult to obtain. Interference filters as large as 38 cm square have been obtained by collocation and mounted on the British 1.2-meter Schmidt telescope for monochromatic images of large nebulae.


Synchronization function

This technology controls the camera, synchronized switching of the IR lamp, filter and color to black. Stability with automatic positioning and anti-shake function, no flicker when the light is at the zero boundary point. Fast switching in one step, will not stop in the middle due to resistance stuck, produce filter offset. No filter shift caused by changes and vibrations such as gimbal rotation and stopping. No bounce due to collision when switching at high speed, resulting in inaccurate filter positioning.

Image color reproduction

Crystal filter can maximize the solution to the problem of false color, color drift, etc. Adding AR-COOTRMG heavy film to the crystal can achieve 98% light penetration. Daytime switch to the crystal filter state, can well sense the visible light, to prevent infrared and other light interference, is the color vivid and realistic, night switch to the filter coated with permeable film, can achieve 100% light penetration. Camera sensing infrared more, and the vast majority of wavelengths of light can be passed, the camera while color to black, so the infrared distance is farther and clearer.

Classification of Filters

Filter products are mainly classified according to spectral bands, spectral characteristics, film layer materials, application characteristics, etc.
Spectral bands: UV filters, visible filters, IR filters.
Spectral characteristics: bandpass filters, cutoff filters, spectral filters, neutral density filters, reflection filters.
Film materials: soft film filters, hard film filters.
Hard film filters not only refer to the hardness of the film, but more importantly to its laser damage threshold, so it is widely used in laser systems, while soft film filters are mainly used in biochemical analyzers.
Bandpass type: The light in the selected band passes through and the light outside the band is cut off. The optical index is mainly the center wavelength (CWL), half bandwidth (FWHM). Divided into narrowband and broadband. For example, narrowband 808 filter NBF-808.
Short wavelength pass type (also called low wavelength pass): light shorter than the selected wavelength passes through and light longer than that wavelength cuts off. For example, infrared cutoff filter, IBG-650.
Long-pass type (also called high-pass): light longer than the selected wavelength passes through, and light shorter than that wavelength cuts off For example, infrared transmittance filter, IPG-800.
The so-called filter is an optical instrument that filters out light of unwanted wavelengths or filters out light of a specific wavelength, so by using a filter, we can control the wavelength or band of light that is blocked or passed through. Filters that allow the passage of a single wavelength are called monochromatic filters, commonly known as infrared filters, ultraviolet filters and visible light filters of specific colors, such as red and blue filters. Filters can be divided into the following categories according to their frequency range: low pass filters, high pass filters, band pass filters, pass stop filters, full pass filters, ND filters, etc.

Low-pass filter

It is a filter that can only pass light with lower than the critical frequency or higher than the critical wavelength (long pass, abbreviated as LP filter), so it can filter the light with higher energy.

High-pass filter

It is a filter that can only pass light with higher than the critical frequency or lower than the critical wavelength (short pass, abbreviated as SP filter), so it can filter the light with lower energy.

Bandpass filter

A bandpass filter is a filter that can only pass light within the upper and lower boundary frequencies, so it can filter light outside a specific energy range. A bandpass filter can be completed by combining a set of high pass and low pass filters.

Bandstop filter

This is a filter that filters light outside of the upper and lower boundary frequencies, so it can filter light within a specific energy range.

Full-pass filter

It is a filter that cannot filter light of a specific wavelength, but can cause time delay.
The so-called ND (Neutral Density Filter) filter refers to the light reduction plate that can continuously change the amount of incoming light, it also belongs to a kind of light reduction plate (Density plate, can be abbreviated as DP), by rotating the ND filter, we can choose the power of the incident light. By rotating the ND filter, we can choose the power of the incident light. The reduction plate is just an optical component that reduces the power. Early sunglasses are a kind of light reducing plate, but nowadays sunglasses have multiple functions, for example: it can filter the light above the ultraviolet ray, then it is a kind of low-pass filter, and some sunglasses also have the function of polarizer (polarizer), using the principle of polarization, so that the light above the ultraviolet ray can not pass, but can make the visible wavelength pass, so as not to prevent the user from viewing Other objects.
In principle, it is still a bandpass filter, so it cannot really control the incident light wavelength precisely until the laser was invented, and the laser pulse width was much narrower than the best filters in the world, so it quickly took the place of the filter.

Thin Film Filters

TiO2 and SiO2 thin-film systems evaporated by electron beam (EB) have important applications. However, with conventional evaporation techniques, the films exhibit distinct columnar structure characteristics even when the substrate temperature is up to 300°C or more. This columnar structure of the film, because the film layer contains a large number of voids, results in a significant drift of the center wavelength of the filter as the film filter absorbs moisture and the refractive index of the film layer increases. To characterize this structural property, the aggregation density P has been proposed, which is defined as the ratio of the volume of the solid portion of the film to the total volume. So it is a physical quantity describing the degree of thin film sparsity.
With the development of ion coating techniques, such as ion-assisted deposition (IAD), reactive ion plating (RIP) and ion beam sputtering (IBS), the aggregation density of thin films has been significantly improved, and it has even been experimentally reported that some films have an aggregation density greater than 1. This means that the density of thin films is higher than that of bulk materials in nature, due to the fact that in films with high aggregation density, often exhibit larger compressive stresses, resulting in films with higher aggregation densities. However, even when the aggregation density of the film is greater than 1, the center wavelength of the filter still drifts. It has been recognized that it is not only the aggregation density but also the temperature refractive index coefficient and thermal expansion coefficient of the film and substrate that affect the drift of the center wavelength of the thin film filter. So the center wavelength drift of the filter can be simply expressed as Δλ = drift caused by moisture absorption in the film void + drift caused by temperature refractive index change + drift caused by thermal expansion.
Obviously, when ion technology is used to increase the aggregation density to 1, the central wavelength drift caused by moisture absorption is already negligible, and the other two factors rise to be the main factors. In this paper, we only focus on the relationship between the optical stability of the three-cavity filter composed of TiO2/SiO2 and the above three factors from the general process. The experimental results show that in the visible region, for a film system with an aggregation density of about 0.92, the central wavelength caused by moisture absorption is the largest of these three factors by an order of magnitude of about 10 nm. For the glued membrane system, the short shift of the central wavelength caused by the decrease of the refractive index of water vapor in the membrane system gap with the increase of temperature is about 1×10-2 nm/°C order of magnitude. And the drift caused by thermal expansion is about 1×10-3nm/°C order.

Drift caused by moisture absorption

Since the film is a columnar structure and there are voids between the columnar structures, the refractive index of the air in the voids before moisture absorption is 1. After moisture absorption, the voids are filled with water vapor and the refractive index becomes 1.333, thus the refractive index of the film layer, and thus the optical thickness and spectral properties are caused to change, which is the optical instability caused by moisture absorption.
By substituting the structure of our prepared film system (HLH2LHLHL)3 and the corresponding refractive indices, and according to our process conditions, the aggregation density of TiO2 and SiO2 is about 0.92, from which the corresponding drift of the central wavelength caused by moisture absorption can be calculated for different central wavelengths of red, green and blue filters. In the case of f=1 (i.e., complete moisture absorption).
The aggregation density of SiO2, a low refractive index material, plays a major role in the drift of the central wavelength in the case of moisture absorption. The difference in the central wavelength drift caused by the difference in the aggregation density of the high refractive index material is only about 1 nm, while the low refractive index material has a variation of about 3 nm. The reason is that after the low refractive index material absorbs moisture, the refractive index rises in a high proportion relative to the original refractive index, which corresponds to a large increase in optical thickness, resulting in a large drift. More importantly, SiO2 is used as the spacer layer of the film system, and the spacer layer has the greatest effect on the central wavelength drift.
In summary, the theory that the evaporation of water vapor that originally occupied the void within the film at elevated temperature leads to a short shift in the central wavelength can better explain the data obtained from our experiments, and it can be deduced from this that the aggregation density of our prepared SiO2 is approximately between 0.92 and 0.95. The theoretical analysis and the analysis of the process conditions are in agreement.

Temperature-induced drift

In addition to the drift of the central wavelength caused by moisture absorption, the change of refractive index of the film layer caused by the increase of temperature and the thickness change caused by the thermal expansion of the film system also cause the change of the optical thickness of the film layer, which leads to the drift of the central wavelength. In addition, because the coefficient of thermal expansion of the substrate is different from that of the film system, the film system will be subjected to elastic deformation by the stress of the substrate under the heat, which will lead to the change of the aggregation density and the drift of the central wavelength. The theory can be used to quantitatively analyze the central wavelength drift caused by the temperature rise. The main factors are the refractive index temperature coefficient of the material, the linear thermal expansion coefficient of the substrate, the Poisson’s ratio of the material, the linear thermal expansion coefficient of the film system, and the aggregation density of the film layer. There is a great lack of data on the temperature dependence of refractive index of various materials, especially for thin film form materials. It is reported in the literature that the temperature variation of refractive index of different materials varies greatly, for example, tellurides show negative values, while the refractive index of general materials increases with the increase of temperature. In our film system, the refractive index temperature coefficient of SiO2 plays a major role since it is SiO2 as a spacer layer. There are also refractive index temperature coefficients of fused silica in the infrared, which are about +1.1 × 10-5/°C at 1550 nm, but it is difficult to find data in the visible region. Based on the above data, we can infer that the refractive index temperature coefficient of SiO2 films in the visible region is about +0.5×10-5/°C. The coefficient of thermal expansion of the substrate is 74×10-7/°C in the range of -30 to 70°C and 86×10-7/°C in the range of 100 to 300°C for K9 glass. The coefficient of thermal expansion of the film system is around 5.5×10-7/℃, and Poisson’s ratio is taken as 0.1.
According to the above theoretical analysis and parameter setting, the temperature drift of the central wavelength of the green filter is calculated to be -0.00088 nm/℃ below 70℃, and above 100℃, the temperature drift of the central wavelength is -0.001459 nm/℃, and the values are slightly different for different color filters, but the magnitude is -1×10-3 nm/℃, and the temperature change of 10℃ The experimentally observed drift is in the order of 1 nm for both single and glued samples, so the above calculated result is not a major factor.
For the samples glued with two sheets, the aggregation density is not equal to 1, where the voids are mostly filled by water vapor, and after gluing, these water molecules are still present and cannot evaporate out of the film. According to the literature, the temperature change of refractive index of water relative to the film material is relatively large. it is on the order of 10-4/°C, one order of magnitude higher than SiO2, and the refractive index decreases faster as the temperature rises. For the aggregation density 0.9, the role of the refractive index temperature coefficient of water molecules and the role of the film layer material has been comparable, or even greater.
The refractive index of water from 20 ℃ to 80 ℃ fell about 0.01, according to the aggregation density of 0.9 to calculate the refractive index of water in the membrane layer caused by the decline in the refractive index temperature coefficient of the membrane layer -2 × 10-5 / ℃, it can be seen that it can completely offset the rise in the refractive index of SiO2 with the temperature, so that the whole membrane system presents a negative refractive index temperature coefficient, at this time the refractive index coefficient of the membrane system becomes – 1.5×10-5 nm/°C, and the temperature drift from room temperature to 70°C is -0.6 nm, which is in the same order of magnitude as the experimental result of 0~-2 nm. For the case above 70°C, there is no data on the change of refractive index of water, but considering that the refractive index of water gradually changes from liquid to gas after 100°C, the decrease of refractive index will be faster, so the short shift of the wavelength of the center of the glued filter with temperature can be reasonably explained from this perspective.
We believe that for unglued monolithic filters, the voids in the columnar structure of the film are almost completely filled with water molecules at room temperature, and when the temperature rises to 70°C, about 80%-90% of the water molecules in the columnar structure are evaporated out of the film, while the remaining 10-20% of water molecules are also evaporated out of the film at 70°C to 120°C. This results in a drift of the central wavelength from 70°C to 120°C. The value of this drift in the experimental data is between 1 and 2.5 nm, which is indeed about 1/5 of the drift value from room temperature to 70°C. The experiments also reflect that the drift from 100°C to 120°C is smaller than the drift in the range from 70°C to 100°C, which is also consistent with our analysis.


Through experiments on the drift of the center wavelength of red, green and blue bandpass filters under the influence of temperature, we have analyzed the causes of this drift. Three factors play a role in this. For unglued filters, the refractive index decrease caused by the evaporation of water molecules originally filled in the void of the film’s columnar structure with increasing temperature is the main factor, which causes a short shift of the central wavelength. This short shift varies with the aggregation density of the film. For a film system with an aggregation density of 0.92, the short shift is in the order of 10 nm. This desorption of moisture is most pronounced in the range of room temperature to 70°C, where 80% to 90% of the water is evaporated, and above 70°C, where the residual 10% to 20% of the water is also evaporated. For glued filters, the reason for the short shift of the central wavelength is that the refractive index of the water vapor filling the film voids decreases with the rise in temperature, and the rate of this decrease is much greater than the rate of the increment of the refractive index of the film material caused by the increase in temperature and the thermal expansion of the geometric thickness, thus causing a decrease in optical thickness and a short shift of the central wavelength. The magnitude of this short shift is about -1 × 10-2 nm/°C. Finally, for film systems with high aggregation density, the refractive index temperature coefficient of the material, and the thermal expansion coefficient of the substrate are important factors in determining the central wavelength drift. By calculation, for the visible range, the magnitude of this drift is around 1×10-3 nm/°C, and the direction is determined by the coefficient of thermal expansion of the substrate.
Based on the above analysis, measures to improve the temperature stability of the film system can be developed. First of all, increasing the aggregation density of the film system is one of the most important means. The increase of aggregation density reduces the influence of moisture absorption, which is the most influential factor on stability. Gluing the film between glass substrates is also a good measure, which can bring down the drift to the order of 10-2 nm/°C. In addition to increasing the aggregation density of the film, choosing materials with small refractive index temperature coefficients, or materials with opposite refractive index temperature coefficients to prepare the film system, and choosing substrates with appropriate thermal expansion coefficients is also a measure, which is especially important in the case of infrared and where the aggregation density is close to one.
SourceChina colorimeter manufacturer

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