filter

Devices used to select the required radiation band

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This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Filter is an optical device used to select the required radiation wave band. One common feature of filters is that no filter can celestial bodies The image becomes brighter because all filters absorb some wavelength , making the object darker.

Chinese name: filter
Foreign name: optical filter, light filter

Type: Optics
Purpose: Used to select the required radiation band
Purpose: Let the natural imaging Darker

catalog

type

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Color filter

This is in various colors plate glass Or gelatin sheet with transmission bandwidth of hundreds of angstroms, which is mostly used for broadband photometry or installed in fixed star In the spectrograph, to isolate overlapping spectral levels. Its main feature is that the size can be made quite large.

Thin film filter

Generally, the wavelength of transmission is long, and it is often used as Infrared filter 。 The latter is based on a certain chip.^[1]

principle

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Standard filter set

The filter is made of plastic or glass and special dyes. The red filter can only let red light pass through, and so on. Glassy transmissivity It used to be similar to air. All colored light can pass through, so it is transparent. However, after dyeing, the molecular structure changes, and the refractive index also changes. The passage of some colored light changes. For example, when a white light passes through a blue filter, it emits a blue light, while green light and red light are rare, and most of them are absorbed by the filter.

effect

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filter

The filter plays a very important role. It is widely used in photography. Why is the main scene always so prominent in the landscape paintings taken by some photography masters, and how is it achieved? This uses filters. For example, if you want to take a picture of a yellow flower with a blue sky and green leaves as the background, you can't highlight the theme of "yellow flower" if you take it as usual, because the image of yellow flower is not prominent enough. However, if a yellow filter is placed in front of the lens to block a part of green light scattered by green leaves and blue light scattered by blue sky, and a large amount of yellow light scattered by yellow flowers passes through, yellow flowers will appear very obvious, highlighting the theme of "yellow flowers".^[2]

characteristic

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Its main feature is that the size can be made quite large. Thin film filter, usually with a longer wavelength, is used as an infrared filter. The latter is based on a certain substrate vacuum The coating method alternately forms a metal medium metal film or all dielectric film with a certain thickness of high refractive index or low refractive index to form a low order, multi-stage series solid Fabry Perot interferometer. The selection of material, thickness and series connection mode of the film is determined by the required central wavelength and transmission bandwidth λ.

wavelength

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Any wavelength from ultraviolet to infrared, λ 1~500 angstrom Interference filter 。 Peak value of metal dielectric film filter transmissivity It is not as high as all dielectric film, but the second peak and side band of the latter are more serious. There is also a circular or long strip variable interference filter in the film interference filter, which is suitable for space Astrometry 。 In addition, there is also a bicolor filter, which is light beam Placed at an angle of 45 °, it can decompose the light beam into two different colors with high and uniform reflection and transmission, and is suitable for multi-channel Multicolor photometry 。 Interference filter generally requires vertical incidence, when Angle of incidence When it increases, it moves in the direction of short wave.

This feature can be used to adjust the central wavelength within a certain range. Due to λ and peak value Transmissivity Both significantly change with temperature and time Narrowband filter You must be very careful. Since it is difficult to obtain a large uniform film, the diameter of the interference filter is generally less than 50 mm. Someone once used the splicing method to obtain interference filters as large as 38cm square, which were installed in a British aperture of 1.2m Schmidt telescope It is used to take monochromatic images of large nebulae.

device

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Synchronization function

This technology can control the camera, infrared light, filter, color to black synchronous switching. The stability has the functions of automatic positioning and anti shake, and the light will not flash when it is at the zero point. Fast switch to the right place in one step, and no midway failure resistance Stuck and pause, resulting in filter offset. The filter will not shift due to changes and vibration such as the rotation and stop of the PTZ. It will not bounce due to collision during high-speed switching, resulting in inaccurate positioning of the filter position.^[3]

Image color restoration

filter

Crystal filter can solve the problems of false color and color floating to the maximum extent. Add AR-COOTRMG heavy film on the crystal, which can achieve 98% light penetration. Switching to the crystal filter state during the day can sense the visible light and prevent infrared and other light interference, which is bright and lifelike. Switching to the filter coated with a transparent film at night can achieve 100% light penetration. The camera senses more infrared rays, and most wavelengths of light can pass through. The camera turns black at the same time, so the infrared distance is farther and clearer.

classification

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Filter products are mainly classified according to spectral band, spectral characteristics, film materials, application characteristics, etc.

Spectral band : UV filter, visible filter, infrared filter;

Spectral characteristic : band-pass filter Cut-off filter , spectroscopic filter, neutral density filter, reflection filter;

Film material : Soft film filter, hard film filter;

Hard film filter not only refers to film hardness, but also its laser damage threshold Therefore, it is widely used in laser systems, and the surface soft film filter is mainly used for Biochemical analyzer among.

Bandpass type : The light of the selected band passes through, and the light outside the passband is cut off. Its optical indexes are mainly central wavelength (CWL) and half bandwidth (FWHM). It is divided into narrowband and broadband. For example, narrowband 808 filter NBF-808.

Short wave pass type (also called low wave pass): Light shorter than the selected wavelength passes through, and light longer than the selected wavelength is cut off. For example, infrared Cut-off filter ，IBG-650。

Long wave pass type (also called high wave pass): The light longer than the selected wavelength passes through, and the light cutoff shorter than the selected wavelength, such as the infrared transmission filter, IPG-800.

Thin film filter

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use electron beam (EB) Evaporated TiO two And SiO two Thin film systems have important applications. However, with conventional evaporation technology, even if the temperature of the substrate is higher than 300 ℃, the film still shows obvious columnar structure characteristics. The columnar structure of the film contains a large number of gaps, so as the film filter absorbs moisture, the film Refractive index The central wavelength of the filter will drift significantly when it rises. In order to characterize this structural property, the aggregation density P is proposed, which is defined as the ratio of the volume of the solid part to the total volume of the film. Therefore, it is a physical quantity describing the degree of film porosity.

With the development of ion plating technology, such as ion assisted deposition (IAD), Reactive ion plating (RIP) and Ion beam sputtering (IBS), etc., the aggregation density of the films has been significantly improved, and even there have been experimental reports that the aggregation density of some films is greater than 1. This means that the density of the film is higher than that of the bulk material in nature. The reason is that in the film with high aggregation density, there is often a greater compressive stress, resulting in a higher aggregation density of the film. However, even if the concentration density of the film is greater than 1, the central wavelength of the filter will still drift. It has been recognized that not only the concentration density, but also the temperature refractive index coefficient and Coefficient of thermal expansion 。 Therefore, the central wavelength drift of the filter can be simply expressed as Δλ=drift caused by moisture absorption in the film gap+drift caused by temperature refractive index change+ thermal expansion Drift caused by.

Obviously, when the concentration density is increased to 1 by ion technology, the central wavelength shift caused by moisture absorption can be ignored, while the other two factors rise to be the main factors. This article focuses on the investigation of TiO from the general process two /SiO two The relationship between the optical stability of the three cavity filter and the above three factors. The experimental results show that in the visible light area, for the film system with an aggregation density of about 0.92, the central wavelength caused by moisture absorption is the largest among the three factors, Order of magnitude At about 10 nm. For the glued film system, the central wavelength shift caused by the decrease of water vapor refractive index in the gap of the film system with the rise of temperature is about 1 × 10 -2 Nm/℃. The drift caused by thermal expansion is about 1 × 10 -3 Nm/℃.

Drift caused by moisture absorption

Table 1 Calculated values of central wavelength drift caused by moisture absorption effect of different aggregate densities of materials

because film It is a columnar structure, and there are gaps between columnar structures Refractive index 1. After moisture absorption, the void is filled with water vapor, and the refractive index becomes 1.333. Therefore, the refractive index of the film, and then the optical thickness and spectral characteristics all change, which is the optical instability caused by moisture absorption.

The prepared membrane structure (HLH2LHLHL) three And the corresponding refractive index. According to our process conditions, TiO two And SiO two The concentration density of is about 0.92, so for red, green and blue filters with different center wavelengths, the corresponding center wavelength drift caused by moisture absorption can be calculated. In the case of f=1 (i.e. full moisture absorption), for TiO two And SiO two See Table 1 for a series of center wavelength shifts calculated for different aggregation densities.

It can be seen from the table that low refractive index material SiO under moisture absorption two The central wavelength drift is mainly affected by the concentration density of. The difference of center wavelength shift caused by different aggregation density of high refractive index materials is only about 1 nm, while that of low refractive index materials is 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 is equivalent to a large proportion of the increase in optical thickness, resulting in a large drift. More importantly, SiO two As the spacer layer of the film system, the spacer layer has the greatest impact on the central wavelength shift.

In conclusion, the theory that the evaporation of the water vapor that originally occupied the void in the film with temperature rise leads to the short shift of the central wavelength can better explain the data obtained in our experiment, and we can deduce the SiO prepared by us two The aggregation density is about 0.92~0.95. The theoretical analysis is consistent with the analysis of process conditions.

Temperature induced drift

Table 2 Refractive index temperature coefficient of quartz crystal

Table 3 Variation of refractive index of water at different temperatures with wavelength

In addition to the central wavelength shift caused by moisture absorption, the change of the refractive index of the film caused by temperature rise, and the film system thermal expansion The resulting thickness change will also cause the change of the optical thickness of the film, resulting in the drift of the central wavelength. Moreover, because the thermal expansion coefficient of the substrate is different from that of the film system, the film system will undergo elastic deformation under the effect of substrate stress when heated, which will change the aggregation density and lead to the drift of the central wavelength. The theory can be used to quantitatively analyze the central wavelength shift caused by 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. There is a lack of data about the refractive index of various materials changing with temperature, especially for thin film materials. According to literature reports, the refractive index temperature of different materials varies greatly, for example, telluride shows a negative value, while the refractive index of general materials increases with temperature. In our film system, due to SiO two As a spacer layer, SiO two Of Temperature coefficient of refractive index Play a major role. There are crystal quartz in the literature visible light See Table 2 for the refractive index of o light and e light within the range. There is also the temperature coefficient of refractive index of fused quartz in infrared, which is about+1.1 × 10 at 1550 nm -5 /℃, but it is difficult to find the data in the visible light area. According to the above data, we can infer the visible light region SiO two The refractive index temperature coefficient of the film is about+0.5 × 10 -5 /℃. The thermal expansion coefficient of the base plate is 74 × 10 for K9 glass in the range of - 30~70 ℃ -7 /℃, 86 × 10 in the range of 100~300 ℃ -7 /℃。 The thermal expansion coefficient of the film system is 5.5 × 10 -7 /℃, Poisson's ratio is 0.1.

According to the above theoretical analysis and parameter setting, it is calculated that the temperature drift of the central wavelength of the green filter is -0.00088 nm/℃ below 70 ℃, and -0.001459 nm/℃ above 100 ℃. For filters of different colors, the values are slightly different, but the magnitude is - 1 × 10 -3 The temperature change of nm/℃ and 10 ℃ will only cause - 10 -2 The drift in the order of nm, while the drift observed in the experiment is in the order of 1 nm for both monolithic and glued samples, so the results of the above calculation are not the main factor.

For the double glued sample, when the aggregation density is not equal to 1, most of the voids are filled by water vapor. After gluing, these water molecules still exist and cannot evaporate out of the film. According to the literature, the refractive index temperature of water changes relatively Film material It is relatively large. See Table 3 for specific data. Its magnitude is 10 -4 /℃, compared with SiO two The refractive index decreases faster with the increase of temperature. For the aggregation density of 0.9, the effect of temperature coefficient of refractive index of water molecule is comparable to that of film material, or even greater.

From the table, we can see that the refractive index of water decreases by about 0.01 from 20 ℃ to 80 ℃. According to the aggregation density of 0.9, the temperature coefficient of the refractive index of the film is - 2 × 10 due to the decrease of the refractive index of water in the film -5 /℃, which can completely offset SiO two The rise of refractive index with temperature makes the whole film system present negative refractive index temperature coefficient At this time Refractive index The coefficient becomes - 1.5 × 10 -5 Nm/℃, the temperature drift from room temperature to 70 ℃ is -0.6 nm, which is in the same order of magnitude as the experimental result of 0~- 2 nm. For the temperature above 70 ℃, there is no data on the refractive index change of water. However, considering that the refractive index will decline faster when water gradually changes from liquid to gas after 100 ℃, it can reasonably explain the short shift of the central wavelength of the glued filter with temperature from this perspective.

We believe that, for an un glued single filter, the gaps in the cylindrical structure of the film are almost completely filled by water molecules at room temperature. When the temperature rises to 70 ℃, about 80%~90% of the water molecules in the cylindrical structure are evaporated out of the film, and when the temperature rises to 70 ℃, the remaining 10~20% of the water molecules are also evaporated out of the film. As a result, the central wavelength shifts from 70 ℃ to 120 ℃. In the experimental data, the drift value is between 1 and 2.5 nm, which is about 1/5 of the drift value from room temperature to 70 ℃. The experiment also shows that the drift from 100 ℃ to 120 ℃ is less than the drift from 70 ℃ to 100 ℃, which also conforms to our analysis.

research conclusion

Through the experiments of red, green and blue band-pass filters under the influence of temperature, we have analyzed the reasons for this drift. Three factors play a role. For the un glued filter, the main factor is the decrease of refractive index caused by the evaporation of water molecules originally filled in the pores of the film columnar structure with the increase of temperature, which causes the short shift of the central wavelength. This short shift varies with the density of the film. For the film system with an aggregation density of 0.92, the value of short shift is on the order of 10 nm. This process of moisture desorption is most obvious in the range of room temperature to 70 ℃, 80% to 90% of the water is evaporated, and above 70 ℃, the remaining 10% to 20% of the water is also evaporated. For the glued filter, the reason for the short shift of the central wavelength is that the refractive index of the water vapor filling the film gap decreases with the increase of temperature, and the rate of this decline is far greater than the rate of the increase of the refractive index of the film material with the increase of temperature and the thermal expansion of the geometric thickness. Therefore, the optical thickness decreases and the central wavelength shifts short. The magnitude of this short shift is about - 1 × 10 -2 nm/℃。 Finally, for the film system with high density, the refractive index of the material temperature coefficient The thermal expansion coefficient of the substrate is an important factor determining the central wavelength shift. By calculation, for visible light The magnitude of this drift is 1 × 10 -3 Nm/℃, and the direction is determined by the thermal expansion coefficient of the substrate.

According to the above analysis, measures can be taken to improve the temperature stability of the film system. First of all, improving the aggregation density of the film system is the most important means. The increase of aggregation density reduces the influence of moisture absorption, which is the biggest factor affecting the stability. Gluing the film between glass substrates is also a good measure, which can reduce the drift to 10 -2 Nm/℃. In addition to improving the aggregation density of the film, select materials with small refractive index temperature coefficient or materials with opposite refractive index temperature coefficient to prepare the film system, and select a substrate with appropriate thermal expansion coefficient is also one of the measures, which is particularly important in the case of infrared and close to one aggregation density.^[4]

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