The red light filter can only be made of plastic or special dye, and so on. The refractive index of the glass sheet is similar to that of air, and all color light can pass through, so it is transparent.
However, after dyeing with dye, the molecular structure changes, the refractive index also changes, and the passage of some color light changes. For example, when a white light passes through a blue filter, it emits a blue light, while there are very few green and red light, and most of them are absorbed by the filter.
The filter plays a great role. Widely used in photography. Why is the main scene of some landscape paintings taken by photography masters always so prominent and how to do it? This uses filters. For example, if you want to shoot a yellow flower with blue sky and green leaves in the background, you can’t highlight the theme of “yellow flower” if you shoot it as usual, because the image of yellow flower is not prominent enough.
However, if you put a yellow filter in front of the lens to block the green light emitted by some green leaves and the blue light emitted by the blue sky, and let the yellow light emitted by the yellow flower pass through in large quantities, the yellow flower will be very obvious and highlight the theme of “yellow flower”.
Thin film filter is divided into thin film absorption filter and thin film interference filter. The former is to use chemical etching on a specific material substrate to make the absorption line just at the required wavelength.
Generally, the transmission wavelength is long and is mostly used as infrared filter. The latter is to alternately form a metal dielectric metal film or an all dielectric film with a certain thickness with high refractive index or low refractive index on a certain substrate by vacuum coating method, so as to form a low-level and multistage series solid Fabry Perot interferometer. The choice of film material, thickness and series mode depends on the required central wavelength and transmission bandwidth λ determine.
The peak transmittance of metal dielectric film filter is not as high as that of full dielectric film, but the secondary peak and sideband problems of the latter are more serious. There is also a circular or strip-shaped variable interference filter in the thin-film interference filter, which is suitable for space astronomical measurement.
In addition, there is a two-color filter, which is placed at an angle of 45 ° with the incident beam. It can decompose the beam into two different colors with mutually perpendicular directions with high and uniform reflection and transmittance. It is suitable for multi-channel and multi-color photometry. Interference filters generally require vertical incidence. When the incidence angle increases, they move to the short wave direction. This feature can be used to adjust the central wavelength in a certain range. Due to λ And peak transmittance vary significantly with temperature and time.
Care must be taken when using narrow-band filters. Because it is difficult to obtain large-scale uniform film, the diameter of interference filter is generally less than 50mm. Some people have used the splicing method to obtain interference filters as large as 38 cm square, which are installed on the British 1.2-meter Schmidt telescope to take monochromatic images of large-area nebulae. This technology can control the synchronous switching of camera, infrared lamp, filter and color to black. Stability has the functions of automatic positioning and anti shake. When the light is at the zero boundary point, it will not flicker. Fast switching in one step, without stopping due to resistance, resulting in filter deviation.
The filter will not shift due to changes and vibration such as PTZ rotation and stop. When switching at high speed, it will rebound due to collision, resulting in inaccurate positioning of filter position.
Image color restoration function of filter. Crystal filter can solve the problems of false color and color floating to the greatest extent. Adding ar-cootrmg heavy film on the crystal can achieve 98% light penetration. Switching to crystal filter during the day can well sense visible light and prevent infrared and other light interference. It is bright and lifelike. Switching to filter coated with 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 longer and clearer.
