Optical filters
2022-01-24
Filter is an optical device used to select the required radiation band. A common feature of filters is that no filter can make the imaging of celestial bodies brighter, because all filters will absorb certain wavelengths, making objects darker.
Color filter
These are flat glass or gelatin sheets of various colors with a transmission bandwidth of hundreds of Angstroms. They are mostly used in broadband photometry or installed in stellar spectrograph to isolate overlapping spectral levels. Its main feature is that the size can be made quite large.
Thin film filter
Generally, the transmission wavelength is long and is mostly used as infrared filter. The latter is on a certain film base.
Principle
The filter is made of plastic or glass with special dyes. The red filter can only let the red light pass through, and so on. The transmittance of glass sheet is similar to that of air, and all colored 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 colored 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.
Effect
The filter plays a great role. Widely used in photography. Why is the main scene always so prominent in the landscape paintings taken by some photographers and how to do it? This uses filters. For example, if you want to take a yellow flower with a camera, the background is blue sky and green leaves. If you take it as usual, you can't highlight the theme of "yellow flower", because the image of yellow flower is not prominent enough. However, if a yellow filter is placed in front of the lens to block the green light scattered by some green leaves and the blue light scattered by the blue sky, and let a large number of yellow light scattered by yellow flowers pass through, the yellow flowers will be very obvious and highlight the theme of "yellow flowers"
Characteristic
Its main feature is that the size can be made quite large. Thin film filters are generally used as infrared filters because of their long transmission wavelength. The latter is a low-level, multistage series solid Fabry Perot interferometer by alternately forming metal dielectric metal films or all dielectric films with a certain thickness with high refractive index or low refractive index on a certain substrate by vacuum coating method. The choice of film material, thickness and series mode depends on the required central wavelength and transmission bandwidth λ determine.
Wavelength
Capable of any wavelength from ultraviolet to infrared λ Various interference filters of 1 ~ 500 Angstroms. 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 and can decompose the beam into two different colors with mutually perpendicular directions with high and uniform reflection and transmittance, which is suitable for multi-channel and multi-color photometry. The interference filter is generally required to be incident vertically. When the incident angle increases, it moves to the short wave direction.
This feature can be used to adjust the central wavelength in a certain range. because 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-size 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.
