Optical filters are ubiquitous in our daily lives, from precision and optical equipment, display devices to optical thin film applications in everyday life; For example, the glasses, digital cameras, various household appliances, infrared sensing devices, and applications in autonomous vehicles that we usually wear are all manifestations of the application of optical thin film technology products.
Filter products are mainly classified according to spectral bands, spectral characteristics, film materials, and application features.
The principle of filter:
A filter is made of plastic or glass with special dyes added. A red filter can only allow red light to pass through, and so on. The transmittance of glass sheets was originally similar to that of air, allowing all colored light to pass through, making them transparent. However, after dyeing, the molecular structure changes and the refractive index also changes, resulting in changes in the passage of certain colored light. For example, a beam of white light passing through a blue filter emits a beam of blue light, while green and red light are very rare and mostly absorbed by the filter.
Characteristics of filter:
Its main feature is that the size can be made quite large. Thin film filter, with a longer wavelength of transmission, is commonly used as an infrared filter. The latter is a low order, multi-stage series solid Fabry Perot interferometer formed by alternately forming metal dielectric metal films or all dielectric films with a certain thickness on a certain substrate using vacuum coating method. The selection of material, thickness, and series connection method for the membrane layer is determined by the required center wavelength and transmission bandwidth λ.
Spectral band of filter:
UV filter: Its main feature is to allow light with a certain bandwidth near a certain wavelength (wavelength less than 400nm) to pass through, while cutting off light in other ranges.
The visible filter and visible light range from 400nm to 700nm, which can be cut off in the visible light band or highly transmitted in the visible light band. It can be customized and produced according to specific needs.
Infrared filter: Its main feature is the absorption of infrared rays by the infrared band absorption plate, and the penetration of visible light. It is widely used in monitoring systems, infrared devices, automatic optical detection equipment, imaging equipment, monitoring systems, counterfeit inspection equipment, infrared cameras, and other fields.
Spectral characteristics of filters: bandpass filter, cutoff filter, spectral filter, neutral density filter, reflective filter; Film layer materials for filter: soft film filter, hard film filter; Hard film filter not only refers to the hardness of the thin film, but more importantly, its laser damage threshold, so it is widely used in laser systems, while soft film filter is mainly used in biochemical analyzers. Filters are divided into color filters (flat glass or gelatin sheets of various colors, with a transmission bandwidth of several hundred angstroms, often used in broadband photometry or installed in stellar spectrometers to isolate overlapping spectral levels) and thin film filters (with longer transmission wavelengths, often used as infrared filters)
