Optical filters are sheet-like optical elements with filtering effects. They can effectively filter, split, attenuate and reflect optics. In different application fields, the purpose of filtering is not just for simple filtering. Its “filtering” function can vary according to the actual application situation.
Categorical features
From the overall classification, they can generally be divided into color filters and coated filters. Color filters are generally made by adding special dyes to give them optical properties, and coated filters obtain certain optical screening properties through their special structural film layer materials.
From the spectrum band, it is generally divided into:
Ultraviolet filter 180-400nm
Visible light filter 400-700nm
Infrared light 700nm-14μm
Infrared filters can also be divided into near infrared 700nm-1.5μm, mid-infrared 1.5-6μm, far infrared 6-14μm
This is a spectrum classification formed by the production of filters, which is slightly different from our light bands. These differences are specifically related to the base material and coating process of the filter.
Spectral classification is easy to understand, that is, it is used to filter specific bands within the spectrum range of this type. For example, ultraviolet filters are used to filter light in a specific ultraviolet range, and the same is true for visible light and infrared.
According to the spectral type, they are generally divided into: bandpass filters, cutoff filters, notch filters (also called band-stop filters, negative filters).
Bandpass filters, that is, they have one (single bandpass) or multiple (multi-bandpass) passbands in a certain range in the spectrum, mainly based on the constructive interference condition of the Fabry-Perot cavity, which allows the central wavelength and the small range on both sides of the central wavelength to be effectively transmitted, while the light outside the passband will be blocked from transmission.
According to the size of the passband range, we generally divide it into broadband filters and narrowband filters. In general, narrowband filters need to be emphasized separately, and their passband range is less than 5% of their central wavelength. Broadband filters generally have a wider adaptability, while narrowband filters have higher precision. The smaller the passband range, the more accurate it is.
Cut-off filter. Cut-off filter generally refers to a filter that cuts off light above or below a specific range. The filter that passes shortwave light and cuts off longwave light is generally called a shortwave pass filter; the filter that passes longwave light and cuts off shortwave light is generally called a longwave pass filter.
The most common short-wave filter is the infrared cut-off filter in our mobile phone lens, which is a short-wave filter that cuts off infrared light. The second is security monitoring, where the infrared cut-off filter can effectively block the impact of daytime infrared light on digital image sensors. The second is the common long-wave filter in life, such as the home photon beauty instrument, which is equipped with a filter for long-wave therapeutic beauty light. Of course, some targeted beauty uses a bandpass filter.
According to the functional principle, they are generally divided into: absorption filters, reflection filters, spectroscopic filters, neutral density filters (also called ND attenuation filters), polarization filters, and interference filters.
Absorption filters are formed by adding special dyes or light-absorbing substances to resin materials or glass materials during the production process to form filters with a certain ability to absorb light. Common representatives of absorption filters include colored glass and neutral density filters;
Colored glass is a common form of absorption filter. By adding colorants such as certain colored metal oxides to the glass, the glass has a strong absorption capacity for light in a certain wavelength range, while the transmitted light is monochromatic. This filter is widely used in photography, display and other fields.
Neutral density filters, also called ND filters, are usually made of multiple layers of thin films and can evenly absorb all wavelengths of light. The intensity of the light transmitted is about 1/n of the input light (n is the ND value). This filter is mainly used to adjust the light intensity and is widely used in photography, videography and other fields.
(650nm reflector)
Reflective filter, the biggest feature of the reflective filter is its high reflection. The biggest feature of the reflective filter is its internal reflector structure. This structure enables the filter to show high reflection in a specific wavelength range, and high transmission in other wavelength ranges.
(1550nm reflector)
For example, a common reflective filter that reflects in a fixed wavelength band, such as a 1550nm reflector, has high reflection in a certain range of 1550nm, but may have high transmittance in other wavelength ranges. This characteristic makes the 1550nm reflector widely used in laser systems, optical communications and other fields, such as output coupling mirrors for lasers, optical isolators, etc.
Spectroscopic filters, the most common products of spectroscopic filters are dichroic mirrors and some proportions of spectroscopic mirrors (such as 5:5 spectroscopic mirrors – half-transparent and half-reflective). Dichroic mirrors, also known as two-color mirrors, are often used in laser technology. Its characteristics are that it almost completely transmits light of a certain wavelength, and almost completely reflects light of other wavelengths. It uses a hard film production process to isolate and identify specific fluorophore analysis, and can change the direction of the optical path of a specific laser to split and combine the light to save energy and improve the accuracy of the equipment. Its characteristics are small laser loss, high projection, high reflectivity, and accurate wavelength positioning.
(Transmit blue-green and reflect red dichroic mirror)
Polarizing filters, also called polarizing films, work in a similar way to Venetian blinds. They can selectively allow light vibrating in a certain direction to pass through. This property makes polarizing films very effective in changing the balance of light and reducing glare. In color and black-and-white photography, they are often used to eliminate or reduce strong reflections from non-metallic surfaces, thereby eliminating or reducing light spots. For example, in landscape and scenery photography, they are often used to show the texture of objects in strong reflections, highlight the scenery behind the glass, darken the sky, and show blue sky and white clouds.
Interference filter is an optical film that uses the thin film interference principle of light to only allow light in a specific spectral range to pass through. It is usually composed of multiple layers of thin films. When light passes through the multi-layer thin film structure of the interference filter, light of different wavelengths reflects and interferes between the film layers. This interference causes light waves of specific wavelengths to enhance each other, while light waves of other wavelengths cancel each other out. In this way, only wavelengths that meet specific conditions can pass through the filter, while light of other wavelengths is reflected or absorbed.
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Based on the film type classification, we can simply divide it into soft film filters and hard film filters.
A soft film filter is an optical element composed of multiple layers of thin films, which may be made of materials such as zinc sulfide, cryogen, or even silver. These films have relatively low hardness, and by precisely controlling the material, thickness, and structure of the film layers, the transmission characteristics of light waves can be precisely controlled. When light passes through a soft film filter, light waves of different wavelengths will interfere and diffract in the film layers, causing light waves of specific wavelengths to be reflected, absorbed, or transmitted.
(Sapphire filter)
A hardcoat filter is an optical element with a special film layer that has high hardness and laser damage threshold. This high hardness allows the hardcoat filter to withstand high-intensity laser irradiation without being easily damaged, so it is widely used in laser system applications. The film layer of a hardcoat filter is usually composed of multiple layers of special materials, which are precisely designed and prepared to achieve precise control of specific wavelength lasers. When the laser passes through the hardcoat filter, the film layer absorbs, reflects, transmits or scatters the laser, thereby achieving control of the laser spectrum.
Based on the substrate type, we can simply divide them into two categories, plastic-based filters and glass-based filters, followed by some single crystal substrates, etc. Different substrate structures also lead to the optimal parameters of the filter performance after coating.
(Infrared acrylic filter)
Plastic filters are based on polycarbonate, polyethylene and other plastics. They are light, low cost, easy to process and mold, and are particularly suitable for large-scale production. However, they have low temperature resistance and chemical stability, and limited transmittance. They are easily affected by the environment and age, and may need to be replaced frequently. They are suitable for short-term use or low-intensity applications, such as outdoor shooting, toys, decorations, etc.
(Heat absorbing glass)
Glass filters are based on optical glass. Due to different glass grades (such as BK7, K9, etc.), their properties are also different. Glass-coated filters have excellent optical properties and chemical stability, are wear-resistant and corrosion-resistant, can be used for a long time in harsh environments, are not easily affected by temperature changes, and have ideal high transmittance in the visible and near-infrared bands, which can meet the needs of most optical applications.
Different substrates have their own applicability and shortcomings. For example, in far-infrared applications, the defects of glass substrates will also overflow. At this time, it is necessary to choose other alternative substrates, such as silicon and germanium.
| Substrate Name | Applicable band range (μm) | Main features and applications |
| ZnS | 0.39~13 | It has high transparency and thermal conductivity, and is suitable for making infrared windows and lenses. It is also used in the manufacture of deep ultraviolet and short-wavelength ultraviolet light sources. |
| ZnSe | 0.614（or 0.515） | Yellow transparent polycrystalline material with high transparency and thermal conductivity, suitable for infrared optical components, especially optical devices in high power CO2 laser systems. |
| CaF₂ | 0.15~8.5（or 0.11~8.5）or Visible light（0.4~0.7）、NIR（0.7~1）、UV（0.2~0.4） | It has high refractive index uniformity and low dispersion, and has good light transmittance in the ultraviolet, visible light to mid-infrared regions. It is suitable for optical components such as infrared lenses and windows. |
| Ge | 212（or 12、214、14300） | It has a wide light transmittance range, covering a wide band from short-wave infrared to long-wave infrared, and is widely used in infrared imaging systems and infrared spectrometer systems. |
| Si | 1.2~8 | The transmittance range in the infrared band is mainly concentrated between 1.2 microns and 8 microns, among which 1.24μm is the near-infrared band and 48μm is the far-infrared band, which is suitable for infrared sensors, infrared imaging systems, etc. |
| sapphire | 0.15~6 | The optical penetration band is very wide, covering the ultraviolet, visible, near-infrared and mid-infrared bands, and has excellent optical transmittance performance in the range of 1.0~5.5μm. |
| Optical Glass | 0.35~2.5 | It has stable optical properties and high light transmittance, and can provide stable filtering effects over a wide wavelength range. It is often used in thermal imaging systems, infrared sensors, infrared cameras and other equipment. |
| Resin (such as PC, PMMA) | Depends on the type of resin and formulation, usually covers visible light and part of near infrared | Easy to process, suitable for general mass-produced applications such as infrared filters. |
This is a brief introduction to the understanding of filters. For more exciting content, please pay attention to the latest updates in our news section!
