What are Filters? Let’s Learn about 7 Common Filters and Their Applications!

With a variety of research and development, optical technology is advancing, where filters play an important role in various fields such as spectral analysis, imaging technology, communications and environmental monitoring, etc. In this article, we will lead you to learn about filters and their principles, and introduce common types and applications of filters, drawing you a brand-new picture of them!
 

What are filters? What are the principles of filters?

A filter is an optical element with specific materials that selectively absorbs, transmits or blocks specific wavelengths and bands, enhancing the intensity of optical signals or adjusting the color effect and spectrum. Typical applications of filters are detectors, optical instruments, as well as various consumer electronics such as video cameras, cameras, projectors and other common appliances.
 

What types of filters are available?

Short Pass Filters (SPFs)

SPFs are designed to allow shorter wavelengths to pass through and selectively block or reflect unwanted wavelengths. Common SPF types are IR cut filters that reduce infrared interference, polarizing films that change the brightness and darkness of light and also convert unpolarized light into polarized one, and polarized filters that create excellent contrast, outstanding resolution, and wide viewing angles. Common SPF applications are lenses of video equipment and surveillance cameras, cold light lamps, LCD panels, etc.

What is polarized light?
Natural light is converted into polarization light when certain vibration direction of the light is selectively filtered or limited, typically making vibration of the polarized light limited to a specific direction. Common types of polarized light are linear, elliptical and circular polarization.

Long Pass Filters (LPFs)

LPFs are mainly designed to allow longer wavelengths to pass through and filter or reflect shorter wavelengths. Typically, they are used in conjunction with SPFs to meet a wide variety of optical needs. Common LPF types are: cold mirrors that are made of IR pass filters that allow IR wavelength or heat to pass through, blue-light cut filters that reflect blue light at wavelengths of 400 to 500nm, and UV cut filters that block ultraviolet light. LPFs are widely used in various applications, including Illuminating devices, medical devices, optical instruments, optical communication technologies, optical/quantum computers, etc. LPF customizations are available from some manufacturers based on customers’specifications.
 

Neutral Density Filters (NDFs)

NDFs are mainly designed to attenuate a wider spectral range of light, and can be categorized into absorbing and reflecting types, which are characterized by high transmittance, excellent spectral flatness, high accuracy and neutral within a visible spectrum. Absorptive NDFs are useful in the light control applications, such as instruments and measurement equipment with exposure energy controls. On the other hand, reflective NDFs will be avoided to use in high-power equipment such as lasers and photometers, which may cause device damages or inaccurate measurements.
 

Bandpass Filters (BPFs)

BPFs are designed to allow light of certain wavelengths or bands to pass through and filter the rest light. They have different optical properties mainly defined by their center wavelength and half-bandwidth, and can be categorized into narrow bandpass filters (bandwidth <30nm) and wide bandpass filters (bandwidth >60nm) based on their bandwidth. Typically, a combination of high and low BPFs is used in a variety of industries such as biomedical, optical, agricultural, or those that require the use of detectors.
 

Notch Filters (NFs)

NFs are also known as band-stop filters, and designed to reflect light of specific wavelengths and bands and allow light of other wavelengths and bands or light within the upper and lower frequency limits to pass through, thus effectively attenuates light in a specific range. Their principles of operation are exactly the opposite of BPFs, and they are very suitable for products that require to filter out light sources of specific bands, such as lasers.
 

Monochromatic Filters (MFs)

A MF is designed to allow light of a single wavelength or a specific range of wavelengths to pass through while blocking light of rest wavelengths. Typical MFs include UV cut filters, blue light cut filters, IR cut filters and VIS cut filters of specific colors, etc. Depending on applications and needs, MF customizations are available for color adjustment of screen, selection of a single wavelength, and spectral analysis.
 

Color Filters (CFs)

CFs are optical components designed to produce graphics or color images, typically being composed of photoresist, ITO conductive film and glass substrate with red, green and blue colors. When light passes through a CF, color information will be carried. As a result, CFs are typically used in devices such as electronic papers, organic displays (OLEDs), flat panel displays, and liquid crystal displays (LCDs).
 

Filter Applications

Lenses of Cameras

Filters used in lenses of cameras help photographers control the properties and colors of light. There are many different types of filters. For example, polarized filters and neutral density filters are used to eliminate reflected light or reduce the amount of incoming light for long exposures, creating special effects; color filters enhance the saturation of specific colors; and other common filters like UV protectors that are ultraviolet light blocked, fingerprint-proof, and wear-resistant.
 

Optical Instruments

There are a wide range of types and applications of optical instruments, such as microscopes, spectroscopes, telescopes, optical glasses and spectrometers, etc., which require filters to improve performance or achieve specific optical functions. For example, optical filters are used in a spectroscope to selectively analyze light of a specific wavelength, polarized filters are used in a microscope to observe the crystal structures, and some customized coated filters can even achieve high or low pass performance to improve resolution and accuracy.

〈Extended reading：What is the Principle of Optical Coatings?〉
 

Illuminating Devices

In Illuminating devices, color filters are the most commonly used for change and adjustment of color temperature, color and intensity of the light source, or color separation films are used for blocking transmittance of specific colors to create different visual effects.
 

Panels for 3C Products

Typically, color filters are used in the display panels, liquid crystal displays (LCDs) and organic light emitting diodes displays (OLEDs) for 3C products (i.e. computers, communications and other consumer electronics) to present colorful images by controlling the light transmittance of different color channels. In addition, in order to protect consumers from eye discomfort caused by watching 3C displays for a long time, blue light filtering displays have been launched to filter out the blue light that can cause damages to the eyes.
 

Detectors for Industries

A wide range of filters are used in industries, such as science, environmental monitoring, industrial testing, and the defense industry, for selective absorption or block of specific spectral bands, or designation of the incidence angle of light (AOI). They are not only useful in applications of analysis, measurement or detection, but also directly affect the sensitivity, signal, and accuracy of the detectors.
 

Conclusion

With the advancement of filters and their manufacturing substrates, there are more innovative functions being developed, while related products become more precise and accurate. YANG EN TECH CO., LTD. provides a variety of optical film designs and process integration services such as filter processing and manufacturing, glass processing and glass finishing, and coated film processing. Equipped with professional manufacturing machines and fixtures from famous domestic and foreign manufacturers, incorporating our extensive practical experience and rigorous product testing standards, we are confident to provide customers with high-quality filters that meet their needs. Please Contact Us if you have any requests for filter manufacturing.