Optical filters are passive optical devices that consist of specialized optical coatings applied onto a substrate. The coatings modify the refractive index of the substrate, enabling them to reflect, transmit, or absorb incoming light depending on its wavelength. This quality is useful for various optical tools and systems, such as chemical analysis units and microscopes.
Optical filters are available in many variations, each of which possesses distinct characteristics that make it suitable for particular applications. Below, we provide an overview of some of the different types available.
Absorptive filters have coatings made from organic and inorganic materials. These materials enable the filter to absorb the undesirable wavelengths and transmit the desirable wavelengths. This design ensures that no energy is reflected back toward the light source.
In contrast to absorptive filters, dichroic filters—also called thin-film filters or interference filters—have coatings that enable them to reflect the undesirable wavelengths and transmit the desirable wavelengths. The thickness and properties of the coatings determine which wavelengths are reflected and which wavelengths are transmitted. These types of optical filters are highly accurate, enabling users to target a small range of wavelengths.
Notch filters—also called band-stop filters or band-reject filters—are designed to block a specific frequency band (i.e., the stopband frequency range). Any wavelengths above or below this range are allowed to pass through freely. These types of optical filters are ideal for applications involving the combination of two or more signals since they can help isolate out interference.
In contrast to notch filters, bandpass filters are designed to block every frequency except for a small range. They are a combination of shortpass filters and longpass filters—filtering out any wavelengths that are too short or too long. This cutoff range can be lengthened or narrowed by adjusting the number of layers in the filter.
Shortpass filters are designed to transmit wavelengths below a set length determined by the optical coating and substrate. Any wavelengths that are longer than that point are blocked. These types of optical filters are commonly used to isolate specific higher regions of a broad spectrum and in conjunction with longpass filters for bandpass filtration applications. Typical applications include chemical analysis systems.
Longpass filters are designed to transmit wavelengths above a set length determined by the optical coating and substrate. Any wavelengths that are shorter than that point are blocked. Typical applications include fluorescent spectroscopy systems. Additionally, they are commonly used in conjunction with shortpass filters for bandpass filtration applications.
Thin-Film Optical Filter Solutions From Evaporated Coatings, Inc.
Want to learn more about optical filters and how to choose the right one for your optical needs? Turn to the experts at Evaporated Coatings! We specialize in the supply of high-precision optical coatings. By helping customers select the right coating and applying it to their substrates, we can make custom optical filters for virtually any application.
Check out our custom optical filters page to learn more about our thin-film coating capabilities. To discuss your optical filter requirements with one of our team members, contact us today.
Thin film coatings, such as antireflective (AR) coatings, are made from various materials, such as metals, oxides, and compounds, and are deposited in layers onto a substrate. Thin film coatings can be deposited in both single and multiple layers, and the configuration you choose determines how it will manipulate different wavelengths of light.
Thin film coatings have many different characteristics, which are used to improve or alter some element of the substrate’s capabilities. The design and configuration of thin film coatings heavily depend on performance requirements, and the proper design is crucial to the functionality and overall success of your application.
Single Layer AR Coatings
Single-layer AR coatings can have different refractive indices depending on the material. For example, single-layer AR coatings of magnesium-fluoride have a refractive index of 1.38. Applying the coating to a substrate with a 1.9 refractive index provides 0% reflection.
Single-layer AR coatings of magnesium-fluoride can be adjusted to perform with various wavelengths and typically prevent reflection of 550 nm lasers. Single-layer AR coatings are prevalent, but complex applications may require multi-layer AR coatings.
Double & Triple Layer AR Coatings
Two or more AR coating layers can overcome the limitations of a single layer AR coating. Combining high and low index coatings, such as 2.3 and 1.38 produces a narrow bandwidth and close to 0% reflection. Three-layer coatings create a broadband AR coating using two high and a single low index coating, such as 2.1 and 1.38.
Some substrates cannot achieve the necessary refractive index with a single coating. Multi-layer coatings allow manufacturers to use more available materials to block a more diverse range of incident angles and wavelengths. It is vital to consider the ideal materials when selecting a two or three-layer AR coating, as the refractive indexes available are limited and deposition is imperfect.
When designing a configuration for thin film coatings, consider the following factors:
Thin film coatings offer increased performance at lower angles of incidence.
Longwave pass (LWP) filters allow for greater transmission and typically higher-performance than shortwave pass (SWP) filters. LWP filter substrates may be more expensive than those in SWP filters, but they enhance manufacturing tolerance and use more simple AR coatings.
Designing a coating with a greater than 2:1 bandwidth ratio increases the difficulty. It requires more layers and increases the percentage of reflection, with a higher reflection penalty when coating 30° and 45° angles of incidence.
Designs may not always translate well during manufacturing. When a design requires more than 12 layers, the deposition process can be unmanageable in practice. Coatings that require more than three materials or a 10 nm or less thickness can also result in unsuccessful manufacturing.
Specify only necessary coating requirements. A range that is wider than necessary can be detrimental to performance.
It is also necessary to consider the substrate texture. Substrates with lithography or etching require an AR coating with an approximate profile with a smaller height and width than the shortest wavelength.
Single wavelength coatings are typically easier to manufacture compared to multiple wavelength coatings. Specific materials must be chosen for each wavelength, increasing the cost and complexity, especially when transmitting long and short wavelengths.
Thin Film Coatings From Evaporated Coatings, Inc.
When designing thin film coatings, such as AR coatings, films can be deposited in single and multiple layers to suit various substrates and applications. There are essential considerations when designing a thin film coating that will offer the performance you expect. At Evaporated Coatings, Inc., we are a leader in thin film coatings with over 50 years of experience in optical coating solutions. We can work with you to design and deposit custom AR coatings based on the needs of your application.
For more information, or for help with your thin film coating design, contact us today.
Optical microscopy is a technique that allows the viewing of samples more closely using optical microscopes. It relies on light and one or more lenses to magnify samples. Optical microscopy is remarkably versatile, increasing the detail and contrast of a microscopic specimen. A range of applications rely on simple and complex microscopy techniques.
Fluorescence microscopy is a type of optical microscopy that uses a fluorescent dye called fluorophores. When light hits the dye, it induces fluorescence rather than scattering or absorbing light, making tissue, cells, and proteins visible under a microscope. Fluorophores absorb energy from a specific wavelength known as the excitement range resulting in the energy’s re-emission in a wavelength known as the emission range.
Phase contrast is a form of optical microscopy that allows operators to view transparent specimens with enhanced contrast. Transparent samples, cells, and microorganisms are viewable in high-contrast without fixing or staining the samples. The technique allows viewers to see live specimens in their natural state.
Differential Interference Contrast
Differential interference contrast (DIC) introduces contrast to samples with minimal contrast using optical microscopy. It provides a near 3D appearance to the specimen, allowing viewers to see a contrasting image in high resolution. DIC uses infrared light for its long wavelengths, allowing the light to penetrate thick samples.
DIC creates a contrasting image when light passes through a polarizing filter and another polarized optical device. The polarized light passes through an objective-specific prism where the light beam is split and passes through a condenser. The condenser focuses the beams of light on specific points of the specimen.
The light beams pass through the specimen at various locations and various wavelengths. They move on to an objective lens that refocuses the beams on the rear of the focal plane. The nosepiece prism combines the beams, and the beam passes through the analyzer. The analyzer causes destructive and constructive interference, bringing the beams to the identical axis and plane. The light travels to the camera for the viewing of the DIC image.
Brightfield and Darkfield Illumination
Brightfield illumination presents a dark specimen on a bright background to create contrast. It is a simple technique that positions a light source below the sample. Light passes through the specimen to an objective lens and optical sensor. The darkness of the specimen increases with the specimen’s density. The more dense the sample is, the more pronounced the image will be.
Brightfield illumination is the result of these four key elements:
Eyepiece or Camera
Darkfield illumination creates a light specimen image on a dark background, contrasting the brightfield illumination technique. This technique enhances a specimen’s contrast without staining, allowing observation of living specimens.
Darkfield illumination begins with a light source that is obstructed by a dark field patch stop as it enters the microscope. The light is reduced to a ring where the condenser lens focuses it onto the sample. When the light hits the specimen, it transmits or scatters. The objective lens permits scattered light but blocks transmitted light with help from the dark illumination block.
Optical Microscopy Solutions From Evaporated Coatings
Optical microscopy improves specimen viewing by magnifying microscopic samples and enhancing their visibility with techniques and lenses. Fluorescence, phase contrast, brightfield illumination, darkfield illumination, and DIC allow optical microscopes to deliver a closer image in various applications.
At Evaporated Coatings, we specialize in high-quality optical coatings. We manufacture a range of optical filters, including excitement and emission filters for fluorescence microscopy. Our designers can help you find a cost-effective and high-performance solution, and our technicians use leading technology to manufacture the substrate and coatings you require. Contact us today to learn more about our high-quality optical coating solutions.
Ion Beam Sputtering Coating (IBS) uses an ion source to deposits or sputters a thin film onto your targeted material (either metal or dielectric) to create a dielectric or metallic film. Since an ion beam is mono-energetic and collimated, it creates very precise control over the thickness of the film.
A typical configuration of IBS systems includes the substrate, a target, and a gridded ion source, with the ion beam being focused on a target material, and a nearby substrate being the sputtered target material.
What Is Ion Beam Deposition?
IBS, otherwise known as ion beam deposition, is a process that deposits a thin film of dielectric or metallic material onto a substrate while allowing for extremely fine control over the coating thickness. During this process, an ion beam or source deposits, or sputters, material from a supply onto the workpiece in a dense, consistent pattern.
Ion beam deposition processes are uniquely advantageous because operators can control everything from the sputtering rate to the ionic energy and density. This allows for complete control of the microstructure and film stoichiometry of the deposited layer. For applications that demand precision, such as with semiconductors, IBS outperforms alternative sputtering processes like physical vapor deposition.
What Is Assisted Ion Beam Deposition?
Assisted ion beam deposition uses two simultaneous processes — IBS and ion implementation — to create an intermixed coating. This process allows for a fine degree of control and can form gradually thickening or thinning transitions between the film layer and the underlying substrate’s original surface layer. Assisted ion beam deposition also gives the deposited film a much stronger bond.
The Main Advantage of Ion Beam Sputtering Coatings
One of the advantages of IBS is the control you get over several parameters. These include ion current density, ion energy, and the angle of incidence to help with the control of film microstructure. This is the main advantage and difference of sputtering processes, which makes IBS a great choice for any challenging applications you may have.
Additional Benefits of IBS Coatings
IBS coatings are known for providing precision control and high-density deposition layers. Other benefits of this coating method include:
High Energy Bonding. The IBS process provides enough kinetic energy to create a durable bond between the substrate’s surface and the coating.
Uniformity. Sputtering is typically emitted from a larger target surface area, ensuring a more uniform application when compared to vacuum coating and other alternative methods.
Versatility. IBS can provide a coating for nearly any material, even those with high melting points. This makes it an excellent choice for projects that require very particular coating properties.
Limitations of IBS Coatings
While IBS coatings have several unique advantages, there are some limitations to consider before choosing this method. These limitations include:
Bombardment Target Area. The target area is generally too small for easy use with larger substrates and projects with large surface areas.
Deposition Rate. The deposition rate is low, making IBS coatings potentially too slow or too expensive for some projects.
Complexity. IBS coating systems are complicated and require extensive training. This can further increase the time and expense of the production process.
Ion Beam Sputtering Coatings From Evaporated Coatings
At Evaporated Coatings, Inc., we specialize in providing high-quality optical coatings, AR coatings, depositions, and more. We work with each of our clients to select the right coating process based on each project’s budget, unique requirements, and intended applications. Contact us today to learn more about our design, preparation, and coating services, or visit this page to learn more about our IBS services.
Thin-film optical filters are optical devices consisting of alternating thin layers of specialized optical coatings deposited onto a substrate (e.g., optical glass). The coating layers alter the refractive index of the substrate, changing the direction of the various wavelengths in incoming light as it passes through one layer to the next. Either reflection, transmission, or absorption can occur depending on the wavelength(s) of the incoming light and the type of optical filter employed.
Types of Thin-Film Optical Filters
Thin-film optical filters are suitable for use with light in the ultraviolet (UV) to infrared (IR) wavelength range. They can be classified into five basic categories based on their spectral shape: bandpass filters, notch filters, shortpass edge filters, longpass edge filters, and dichroic filters. For more information on the various types available, check out this thin-film optical filter blog post.
Applications of Thin-Film Optical Filters
Thin-film optical filters are highly customizable, which allows them to be designed and built for effective performance in extremely specific or unique light-based applications. They are used across a wide range of industries in a variety of devices, equipment, and systems for many different use cases, including, but not limited to, the following:
Biological image detection. They can filter luminescence to facilitate the operation of biological imaging devices.
Chemical analysis. They can isolate specific emission or absorption ranges from incoming light to aid in chemical identification and analysis operations.
Contrast enhancement. They can increase the contrast between objects during scanning and other imaging operations to improve identification, recognition, and verification.
Laser systems. They can manipulate the beam of light generated in laser systems.
They can separate and modify signal transmissions in telecommunication systems.
Visible light coloring. They can add or enhance the hue of visible light to achieve a specific aesthetic effect.
Thin-Film Optical Filter Solutions From Evaporated Coatings, Inc.
Thin-film optical filters find use in a variety of light transmission, blocking, and absorption applications. As their design and construction—i.e., the optical substrate and coating used—vary depending the application, some customers may find it challenging to select and source a product that meets their needs. Fortunately, the experts at Evaporated Coatings are here to help.
At Evaporated Coatings, Inc., we specialize in the supply of high-precision optical coatings. Equipped with over 50 years of industry experience, we can help customers select the right coating and apply it to their substrates. Visit our custom optical filter page to learn more about our thin-film coating capabilities. If you want to discuss your optical filter requirements with one of our experts, contact us today.
Beamsplitters—also referred to as beam splitters or power splitters—are optical devices designed to split incident light into two or more separate beams. They can also be used in reverse to combine two or more separate beams into a single one.
Some of the key properties to keep in mind when choosing a beamsplitter for an application include:
Splitting ratio: the amount of light that is transmitted vs. the amount of light that is reflected
Wavelength range: the finite range of wavelengths the device accommodates
Optical loss: the output power compared to the input power
Spatial configuration: how the output ports are positioned relative to the input beam
Aperture: the size of the area that allows light to enter the device
How Does a Beamsplitter Work?
As indicated above, beamsplitters are used to split incident light into two or more separate beams. The splitting process is dependent on the wavelength, intensity, or polarity of the incoming light and the design and configuration of the beamsplitter. Regardless of these factors, however, all beamsplitters follow the same basic principles: incoming light is split into two or more beams with one or more continuing forward through the optical component (i.e., transmitted) and one or more directed at an angle out of the optical component (i.e., reflected).
Due to their ability to split light into separate beams based on controlled reflected/transmitted (R/T) ratios, beamsplitters find use in a wide range of light-based devices and equipment, including, but not limited to, the following:
Cameras and projectors
Head-up displays (HUDs)
Laser alignment and attenuation systems
Medical imaging systems
Types of Beamsplitters
While all beamsplitters perform the same basic function—i.e., splitting light into separate beams—how they do so varies depending on their design. For example:
Standard beamsplitters split incident light without regard to the wavelength, polarization state, or intensity. They are generally used for one-way mirrors and illuminating assemblies and subassemblies.
Dichroic beamsplitters split incident light by wavelength. They are typically employed as laser beam combiners or broadband hot/cold mirrors.
Polarizing beamsplitters split incident light by polarization state. They are ideal for use in photonic instrumentation systems.
Non-polarizing beamsplitters split incident light by intensity. They are suitable for applications that utilize polarized light.
Standard, dichroic, polarizing, and non-polarizing beamsplitters are available in a variety of configurations. Some of the most common include:
Cube beamsplitters. Cube beamsplitters consist of two right-angle prisms connected at the hypotenuse with a semi-reflective coating at the point of connection. They are suitable for applications that require simple mounting mechanisms and durable optical components.
Plate beamsplitters. Plate beamsplitters are made from flat and thin glass plate with a special coating on the first surface of the substrate. They are typically used for light with a 45-degree angle of incidence and can be built to support multiple incident ratios.
Pellicle beamsplitters. Pellicle beamsplitters are made from very thin substrates. This design allows for minimal beam offset.
Polka dot beamsplitters. Polka dot beamsplitters feature a pattern of reflecting dots on the glass surface. This design allows for geometrical beam splitting that is not angle-dependent.
Custom Beamsplitter Coating Solutions From Evaporated Coatings, Inc.
Beamsplitters serve a critical function in a wide range of light-based applications. Ensuring they operate as intended necessitates verifying the base substrate has the proper mechanical properties and utilizing the right optical coating. If you’re looking for an experienced and knowledgeable supplier for your beamsplitter coating needs, turn to the experts at ECI.
At Evaporated Coatings, Inc. (ECI), we’ve supplied high-precision optical coatings for over 50 years. This extensive experience allows us to manufacture beamsplitter coatings for a wide range of R/T ratios, wavelength ranges, angles of incidence, polarization states, incident mediums, and temperature sensitivities and apply them to various customer-supplied substrates (e.g., glass, plastic, molded polymer, semiconductor materials, fibers, and fiber optic devices).
Visit our beamsplitter page to learn more about our beamsplitter coating solutions. To discuss your beamsplitter requirements with one of our experts, contact us today.
A thin-film optical filter is produced by placing thin layers of substances that contain specialized optical properties in an alternating fashion on top of a membrane; glass that made specifically for optical purposes, for example. As light passes through an optical filter, its wavelengths change directions as they pass through each layer of the filter. The thin-film coating alters the refractive indices, which results in internal interference, a process that helps to minimize interference from internal reflections. The wavelengths of light can pass through, absorbed, or reflect off of the filter. The kind of optical filter and the wavelength will determine how the light reacts to the filter.
There are different types of optical filters. Some can transmit light, while others can reflect it, and still others can block it completely. All types can process any wavelength, from the UV range to the IR range. Generally, optical filters are categorized into five key groups according to the spectral shape of the filter.
1. Bandpass filters. These optical filters transmit a variety of wavelengths while also blocking out the neighboring light. 2. Notch filters. Notch optical filters block out a range of wavelengths while transferring the light on both sides. 3. Shortpass edge filters. Optical filters that belong to this category transmit short wavelengths of light and block out longer ones. 4. Longpass edge filters. Short wavelengths of light are blocked by longpass edge filters, while longer wavelengths are transmitted through them. 5. Dichroic filters. Certain kinds of wavelengths are reflected by dichroic filters, while other ranges pass through them.
Typically, edge, bandpass, and notch optical filters work at small angles of incidence (AOI), such as 0 degrees. Dichroic filters, however, are designed to work at an AOI of 45 degrees or larger and are designed in edge, bandpass, or notch arrangements.
Multiband configurations of optical filters can also be made. Multiband optical filters are bandpass that allow multiple passband. These filters contain several blocking regions and they diffuse all neighboring wavelengths of light. Polychroic filters, for example, are dichroic filters that contain several notches or bands.
While the majority of optical filters are grouped into the above mentioned categories, customized filters can be made. Custom filters can have any type of spectral shape you can think of; for instance, light waves from a xenon lamp can be made to resemble the spectrum of light the sun produces when passed through a customized filter. Other types of specially made optical filters can correspond with random spectral shapes.
Since optical filters are so versatile, they can be used in a variety of ways, including:
Selecting the optimal optical filter for your shot improves the contrast in images, reducing the processing times required to extract the relevant data from the image. Achieving the highest possible image contrast is the single-most critical factor when designing a machine-based vision system.
Your choice of aperture size, illumination level, and the quality of your lens all play a significant role in determining the performance of your system. It’s tempting for designers to enhance performance by upgrading lenses or lighting units. However, these additions can add considerably to your costs.
Fortunately, there’s a way to enhance your system performance and image quality through a more affordable option. Filters offer you the opportunity to improve your image, provided you carefully evaluate any spectral components of your target object. Filters improve performance while providing a minimal impact on the other elements of your image design.
Understanding the Different Types of Filters
There is a range of filters available for designers. The filters receive more definition according to the structure of the transmission curve.
A long-pass filter blocks short wavelengths while allowing long wavelengths to pass through.
A short pass filter works oppositely, blocking longer wavelengths while allowing shorter ones to pass through.
A band pass filter transmits central wavelengths, blocking both shorter and longer wavelengths.
A notch filter is the opposite of a bandpass filter, passing the shorter and longer wavelengths while blocking the wavelength band.
Within each of these filter groups, types of filters available depending on the technology solutions used in its creation. For instance, the colored glass filter is unavailable in notch varieties.
Designers have a vast array of filters available for use on projects. Some of the more common filters include the following.
The use of a colored glass filter is an affordable and pragmatic solution for enhancing the contrast in applications. However, this practice has limitations on images where broad spectral characteristics distinguish objects, such as in the separation of purple and orange objects.
An interference filter transmits the specific range of wavelengths, and they offer more precision in use over colored glass filters. An interference filter provides the designer with a nanometer-level control over the transmission of all wavelengths. The same level of accuracy isn’t possible with a colored lens.
Polarization and neutral density filters may also assist with improving performance in specific imaging situations. Properly incorporating filters into your system requires designers to understand and comprehend the limitations and potential of each of the types of filters available to you.
Colored Glass Filter
Spectral discriminations caused by the use of a colored glass filter occurs due to the dopants present in the glass. The concentration and selection of dopants determine the transmission wavelengths and the filter attenuation.
A colored glass filter offers the designer an affordable solution for many design applications that have relaxed requirements on performance and are angle-independent. The optical transmission never shifts, even with the use of wide-angle lenses, or when tilting on the system’s optical axis.
It’s important to note that a colored glass filter features a slow transition between the transmission and blocking wavebands, with transmission curves appearing less steep than with using a coated interference filter.
There are plenty of types of color filters, including a daylight blue filter for balancing colors during the use of color sensors and polychromatic light sources.
Infrared (IR) Filters
IR filters are suitable for use in machine vision applications in color and monochrome cameras. Most machine vision cameras feature silicon image sensors that respond to infrared wavelengths. Near-infra-red wavelengths commonly occur due to overhead fluorescent lighting systems, creating inaccuracies in the camera sensors.
It’s for this reason that most color imaging cameras come with IR-cut filters as standardized equipment mounted over the sensor. Monochrome camera systems will experience massive degradation of the contrast in the image due to the presence of IR light.
For identifying small shifts in color, spectral discrimination of interference filters is a necessity due to the filter’s ability to create sharp transitions between wavelengths they block and transmit. The wavelength-selective interference filter consists of an alternating dielectric layer of low and high refraction indices depositing on a specific surface.
The uniformity and quality of the surface create a baseline optical quality for the interference filter while defining the wavelength limitations based on the transmission characteristics of the surface.
Dielectric layering produces detailed spectral characteristics from the filter, creating a destructive interference between the wavelengths that aren’t within the transmission band. As a result, it blocks the wavelengths from transmission through the interference filter.
Neutral Density Filters
Gain control over the image brightness without altering the settings for your exposure time of f/#. Both reflecting and absorbing types of neutral density filters can help you lower light transmitted to the sensor in the lens.
These filters are excellent for use in situations like capturing an image during welding. The neutral density filter reduces the intensity of the light, without compromising any of the other colors or contrast in the image.
These special filters decrease optical density away from the radial distance of the center of the image. These filters are excellent for handling image hotspots caused by reflections.
Limitations on Your Filters
Hard-coated filters get their performance from the specialized coating on the filter, but this same technology also creates limitations on the use of these filters in imagery. Interference characteristics depend primarily on the relationship between the length that light waves travel through a specific medium and the given light of the wavelength.
When traveling through interference coatings at an unfavorable angle, the light path changes through each layer of the lens, resulting in the modification of the filter’s wavelength selectivity. An interference filter functions and performs based on the distance that the light travels upon the filter.
With the proper angle of incidence, light waves will incident the filter, and destructively interfere, blocking them from passing through the filter. All interference filters feature a specific Angle Of Incidence (AOI), with most manufacturers setting it at 0°.
The angular field of view defines the acceptance angle when placing a filter in front of the lens. With a short focal length lens, light transmitted through a filter displays an undesirable effect where the slope decreases due to passband shifts down in the wavelength.
The common moniker for this effect is “Blue Shift.” For instance, a 4.5mm focal length wide-angle lens has a more significant blue shift than narrow-angled 50mm focal lenses. Designers will find that the filter behaves differently at various field points due to changes in the wavelength ranges: the further out, the more noticeable the blue shift.
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The Governor of Pennsylvania issued an Executive Order dated March 19, 2020, which required the closure of all businesses in the state except those that are “life sustaining” businesses. Business guidance was updated on March 21, 2020 and aligned with the Department of Homeland Security’s Cybersecurity and Infrastructure Security Agency (CISA) to maintain continuity of operations of the federal Critical Infrastructure Sectors.
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Machine vision, also sometime referred to as computer vision, is an extremely important field of computer science that is likely to play a gigantic role in the direction of technology and society moving forward.
But while many people will focus on the software aspect of machine vision, it is all-too-easy to forget the equally important practical elements that will influence overall performance. One such example is the use of optical coatings and filtering: both of which will either extend or severely limit the possible applications for this highly exciting technology.
Computer Vision and Optics
Computer vision/machine vision is the ability of a computer to ‘see’. It does this by using a camera to create a digital image, and then analysing the data that is contained in that image. This is a technology that has been used for a long time, but in the last few years it has been rapidly increasing in importance. That’s because machine learning has enabled rapid improvements in this field, to the point that a computer can not only ‘see’ but also understand precisely what it is seeing: using this information to identify elements in a scene, or even to navigate in 3D space.
When you think of computer vision, you might think about robotics: specifically, robots that move through a room. This is one application to be sure, but others also include VR, facial recognition, digital assistants, data processing, social media, and much more.
VR for example uses computer vision in order to understand 3D space, thereby keeping the user safe while they enjoy their immersive experience, while also tracking their virtual movements to their real-world ones.
In order for all this to work, filtering is needed. These filters are created by doping glass materials with elements that can help to alter the absorption and transmission spectra.
The precise elements, or ‘dopants’, depend precisely on the wavelength that is desirable for the application.
The role of these filters is several fold. In some cases, filtering might be used in order to help protect the substrate underneath. For instance, a coating can prevent bright sun from damaging machinery and this could be important for a drone that is being flown in harsh weather conditions.
Likewise though, filtering can help to provide the first steps in the computational processes that allow machine learning to occur. When navigating through a virtual space for instance, a computer program will only need to look for contrast – which typically denotes an edge. The right filter can help to increase the contrast of the image, thereby making it easier to navigate the scene with less on-board processing necessary.
Another type of filter might be used in order to provide data not visible to the human eye. For example, an IR light can create a false color on a camera that can degrade the color reproduction and therefore many imaging cameras will use an IR-cut filter for the sensor.
Conversely, some technologies will use invisible light waves such as IR precisely because they can’t be seen by the human eye. An example is the ‘Leap Motion’ hand tracker.
Types of Coating
There are many types of coated filters used in this technology. Typically, coated filters are intended to offer sharper cut on and off transitions and higher transmissions. They are superior in these ways to other colored glass filters.
Every coated filter will go through a unique manufacturing process that ensures it meets performance targets. Wavelength-selective filters are manufactured using the deposition of dielectric layers added to the substrate. These have high and low refraction indices respectively and can combine to produce a range of desired results.
Surface quality and uniformity are extremely important factors when choosing the substrate as this can drastically impact on the performance and longevity of the coating.
There are a wide range of different types of filters, which include bandpass, longpass, shortpass, and notch filters. These each have specific blocking ranges. The explanation is in the name in each case, where the ‘long pass’ filter allows the longest wavelengths to pass through, blocking the shorter wavelengths. Short pass will block longer wavelengths and allow the shorter wavelengths to pass through. Bandpass filters block both longer and shorter wavelengths while only allowing a selected wavelength band in the middle to pass through. Notch filters will only allow wavelengths at either end of the spectrum to pass through while only blocking a selected wavelength band in the middle. Think of this as being like a cut-out or ‘notch’ in the middle of the signal.
Using these coated filters on cameras and other technologies is a relatively straightforward process, but the unique application of machine vision can introduce some unique difficulties. For example, these filters are designed for a specific Angle of Incidence (AOI) which is normally 0 degrees. This means that only light hitting the lens head-on will be blocked at the right wavelengths. This issue is particularly pronounced with the use of wider angle lenses – as is common in machine vision. Solutions can be applied however, such as moving the lens itself, or having multiple layers of glass. Alternatively, multiple lenses may be used rather than one larger one. Software correction can also help to reduce the noise.
Even with that limitation in mind however, coated filters will offer superior performance in most cases and is almost always preferred.
As with all things though, it is important to consider the precise application and other goals of the project. There are huge varieties of different types of machine vision, and using the right filter will depend on the environment, the goals, and the type of image analysis.