How Protective Windows and Coatings Influence Performance of Machine Vision Systems
For over 30 years, Midwest Optical Systems has been developing expertise in optical design, fabrication and inspection. The company offers a variety of options for protective windows, which are tailored to a wide range of applications and budgets. The base material used for each type of protective window dictates its suitability for a given spectral range and end use. There are different materials available that excel in transmitting wavelengths in the ultra-violet, visible, and near-, mid- and far-infrared wavelength ranges. These include economical acrylic options to more costly alternatives for ultraviolet (UV) and long wave infrared (LWIR) applications.
This family of protective window products includes options for highly challenging environments, such as those with extremely high or low temperatures, excessive dust and debris, frequent exposure to scratching, and thermal or mechanical shock. Chemically resistant options are also available. Further- more, the full range of precision mounting capabilities by MidOpt can be used to optimize adaptability, enabling protective windows to be effectively used in a diverse array of applications and configurations.
Also available are oleophobic (anti-oil, anti-smudge), hydrophobic and anti-reflective (A/R) coatings, which, when applied to a chosen protective window material, can enhance transmission and cleanability.
The Strength of Sapphire
LP190 sapphire protective windows transmit wavelengths in the ultraviolet, visible, and short- to mid-wave infrared. This material is exceptionally durable and scratch resistant. With a surface built to withstand harsh environments, this material holds up even when exposed to heat, high impact, solvents, and other chemicals.
These qualities make LP190 windows an excellent fit for use in embedded vision systems, such as those used to monitor the operation of welding equipment, CNC metalworking equipment or forklifts. Utilizing a protective window that is durable enough for these taxing environments ensures maintenance of image clarity while reducing down time by minimizing the need for frequent replacement.
LP286 protective windows are made of heat resistant Borofloat. Due to its low coefficient of thermal expansion and shock resistance, LP286 can withstand temperatures up to 450°C for prolonged periods of time. Economical yet durable, this product resists staining and damage from acids, alkalis and other harsh chemicals. It transmits wave- lengths in the near-UV, visible and near-infrared ranges.
LP286 windows come standard with both rugged, broadband multilayer A/R coatings and a durable, high efficiency oleophobic coating. Uncoated glass will only transmit about 91% of white light that passes through its surfaces. However, with A/R coatings, LP286 transmits 98% or more of incident light. In addition to minimizing reflection losses, A/R coatings improve performance in machine vision systems by enhancing contrast and eliminating ghost images.
The durable anti-smudge coatings by MidOpt found on LP286 and other protective windows repel oils and moisture, simplifying and greatly speeding up the process of cleaning these windows. This then minimizes scratching and surface degradation over time, enhancing longevity while preserving image quality.
With ultra-low thermal expansion and exceptional shock resistance, optically flat LP170 protective windows withstand operating temperatures up to 1,100°C. Quartz-like, ultra-pure synthetic fused silica has superior optical properties and excellent scratch and chemical resistance. These windows transmit light from UV to near-infrared wavelengths. LP171 windows are provided with wave- length-specific multilayer A/R coatings.
LP8000 protective windows have a 7.5-12-micron useful wavelength range and abrasion-resistant, diamond-like hard carbon A/R coatings. They are offered in sizes ranging up to 110-millimeter diameter. These precision ground flat windows represent a superior option for LWIR camera protection. They frequently find use in flame detection, medical, automotive, pyroelectric, defense, and space applications. LP8000 windows resist alkalis, acids, and other chemicals. They are commonly installed in enclosures used to protect infrared imaging equipment from airborne particulates, humidity, saltwater and other harsh environments.
Made from optical crown glass, LP345 protective windows are double-side ground and polished to crystal clarity and precision optical flatness, then enhanced with rugged, multi-layer A/R coatings. They are free from image distortion, bubbles, and inclusions. LP345 windows have a peak transmission of at least 98% and can be used in the near-UV, visible and near-infrared ranges. These protective windows are environmentally stable and highly stain resistant.
MidOpt acrylic protective windows are made with highly transparent acrylic. Two affordable, durable, optical-grade options – AC370 and AC400 – are available for the visible to near-infrared wavelength ranges. Both come standard with multilayer anti-reflective and oleophobic coatings.
Most other acrylic materials that are utilized are made from cell cast acrylic. Cast acrylic substrates provides several key ad- vantages over extruded acrylic. Cast material is better equipped to withstand constant contact with water and long-term mechanical loads. Additionally, it has a superior surface finish with a surface roughness of better than 0.02 μ, reducing low angle distortion and providing high isotropic transmission (92% minimum), all while exhibiting minimal autofluorescence. When called for or specified, high vacuum evaporation methods are used to deposit coatings on both surfaces of the cast acrylic sheet material.
Preparation for these coatings first requires that UV- cured hard coatings be applied, affording 50 times the scratch resistance of untreated acrylic and considerably improved pencil hardness (6H or better) over acrylic sheets that are treated with dip coatings. These coatings also make the acrylic sheet impervious to solvents, fuels and chemicals (such as acetone, methylene chloride and MEK). Aside from their lower cost, acrylic protective windows offer several other benefits. They weigh approximately half as much as comparable glass windows. Using precision laser cutting, they can be very quickly and inexpensively custom cut to almost any size and shape. And again, when hard-coated, they are highly resistant to abrasion and scratching.
MidOpt offers three stock thicknesses for clear acrylic protective windows, however custom thicknesses can also be made available. AC370 is one millimeter thick, while AC380/AC400 are offered in both two- and three-millimeter thicknesses. All of these acrylic options block nearly all UV light, while also transmitting at least 98% of visible light. They are suitable for covering screens, camera enclosures and on-axis lighting modules. Furthermore, they are essential for use in FDA/EFSA applications, where glass is prohibited, and thickness and weight must often be minimized.
For applications in which glass can be utilized and is desired over acrylic, industrial-grade glass protective windows are another low-cost option. They can be used for visible to near-infrared applications. LP330 windows come without coatings, while LP340 windows have multilayer A/R coatings. LP341 material is provided with multilayer A/R and oleophobic coatings.
By Barry Warzak
Founder, Midwest Optical Systems
Bandpass filters are used when imaging to block unwanted ambient light, while passing only the necessary light output to increase contrast and resolution. Interfering light is typically caused by the sun or overhead lighting in a building or factory. A bandpass filter will block this light without sacrificing the vision system’s monochromatic light. But not all bandpass filters function the same.
Palatine, IL. (May 28, 2020) – The Backlight fluoreSHEET™ by MidOpt is used to provide affordable, indirect illumination for machine vision applications. Unlike other backlighting technologies, the Backlight fluoreSHEET™ does not require power input. A backlighting effect is created when an orange Backlight fluoreSHEET™ is coupled with a blue LED light and a MidOpt Orange Bandpass Filter. The fluoreSHEET™ emits an orange fluorescence when a blue LED shines on it from the front. The Bandpass Filter captures the orange emission and blocks the blue LED excitation, resulting in a bright white diffuse background within a monochrome image.
Optical filters are important components for machine vision applications. They’re used to maximize contrast, improve color, enhance subject recognition and control the light that’s reflected from the object being inspected. Midwest Optical Systems (MidOpt) sets the industry standard for filter performance, and uses a cutting-edge Robotic Assembly & Inspection System, which ensures quality and repeatability in their manufacturing and inspection process.
Consistent Quality Inspection: The MidOpt Robotic Inspection Station guarantees consistency and uniformity when inspecting scratches, digs and other cosmetic defects. A greater variety of inspection lighting is utilized, increased magnification and additional inspection steps are employed, and the variability of individual perception is removed from the inspection equation. This results in pristine optical surfaces and superior image quality.
Torque Controlled Mounting: When assembling optically flat, precision polished filters into tightly screwed down filter mounts, interferometric measurements reveal drastic deformations caused by the stress of assembly. The resulting optical distortion can significantly impact image quality. The MidOpt Robotic Assembly Station allows each filter to be gently and securely held in the filter mount by the retaining ring. After ideal torque is applied, controlled amounts of thread-locking compound are dispensed to prevent the retaining ring from shifting.
Palatine, IL. (May 28, 2020) – Each camera features a Midwest Optical Systems LP780 IR longpass filter that blocks most light under 780 nm wavelengths, to make sure light is captured primarily from the IR LEDs, versus capturing ambient light. Blocking visible daylight also eliminates shadow effects in the driver’s immediate area that may interfere with the accuracy of face recognition algorithms.
About Midwest Optical Systems (MidOpt)
Midwest Optical Systems (MidOpt) has more than 30 years of experience and innovation in the fields of optical design, fabrication and inspection. MidOpt is a worldwide leader in machine vision filters and optical solutions. They are represented in over 30 countries and offer more than 3,000 cutting-edge products.
Triple Bandpass Filters are tools that allow us to go above and beyond traditional Normalized Difference Vegetation Index (NDVI) indicators to reinvent the way we monitor crop health and collect more information than ever before.
Traditionally, NDVI was used to determine plant health, which incorporates a camera with a red and near-infrared (NIR) filter—and the equation NDVI = (NIR – Red)/(NIR + Red). While this is a great tool, scientific advances have found that modifying this calculation can provide even more details about crop and field health. Similarly to red light, healthy plants also absorb visible blue light. And in addition to NIR light, healthy plants also reflect some visible green light. These facts enable us to assess crop health quickly, inexpensively and in realtime using converted consumer cameras and unmanned aerial vehicles.
Optical filters are critical components for machine vision applications. They’re used to maximize contrast, improve color, enhance subject recognition and control the light that’s reflected from the object being inspected. A machine vision filter is not just a machine vision filter. Here’s what to look for when choosing a filter for your machine vision system:
Midwest Optical Systems (MidOpt) has more than 30 years of experience in industrial optics and is a worldwide leader in manufacturing filters specially designed for machine vision applications. MidOpt sets the industry standard for filter performance, and their recent addition is no exception to their tradition of excellence and innovation. MidOpt unveils an industry first, introducing cutting-edge robotic technology into their assembly system to ensure quality and repeatability by limiting the possibility of human error in the manufacturing and inspection processes. MidOpt filters offer many distinct advantages, including StablEDGE® technology, improved contrast, optical flatness control and an automation process that ensures filters that are free of imperfections.
Optical filters have long been critical components in traditional machine vision applications. They maximize contrast, improve color and subject recognition, or otherwise control light reflected from an object under inspection. Typically, filters are used for machine vision tasks that do not vary much from product to product or from one subject to the next. However, recent advances in sensors, artificial intelligence, human-robot interaction and other technologies have led to the use of machine vision in more flexible applications such as sorting, material handling and automated assembly. When choosing a filter for these applications, consider the following.
Midwest Optical Systems has introduced new acrylic longpass filters. Acrylic Longpass Filters are frequently used as enclosure windows, to shield lenses and lighting from damage, or to economically control the wavelength emission in large area, broad spectrum light sources. Because of their durability, they are also commonly used in Food & Drug Administration (FDA) and European Food Safety Authority (EFSA) regulated applications where exposed glass over the inspection area is not permitted. AC370 and AC380 offer anti-abrasion, anti-reflection coatings which can also withstand harsh solvents such as acetone and MEK.
MidOpt President, Barry Warzak is interviewed by new industry publication: MVPro Media – The article features StablEDGE® filters and VISION 2016 highlights.
Some applications that have traditionally relied on single wavelength filters and multiple sensors or multispectral imagers are, with the use of dual band filters employed together with other recent technical advances, now becoming far more affordable. With the resulting reduction in cost barriers, some of these technologies are far more accessible and the potential to benefit a far larger number of individuals and organizations is now greater than ever. Applications include Security/surveillance imaging, 3D point cloud imaging, gaming, automotive, NDVI imaging and numerous other applications.
Over the past 150 years or more, fossils and other objects from the past have been collected and archived all over the world by museums, colleges and universities, and many private individuals. Usually considerable work has been put into restoring these objects in an attempt to make them appear whole and more aesthetically pleasing. Michael Eklund of the University of Texas and ThinklabZ has coined the term “Progressive Photonics” to describe a series of reproducible steps and techniques that are designed to enhance long term data yield from paleontological and archeological specimens. Progressive Photonics involves the use of specific types of cameras, lighting, optical filtering, staging, shrouding and other tools and equipment for obtaining and recording these images. It relies on fixed, predetermined methods, settings and sequences. It is a recording process designed to be easy to follow, relatively inexpensive, and one that allows for later manipulation of the images/data as may be deemed beneficial by individual researchers, scientists or collectors. Of paramount importance is that the images as seen by the unaided eye and recorded by the camera are known, specific and repeatable by anyone, at anytime, anywhere in the world. For this reason, the use of Photoshopping or other similar manipulation of digital images during the recording process is eschewed. Progressive Photonics is therefore not only an attempt to improve the scientific yield from specimens by adopting standard methodologies, instrumentation and practices for imaging, it ultimately also attempts to restore confidence in the integrity of the resulting data and conclusions drawn from interpretation of those images.
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MidOpt’s BP850 near-IR bandpass optical filter aids in surgical navigation application. During typical sinus and skull base procedures, surgery is performed under low light conditions. Hence it was important to create a system that is unobtrusive to the surgeon. To illuminate the scene, the NaviENT (system developed by ClaroNav) uses an array of 2×96 Near-IR LEDs of 5mm diameter, emitting at 850nm with a 60° field of view. These are mounted inside a tracking box with the Point Grey Bumblebee XB3 camera. Near-IR light projected onto the scene is then captured by the cameras and filtered with MidOpt’s 850nm near-IR band-pass filters (BP850) prior to processing. Since the BP850 ensures out of band blocking, the optical filter removes unwanted ambient light from the scene.
StablEDGE® optical filters are specifically designed to be less susceptible to effects from angular shifting seen when optical filters are used with short focal length (less than 12mm) camera lenses. This feature is becoming increasingly important as today’s trend in machine vision progresses towards more compact inspection layouts, which utilize less space – forcing the camera and lens closer to the subject. As a result, short focal length lenses are now more extensively used than ever before.
In a special anniversary report from IMVE, we get a closer look at MidOpt’s evolution over the past 30 years, along with newly developing applications and emerging markets for precision optical manufacturing.
MidOpt helped showcase the future of machine vision system technology with their SensorSmart Machine Vision and ID Seminar presentation on filters. Optical filters are the simplest and most effective way to gain control over what enters the system and is where image quality begins. Filters are no longer an accessory in machine vision; they’re a necessity.
Red and blue MidOpt Bandpass Filters are used to compare the sensitivity and contrast of plant health in normalized difference vegetation index (NDVI) applications. Each were placed into a camera that had the internal factory infrared filter removed. Raw images were captured, which proved previous findings that red filters perform better than blue when looking at plant health.
MidOpt Filter Kits were showcased at SPIE Photonics West. The FK220 BN Filter Kit is designed with reduced bandwidth for further wavelength separation and is compatible with LED and laser diode light sources. The kit includes blue, green, orange, light red and dark red filters; four near-infrared filters; a linear polarizer; ultra-high contrast polarizing film; and more.
Attempting to capture high-quality images of extremely fast events can be difficult, but it’s required by many analysis applications. In an effort to capture a high-speed bullet, a camera and illumination system was developed, featuring the BP880 MidOpt Bandpass Filter. This filter minimizes background light to effectively illuminate and “freeze” the motion of the projectile.
After automotive manufacturing, sub-assemblies must be inspected to ensure that the molded locking tabs and clips are in place. To determine if machine vision is an option for this application, a cutting-edge vision system incorporating a MidOpt BP635 Bandpass Filter is designed. This detailed inspection plan successfully identifies if the tabs are properly formed.
Machine vision systems are now commonplace in factory automation systems. When building these systems, developers must carefully evaluate the type of product being inspected and how illumination and optics can be used to increase the contrast and resolution of captured images. Here are the most important steps when choosing the best optical filters for your system.
During an automated wheel and tire verification process, a global manufacturer began experiencing unacceptably high levels of false fails. It was determined that by employing a combination dome light and brick light configuration, a MidOpt Bandpass Filter, and multiple smart cameras, the wheels, tires and resulting assemblies could be successfully verified.
Although optical filters are often an afterthought in machine vision systems, they are a critical element for optimizing lighting. Filters can be used to improve contrast, reduce glare, sort colors, remove barcode interference and more. Learn more about the many advantages of optical filters and how to choose the right one for your system, including real-world examples.
Although optical filters are often viewed as an afterthought in machine vision applications, their ability to optimize lighting and the entire vision system makes them so valuable that they should be considered from the initial planning stages of most applications. The value of optical filters arises from their ability to block all unwanted ambient lighting and pass only the output of light needed for inspection while increasing contrast and resolution…