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.

Borofloat®

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.

Fused Silica

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.

Germanium

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.

Precision Windows

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.

Cast Acrylic

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.

Industrial-Grade Glass

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.

PROTECTIVE WINDOW SERIES

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.