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.
KEY FEATURES OF A QUALITY MACHINE VISION FILTER
SUPERIOR WAVELENGTH CONTROL
Short-shifting occurs when the angle of light passing through a traditional filter increases. This is most commonly seen when the filter is placed in front of a lens with a focal length of 12mm or less (lenses with greater than 50 (+/-25) degree angular fields of view. This accounts for almost 60% of all lenses used today – a number that continues to grow as the demand for space forces inspection footprints to shrink.
MidOpt offers exclusive StablEDGE® technology across a full range of products. Compared to traditional coated interference filters, StablEDGE® designs are less sensitive to angular shifting, are more rugged and offer superior lower wavelength blocking.
UNMATCHED PASSBAND PERFORMANCE
Some filters are marketed as having a high and flat transmission profile to provide more even illumination. This shape can actually lead to an undesirable loss in contrast. In these cases, the weaker tail ends of the LED spectral output curve dictate the fit of the bandpass design, allowing an overwhelming amount of ambient light to pass through in these regions.
To ensure your machine vision bandpass filter is most effective when used in monochrome applications, the position, height and width of the passband should emulate the bell-shaped spectral output curve of the LED illumination being used. All MidOpt Bandpass Filters are designed with this principle in mind.
When a ray of light passes through a glass surface, a portion of the light is reflected, resulting in a 4% transmission loss per surface. This means only 92% of the light actually passes through the glass, and the rest is reflected from the surface, known as reflection loss. MidOpt uses anti-reflection coating on all filter designs, reducing surface reflection to less than 1%. This improves the efficiency of the vision system by increasing transmission, enhancing contrast and eliminating ghost images. MidOpt also offers oleophobic (anti-smudge) coatings, which are oil-resistant and prevent moisture build-up, allowing for easier cleaning.
OPTIMAL PERFORMANCE & REPEATABILITY
With improvements in sensor technology and camera resolution, there’s an increased demand for filters that are free of distortion. All MidOpt Bandpass Filters are double-side polished to achieve optimal flatness.
Surface quality is also important, so the images are free of imperfections. MidOpt exceeds the industry surface quality inspection standard with a 40/20 scratch/dig ratio. And in cases where the filter is placed in front of the sensor, where imperfections are most pronounced, we offer a 10/5 scratch/dig ratio.
To ensure every filter upholds the highest standards, state-of-the-art spectrophotometer technology is used to test filter performance.
CONVENIENT MOUNTING SOLUTIONS
MidOpt offers same-day shipping and stocks over 20,000 mounted filters, ranging in size from M13.25 to M105. A variety of other mounting solutions are also available, including options for applications without filter threads, custom mounting solutions and the MidOpt exclusive 25.4® C-Mount filter that threads into all C-mount cameras behind the lens.
This article was featured in Automation Magazine.