Q&A: Optical filter manufacturer expands into the machine vision world

The following is a conversation with Jeff Carmichael, Technical and Product Marketing Manager, Chroma Technology Corp.

I saw your booth at The Vision Show 2016 in Boston and you walked me through your product demonstration. What prompted you to introduce a line of machine vision filters now?

Well, we’ve been looking at the machine vision and factory automation markets for a while and we feel that our expertise allows us to offer something that was lacking for those seeking improved performance.

Many machine vision applications already use filters though, so what sort of improved performance are you referencing?

Chroma has been manufacturing optical filters for 25 years. Although we’ve supplied filters in many industrial markets, our primary markets have been in the life sciences, and the primary imaging modalities that we’ve served have typically involved fluorescence detection of some kind. We continue to work with all of the major microscope manufacturers as well as manufacturers in markets like flow cytometry, DNA sequencing, High-content-analysis, Raman spectroscopy, surgical instrumentation, and more recently the medical point of care industry.

Most of these applications typically require properties not found in available filters marketed to machine vision applications. Properties like extremely high and broad levels of out-of-band attenuation – called “blocking”, very steep transition slopes, and very high levels of transmission to detect low signal intensities in the presence of high noise levels. You’ll typically see >95% transmission across the full width of a square-wave shaped passband.

We know that most of these filters are over-built in terms of what’s generally required in machine vision, but on the other hand we’re seeing more demanding tasks that require improved contrast and greater levels of blocking of unwanted light. We think there’s also a need for interference filters that don’t suffer from spectral blue-shift or collapse of bandshape when working with larger viewing angles.

So it looks like evolving vision applications will require ever-increasing levels of performance, and that’s right in our wheelhouse.

What is it about these filters that you think will make them suited for machine vision?

We’ve specified 17 single-band filters to start with which correspond to the most popular LED wavelengths and the more common laser diodes. Most passbands are 40nm-wide, while the narrower filters are 20nm-wide.

The main thing that’s different from our other catalog filters is that these are designed to accept a approximately 20° half-cone angle. You’ll often hear that interference filters are very angle-dependent, but they don’t have to be. The stacks of thin film layers can be built in a way that accommodates a wide range of angles.

This means that when using lenses with angular fields of view of 40-50°, you don’t suffer any real loss in performance in terms of contrast from edge-to-edge. The passbands hold pretty well over that range. We thought that made sense to start with, and then we can always improve on that if there’s a need for even greater angle acceptance.

So the idea is that this now allows you to benefit from the other advantages that sputtered interference filters already provide, but while also ensuring dependable performance over a wide range of angles common to vision applications.

Those other advantages include the >20% of increased contrast that these highly transmitting square-wave interference filters provide when compared with more traditional absorption glass filters. Traditional passbands are Gaussian in shape.

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