Filters allow the kind of sharp contrast required in manufacturing applications, when you may need to use machine-vision, for example, to find a product flaw that could easily be missed by the human eye.

Using colored glasses or dyes, optical filters transmit specific parts of the spectrum while reflecting unwanted light.

Generally speaking, a color filter will lighten objects of the same color, while darkening objects of opposing colors.

In a Tech Briefs webinar titled "Machine Vision for Industrial Inspection," a reader asked Jeff Carmichael, a Director of Marketing at the Vermont-based filter manufacturer Chroma Technology:

"How do you know which filters to choose?"

Read Carmichael's edited response to the reader's question below.

Jeff Carmichael, Director of Marketing, Chroma Technology: If you use a white-light source, like a broad white LED, you often don’t have much information to know where to start, in order to choose a wavelength range, to increase and improve the contrast in your image.

Sometimes you do, because you've done something similar before and you know roughly where to look. Even in that case, let’s say you’ve decided blue light is what you need. Which blue light?

How wide a bandpass is probably even more important. How much of the ambient light is present that you need to block out? How narrow is the spectral output of that LED? That helps guide how wide the bandpass should be.

When you don’t really have much guidance, you can use something like a filter kit that has a broad selection of filters. And just empirically play with it. Just take filters out and [do] trial-and-error. Take images and see which ones actually improve contrast of the feature of interest.

"You mentioned that you can use a broad white light source for these filters. Why do you say this when we're generally taught that you should avoid this and use colored LEDS?"

Carmichael: There are a couple answers to that question. First, it's pretty much just historical that many, many years ago, red LEDs, since they were relatively cheap and safe to make, came into vogue. A red LED will limit wavelength. This was before filtration was used very much. That was the way of limiting the spectral range of an image, simply with an LED.

The reason I say a white light is fine to use: I'm not necessarily suggesting that an imaging application on the floor, where fast imaging is required, is where you should use a white light, although it would be perfectly fine.

The filter does all of the filtration work. You don't need to limit the wavelengths that you're illuminating an object with in order to limit the wavelengths the camera sees. The filter does that far better. In applications requiring high speed or really high output, where you're using a filter, you're probably going to want to pair that with an LED that can strobe or overdrive and put out a lot more light over a 20 nm range than you could squeeze out of a white light.

But when you're doing testing and you haven't put the system together yet, use a white light, because then you can test every possible wavelength range. And then at the end, you've optimized the range and you can pick the LED.

What do you think? Have you used optical filters? Share your questions and comments below about how to choose the right optical filter.