Sunday, October 31, 2010

White light for machine vision?

One of the current trends among manufacturers of LED lighting seems to be white light. Everyone seems very excited that they can offer white LED’s, and are rushing to fill the perceived gaps in their product lines.

Well today I’m going to tell you not to buy those white lights.

OK, there is an exception. If you’re doing color machine vision you probably need white light, BUT … as there’s no such thing as a white LED (LED emission is inherently monochromatic,) be sure you understand how the spectral distribution will affect the way colors appear.

So with that caveat out of the way, let’s talk about why I think white lighting should be such a no-no.

From my experience, there are two problems: chromatic aberration and wavelength-surface interaction.

Chromatic aberration refers to the fact that refraction is related to wavelength. So, as light passes through a lens, different wavelengths refract by differing amounts. Of course, the lens manufacturers know about this and try to minimize its impact, but the laws of physics are not easy to evade. Monochromatic light avoids this problem, so why not make life easier for yourself?

Wavelength-surface interactions are more complex. You know that IR light can pass through some materials, don’t you? Well rather than there being a simple transmit-reflect threshold there’s a gradual drop-off in transmission. If near IR light is transmitted through a material, then a fraction of red light will also transmit rather than reflect off the surface. This means that, depending on what you’re illuminating, a shorter wavelength may do a better job of actually revealing the true surface.

To reinforce this point, I recall reading about inspection of CD cases. The task was to find very small cracks, and the developers found that shorter wavelengths did a better job of creating good contrast. (If anyone has a link to the article, please share it with us.)

So the messages here are: don’t use white light unless the application needs white light, and don’t just assume that red light is the best tool for your particular job. Experiment to see what works best.

Now, what about white LED lighting for the chandelier in my hallway that I can’t reach without a tall ladder? That would be interesting.

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