The influence of wavelength

Near IR illumination is interesting, not just because it highlights things that we can’t see with our eyes, but because of the ability of longer wavelengths of light to penetrate materials that are opaque in the visible part of the spectrum.

Take skin for example. Near IR can penetrate skin, which means it can be used to look at things just below the surface of our bodies, such as blood vessels. The article, “Body Vision: IR Applications and Lens Selection in Biomedical Settings,” (BioPhotonics, October 2010,) gives some examples.

Now I appreciate that most of my target audience deal with industrial applications of machine vision, so you may be wondering about the relevance of biomedical imaging to your work. Well here’s the point: IR light can be used to see through things that appear opaque to our eyes, and conversely, some materials that look transparent to us are opaque to IR.

Depending on what you’re doing, this can be either a blessing or a curse. Plastics, for instance, have some IR transmissibility, so a strong backlight might highlight internal features that can’t be seen otherwise. On the other hand, you may find that longer wavelengths – perhaps even just deep reds at 680nm – show some penetration, which will complicate surface inspection. (Metals of course are immune to these problems. Well I think they are. Try it for yourself.)

Bottom line? Next time you’re looking for a way to light that hard-to-inspect part, give IR a try. Or keep well away from IR. Either way, you might be surprised by the result.