The View Up Here

Random scribblings about kites, photography, machining, and anything else

Camouflage, Paint, and Hiding in Plain Sight

Posted by Tom Benedict on 12/06/2017

I’m planning on using my weatherproof recording box to record wind in the rocks and grass at a remote lake up on a mountain, weather permitting. (Weather not permitting, I’ll record something else!) But the place I’m planning to record is primarily browns and reds rather than the greens I used to camouflage my weatherproof gear. It’s too late to change my gear now, but in the future I’ll need to think more about the camouflage I use to hide my stuff in plain sight.

Meanwhile an interesting discussion about camouflage took place on one of the field recording discussion groups on Facebook. In the discussion, someone raised the point that paints, fabrics, etc. that look fine to us in the spectrum we can see may be overly visible or downright jarring in the spectrum visible to some wildlife. The discussion centered around deer, which can see into the UV, but also applies to birds, insects, etc.

Since I’m already in the business of characterizing the reflected spectra of materials, surface treatments, paints, etc. I scanned the paints under discussion to see what their reflected spectra look like. I prepped the samples the same way I did for the SPIE paper: four coats, applied at roughly a 45 degree angle, coming in from the four cardinal directions.

Krylon Paint Reflectivity 250nm - 750nm

All of these are from the Krylon Ultra Flat Camo Paint series, one of which, their flat black, was part of the sample set I scanned for the paper I presented at the 2016 SPIE Astronomical Telescopes and Instrumentation conference in Edinburgh.

From 250nm to 750nm things look relatively normal. The behavior short of 350nm is more a function of the binders than the pigments, and all are relatively similar to the flat black, some even performing a little better in the near-UV.

From 350nm out to 650nm you see the action of the pigments in each of the paints. Khaki is fairly broad-spectrum, reflecting a good amount of reds, greens and blues, tending toward the redder end of the spectrum. The brown (which is quite dark) peaks in the red, with lower reflectivity in the greens and blues. The olive and light green peak at shorter wavelengths, favoring more greens than reds. It all makes sense until you look at the near-infrared.

Krylon Paint Reflectivity 250nm - 2500nm

That’s where it gets really weird. The Krylon flat black paints use carbon as a pigment, so they tend to stay low well into the NIR. The long tails on the brown, olive, and khaki paints aren’t surprising, especially given what I saw with some of the other samples I measured for the SPIE paper.

What’s weird is the light green paint. It’s more reflective in the NIR than any of the other pigments in the visible. I have no idea what they use for a pigment, but it’s got one heckuva NIR signature.

None of which may matter much when it comes to camouflage. Photosynthesizing vegetation is quite reflective in the NIR, so having one out of five colors reflect strongly at NIR wavelengths may actually help the disguised object blend better in plant settings.

In reading further about deer and their ability to see into the near-UV, I learned that there are two things at work: First, deer really can see further into the UV than we can. In the case of reindeer it helps them find the lichens that are one of their primary food sources and it helps deer spot UV-absorbing urine markers from predators.

The other factor at work is that the sensitivity of the blue receptors in deer eyes peak around 400nm, right around the center wavelength at which fabric and paper brighteners fluoresce when exposed to UV light. It’s this effect that makes white cotton shirts, shoestrings, and paper glow under black light.

The Shimadzu spectrophotometer I use to measure samples registers light of any wavelength, so it’s sensitive to fluorescence as well as directly reflected light. But just to be on the safe side I photographed the paint samples under UV illumination, as well as with an IR-converted camera, the same one I used for the SPIE paper.

Paint Three Ways

All in all, I think the Krylon Ultra Flat Camo paints are good to use as camouflage, both for humans and for wildlife. But I need to come up with a better color combination for blending with lava rock than the green scheme I’ve currently got on my gear.

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