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Archive for June, 2017

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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Weatherproof Recording Box – Part 2

Posted by Tom Benedict on 02/06/2017

A little while back I wrote about a weatherproof recording box I’d built so that I could do drop-and-recover field recording without having to care quite so much about the weather. The design had a couple of shortcomings, most of which I’ve been able to address. Since then I had the chance to take the revised box out into the field and put it through its paces. I’m happy with how it performed.

The two biggest issues I had with the first revision of the box were the heating problem I mentioned at the tail end of that article and the fact that with the connectors I was using, I couldn’t actually plug all four channels into my recorder and still have it fit in the box. I had a couple of minor issues as well, including the fact that the lid on the weatherproof outlet cover rattles. Here are all the changes I made to address these problems:

I fixed the rattle in the outlet cover with small snippets of weatherstripping. Nothing fancy, just adding enough spring to take out any tendency to rattle in the wind.

I also added strips of industrial strength Velcro to either side of the box. This makes it easy to use the box as a baffle and mount omnis on either side like tree ears. I’m not entirely happy with how this sounds, though, since the box represents a much harder boundary than tree bark does.

The way I solved the connector issue was to swap out all the Neutrik XLR plugs for low profile connectors from Cable Techniques. Redco Audio sells these for about $15US. They come with black caps, but Cable Techniques makes color caps for the connectors as well, so I was able to maintain the color coordination between the outside panel and the inside connectors.

Low Profile Plugs

While I was at it I bought a right-angle USB cable for the battery pack. In 20/20 hindsight I wish I’d bought one with a right angle connector at both ends instead of just at the micro-USB end. But at this point it’s just a nit-pick. The cable I got works fine.

Right Angle USB

With those two changes, everything now fits in the box with all the foam panels installed.

Everything Fits

Which brings me to the second major modification: In an edit to the last article I said that I’d tested everything with a temperature logger, and in a 21C room the air temperature inside the box had risen to 34C after eight hours of operation. What I didn’t take into account is that the battery and recorder were considerably warmer than the air in the box. The operating range on my DR-70D is 0C-40C. I wasn’t comfortable running it that close to the limit.

Thermal Pad
The foam panels in the box are all removable. They just wedge into place. So I pulled the two side panels and installed thermal pads. Thermal pads are made out of silicone that’s had thermally conductive additives mixed in. They’re graded by their rate of heat transfer. The pads I bought have a decent transfer rate, but aren’t up into exotic territory. (I got mine off of Amazon.) As it turns out they’re probably overkill for the job, but I’d rather err on the side of too cool than not cool enough.

Everything Heat Sinked

With both pads installed and everything loaded into the box, a small block of closed cell foam acts like a spring to keep the battery and the recorder pressed up against their respective pads.

Weatherproof Recording Box in the Field

Last weekend I took it out to an old cane haul road on the Hamakua Coast to record coqui frogs. Unfortunately the road was blocked off about a mile from where I wanted to record, so I had to set in a different location along the road. It looks like logging operations are about to start out there, so this may be the last time I get to record at that location. (The box is weatherproof, but it won’t stop a bulldozer!)

I had some issues with moisture causing noise on the mics early on in the evening, but overall the session went well. I used my SASS with EM-172 capsules for this. EM-172s aren’t typically all that sensitive to humidity, so I’m looking into why the mics glitched. So far I haven’t been able to reproduce it, but I might spend some time monitoring the next time I use this box just so I can hear if it’s happening again.

Meanwhile, here’s a long(ish) track from the test. It covers the sunset transition from primarily birdsong and insects to the coqui frogs dominating the soundscape. And there’s the occasional cow. (I really love the way cow calls reverberate through eucalyptus trees.)

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