Territory marking through UV fluorescence
March 24, 2012
For awhile now I've been considering ways to have robots tag an area for other robots to detect in a way that wasn't readily apparent to the human senses. The application would be for my outdoor solar swarm bots. They would use the marker to signal to other robots that "this is a sunny location in the morning." I wanted something passive, no electronics. No dropping off a bunch of miniature IR beacons around my patio or any such thing. Hmm.
Add to that my fascination with insects such as the firefly, the ant, and the bee. The firefly speaks in a visual way that is easy to see and appreciate. The ant communicates through various temporary pheromones invisibly and a bee's vision extends into the invisible UV wavelengths.
So all this culminates into this little project. The idea is to have some sort of dispensable medium that glows under UV light. Then find an efficient LED that has a peak wavelength in the brightest part of the spectrum emitted by the fluorescing medium. The LED will operate as a photodiode and its signal run through a low current op-amp with adjustablility. There will be a UV LED pointed at the focal point for the photodiode LED. When the UV reactive media passes under the UV light it will glow and be detected by the LED/op-amp setup. The assumptions are:
- This detector is no good if ambient light is around. The detection of a soft fluorescent glow will require sensitivity that will be rendered useless in anything but darkness, ie the black of the night.
- Most material will not glow under UV light. At least materials like concrete, patio tiles, sticks, leaves, etc. If it does it's usually in the green or blue wavelengths.
- Any toxic material is unacceptable being in an uncontrolled outdoor environment.
I had some 380nm UV LEDs from around the time I was tinkering on my firefly ideas. Those ideas kind of puttered out but the unused UV LEDs would finally come in handy. I only have four that put out around 50mcd (these were from when cheap UV LEDs were still kind of rare). They should suffice for what I need.
The hunt was on for a dye that would offer a glow under UV, preferrably in the red hue. Not only for the reasons stated above but also because red is an easier color to detect. I'm really trying to stay away from having a bunch of LEDs hooked up in parallel to help amplify the signal. I found the red dye they use to pour into rivers and such but it's prohibitively expensive. I ran across red UV reactive hair products but would like to be able to control the concentration of the dye. I found dye used to detect leaks in cooling and A/C systems in cars, but again, somewhat expensive and of questionable toxicity. I even learned how to make red glow stick fluid but I thought of all the dolphins out there and decided against it. Then I found a red UV dye that can be added to computer coolant systems for visual appeal. Although it seemed to glow more orange than red I decided to go with some of that stuff.
It's strange but for some reason it's not easy to find the wavelengths UV dyes glow at. Fortunately I remembered using spectroscopes in junior high school so I grabbed one of those. Althought not terribly precise it would allow me to estimate the wavelength to the +/-5nm. I was expecting a pretty well defined emission band so that should suffice.
This is what you see through the spectroscope when looking at daylight. You line up the slit on the right with the light source you want to analyze. You can see it's just like all the science books show; a continuous spectrum from red to blue.
This is actually a shot of sunlight lookng through the dye in water. I was surprised to see the mixture looked bright orange when you looked at it outside in sunlight versus the deep red when seen indoors. I did try to get a first-person view of the sample under my puny artificial UV array but my camera was not sensitive enough to see anything. The spectrum pictured is pretty close though. There is more red (>~610nm) than what you see from the fluorescing glow. I chalk that up to the visible light filtering through the non-fluorescing red element of the dye. I was surprised to find it a very wide band, not like a sodium or mercury emission.
A pic of my two LED array.
I eventually hooked up all four LEDs and it helps a bit. I plan on getting some UV LEDs with thousands of mcd output.
I did some initial testing of the dye/water mixture on the patio and it just dispersed too much. It was clear that it would be necessary to suspend the dye in something so it would stay in a concentrated, detectable form. The wish list of criteria was:
- It would have to have some viscosity. Not too thin, not too thick. Something dispensable from a low-voltage pump possibly.
- Not attractive to animals. No need for the critters to be licking up my markers late at night.
- Nothing toxic. Oh, the critters!
- Clear, transparent. Nothing to interfere with the emissions.
- Dye-friendly. Some dyes won't fluoresce in alcohol, water, etc.
- Fast drying.
- Temporary. UV dye eventually is neutralized by sunlight and bright spots on the patio move with the sun so the medium should be temporary too.
So no oils, corn syrup, epoxy or any glues, cornstarch suspensions, etc etc etc. Eventually I posed my problem to a non-robotic girl friend and she almost immediately came up with clear Aloe Vera gel. Eureka! That just might work.
I since done some testing and found the concentration of the dye is critical. If it's too dense the molecules can work against each other. Not enough and the emission is too dim. My first test I took a good sized blob of aloe gel and added way too much dye to about half of it. With the dye covered stirrer I scooped up some more pure gel and added it to the overdyed gel. I got it where it would glow nice and bright under my UV array and I put the whole thing aside to dry.
It took forever.
That problem aside, I found the dyed blob had dried to the blood-red color of the pure dye and would not respond to UV. However off in the corner, the original blob of aloe gel (the one I dragged the dyed stirrer through), was glowing ferrociously! In visible light you can barely see any color left from the stirrer passing through but under UV it's brilliant. More testing of the dye-to-gel ratios will have to be done.
After all I've seen I'm going with some detection LEDs that have peaks around 610nm. Their outputs vary from 7Kmcd to 15Kmcd with varying viewing angles. It might be a little bit before I obtain them, though. They need to wait to get in on my next whopper component order in the name of frugal shipping and handling expenses. Hopefully with the super bright UV LEDs and the proper mixing of the dye we'll have something detectable.