Tuesday, July 17, 2012

Sad story

The sad story why the unicorn doesn't glow in the dark

As we did't want to do it sparkling way, but aimed for an evolutionary correct unicorn, we thought it might look too ordinary. Why not let it glow in the dark? One might even come up with some explanation for that selection factor apart from secondary sexual characteristics. Even though we have to admit that the Peafowl too is some kind of magical creature.
Options
  1. A single heatbulb at the top. Retro! But hight power...
  2. LED lighting. Still had to power it. In addition the horn isn't a gadget. Otherwise we'd had to add bluetooth.
  3. A Laser. Hannover would have been proud. In fact, next time: we only put up a laser pointing upwards. Horse at day, magical (depending on HeNe, Argon, doubled Nd:YAG, ...) creature at night.
  4. Chemical reaction as in glow sticks. Would only work for a single night.
  5. Fluorescent color pigment. Ok. we'd have to install a UV lightsource somewhere.
  6. Phosphorescent color pigment.
Last is our winner. Sunlight populates an exited state that is forbidden to decay to the ground state. But as quantum mechanics is awesome: Electrons can tunnel to the ground state, with a huge half life of course.
This way the emission of light is delayed for hours. As with all probabilistic decaying things, the emitted light intensity is proportional to exp(-t).

So much for the theory. We applied greenish phosphorescent pigment to parts of the horn and the effect was quite stunning. After some time in the sun, the glowing was strongly visible for some hours after sundown inside the room. Held up against the sky outside the window, the effect was still visible.

From earlier investigations we knew, that the space in front of the university had quite a few light sources. And as the main building's walls are lit the background would be quite bright. That's why our biggest concern (apart from not getting caught installing the horn and the platform not fitting the head) was, that the glowing wouldn't be visible at night.

Sad part, as you might have already noticed is: Glow is not visible.

Investigation

Obvious reason: The environment is too bright. If the unicorn was in a dark forest, as they used to be, no problem.
So, let's get a little deeper into that. We'll discuss two possibilities
  1. The faint glow is simply not bright enough
  2. The decay rate of the effect is faster, than the rate the outside light reduces after sundown
After installing the horn, we flashed it with a DSLR flash. From earlier test we knew, that the glow from that was about as bright as from direct sunlight exposure. In the following picture one can clearly see a greenish glow. It seems that intensity of the light emission is not the issue.

Next: Decay rate. In the following we're going to compare brightness measurements from image sequences we've taken.
The Day1 time-lapse delivers the sky brightness around sundown. Images were taken at ISO400, f/4 and the sky was scaled to 18% grey for comparison. The above image (taken at ISO500, f/3.5) delivers a reference value for the glow's intensity after exposure to light equivalent to direct sun exposure.
In addition we directly measured the phosphorescent pigment. Setup was the following: We took digital images in 30s interval and let the camera's meter measure the glow at 18% grey. ISO200, f/2, which is half the sensitivity as with the day1 images, but 4 times the light through the lens. We compared the resulting exposure times of the images scaled by the mean brightness distribution over the glow.
We plotted both intensities scaled to match in a logarithmic plot to base 2. Every increase by 1 on the y-axis represents a doubling in brightness.
Interestingly the sky brightness went down exponentially (as you can tell from the quite linear plot), the glow's intensity didn't follow the predicted law at all! Only for the decay starting at 10 minutes of the last exposure, one could guess a linear functional relation in the logarithmic plot.
Even though the slew rate for the sky is higher, in other words: The sky get's dark faster than the glow, the ~7 octaves signal ratio (when the glow's decay becomes linear), are way to much for the effect to ever be visible. After ~100 minutes from sundown, the sky's brightness didn't decrease much further, but glow was still > 4 times fainter ( ~2 octaves signal to signal ratio).

If some of you have observed the glow despite this physically correct examination, magic happens. Feel free to leave a comment.

Next, we'll Next time we go for the bluetooth option.

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