Sunday 6 December 2015

BirdBox 2016: fitting the UV LEDs

Although initial testing with a bunch of UV LEDs did not look too promising, I've decided to push ahead and fit them to a bird box anyway.

They were very cheap, and I'd like to say they were easy to retro-fit into last seasons bird box.

But a wiring error resulted in a blown Darlington driver, so it took up more of my Sunday afternoon than I'd expected.

The reason for doing this is covered by an earlier post on the difficulties of telling the difference between male & female bluetits.

Not a hope!

I was hoping that a group of UV LEDs would somehow extend the visible range down to about 350nm, and that the RaspiCam would render these higher frequencies into colours or shades that I could see in the streamed video.

Further research on the web, and a bit of testing with RaspiCam (...and a few colouring pens) leads me to think that this just is not going to work.
Quote from the web: " The same laws of physics that make LEDs light up also limit their maximum wavelength range to a narrow band about 30nm wide. The range for most LEDs is even more narrow than that at less than 10nm. "
I felt compelled to compare a standard RasipCam with a NOiR version, just because so many websites claim the NOiR is good for iR and UV.

How can that be? Infrared and ultraviolet are at opposite ends of the visible spectrum.

The difference in background colour is largely due to exposure and White Balance

Although there is a difference in these two shots, when "leveled" in The Gimp they look the same.

I also found a UV marker pen and used this as a reference.

Although out of focus, the standard RaspiCam image is similar to the NOiR

When viewed with the naked eye under my UV LEDs, the marker pen ink is really bright. I think this is because both marker pen and the UV leds are most responsive around 400nm.

But the RaspiCam/NOiR images render the text as light blue or turquoise. So I suspect the camera's UV response is pretty poor.

Installing the UVs

Anyway, just like Marconi (when everyone told him that radio waves only travel in a straight line) I decided to push ahead anyway.

White leds reflecting light, while UV leds shine straight down

I've added 3 UV LEDs to last years box, but due to space constraints (and a feeling that reflecting UV off of white paint might not be very effective) I have mounted the LEDs facing straight down.

I doubt I'll be using them much once I have done a couple of simple checks with real live birds. Anyway I have not driven them very hard, so hopefully the birds won't be too bothered by them.

These particular UV LEDs have a forward voltage of 3.3Volts. Three in series will drop approximately 10V of my 12V supply, so I used 2 x 100 Ohm resistors in series to limit the current.

Diode current = (12 - 10)/200 = 10mA

Once running in the box, I captured a few more images, initially using a soft fabric place-mat.

White leds only  -  UV leds only  -  White and UV leds together

...and then I wrote my name on a box with UV marker pen...

White leds only  -  UV leds only  -  White and UV leds together

So that's it. If female blue tits use UV marker pens as eyebrow pencils, it should work!

Will I notice anything different when the blue tits arrive in March, who knows?

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