Black Light
Did you know that you can take photographs in the invisible infra-red part of the spectrum using your ordinary digital camera? It turns out you can; allow me to explain how I noticed this strange phenomenon, and give brief details of how you can try it at home.
I was at a party stood next to a brazier and I decided, having got atmospheric shots of pretty much everything else around, to take a picture of the embers glowing romantically away. Deep reds and oranges swam slowly beneath the shadowy mask of burnt wood…but, when I took a photograph, I got a bright pink mess. I tried again. Another bright pink mess.
I have repeated this several times, both at my party and with a fire in a hearth. One Christmas, it struck me. What do fires give off a lot of? Not very much light, but quite a lot of heat. And what comes in between light and heat*? Near infrared!
So, my guess was that the CCD in my digital camera was sensitive to infrared radiation. So, I grabbed a nearby remote control, and gave it a go.
The remote leaves a lovely trail of pink—exactly the colour which originally blighted the brazier photo—and so it would appear that the red and blue sensors in the camera are especially sensitive to this ‘black light’.
Thus, I waited until morning, at which point I grabbed the Wratten 87 filter from my filing cabinet. This very expensive piece of opaque cellophane is something no self-respecting physicist should be without. It looks like a black square of plastic, but is in fact transparent not to visible light, but near infrared. It’s the essential bit of kit, once you’ve checked if your camera is IR-sensitive, if you want to take photographs in black light.
So, I decided to photograph my garden, shown here in both infrared, and with a visible comparison.
The glowing grass is because foliage reflects infra-red very strongly, just as it reflects green. Indeed, if you’re looking for soldiers in camouflage or covered in fake leaves and webbing, infrared is a good wavelength to choose, because the dark soldiers’ plastic leaves will stand out a mile against the bright background of real jungle.
It looks far cooler if made into a monochrome black-and-white image, and one of my favourite weird pictures is of me. Note the fact that, in near infrared, my beard, eyebrows and face all seem to have almost exactly the same reflectivity… Useful information for any crack troops who want to hide on my skin.
*A final note on the EM spectrum:
“Why don’t you take pictures of exciting hot things?!?!” is the most common question you’ll be asked if you tell someone you’ve been taking infra-red photographs. It is true that infrared radiation is what we know as heat—but heat is a small part of the expansive part of the spectrum we call IR, and there is a section in between visible light and heat called ‘near IR’. It is this near stuff which consumer digital cameras can detect. The IR in the above photographs probably has a wavelength of around 700–900—about as far the other side of red as green is from it into the visible spectrum (therefore about half the width of a rainbow above a rainbow, as it were). So sadly, there will be no intriguing photographs of heat emission, just intriguing photos of albino people and glowing grass. Suits me—until I can afford a heat-sensitive camera to play with instead… If none of this makes any sense at all, try reading this patronising explanation from NASA!
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