Archive for May, 2010

IR photography

Posted by 3ricj on 31 May 2010

CCD sensors are very sensitive to IR light. Most cameras filter this out so that color balance is easier and photos look better. However, if all you want is to photograph IR, you can hack your camera such that you can see it.

I modified a Canon SD1200 point n' shot such that it can only be used for photographing non-visible light. Here's how you do that: Disassemble your camera such that you can extract the window right in front of the camera sensor. Make sure you use gloves and keep track of your screws and tiny bits. Don't get dust/fingerprints on your optics. Remove the existing IR blocking filter carefully with a tweezers on the edge. Having things like lens tissue is good to have around to place your optics on. Once you extract this IR blocking sensor, you'll need to replace it with a suitable 'window' of the same size and thickness. With my camera, the window was 7.9mm x 8.88mm by 0.22mm thick. I found that I had glass coverslips which where pretty close in thickness (0.18mm). Using AR coated glass from edmonds optics or thorlabs will give you much better results. Sizing down this window was tricky - - Markr finally got one the right size. If your window is thick you might need a glass cutter; in my case 0.18mm glass can be cut with snips and broken to size easily.

Once you have it cut down to size, you must clean it completely. Lab grade acetone or methanol is the ideal cleaning solvent. Grasp your new window by the edges using tweezers, eyedrop a bit of acetone onto the surface, then quickly blow it off using a can of air. Don't use 'shop air' as it frequently contains quite a bit of water and oil. Repeat as needed until it is clean.

Once your camera is reassembled, you can take 'full color' photos. Most photos will look far more 'red' than before. Check that your focus is still ok - - if you get your window thickness wrong your camera will be near sighted or far-sighted.

For the really fun stuff, you need to filter out visible light. You can do this using a visible light filter (such as a 87 filter from a camera shop, $150) or you can make your own filter using non-exposed developed film. If you find a place which develops medium format film, give them some non-exposed film and ask them to develop it. Make sure you tell them that the resulting pictures will be pure 'black'.. that's what you want. Film which has been prepared like this has one very tricky attribute: It will block nearly all visible light, but pass

most IR light. Now you cut down this film in order to make a filter on your hacked camera. This can be done after the optics on the front of the camera, or could in theory be installed in front of the sensor.

What does this look like?

Here is the back of a 5 dollar bill which has been photographed using a full spectrum camera:

.. and here it is in 'pure' IR:

Note the interesting lack of print over sections of the bill.

For extra credit, you can also filter out the light on your flash, such that it only emits IR. "non-LED" Camera flashes produce quite a bit of IR, so you can use the same film filter to block the visible light on your camera flash and only produce IR. The resulting camera can take shots at night without a visible flash - - all sorts of fun purposes for this.

Here's an example of a tree photographed in pure IR:

More examples can be found here, or by googling IR photography.


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