Saturday, April 25, 2009

A New Window On The Universe - IR!

I've been a bit of an infrared (IR) nut since I started doing photography in high school. Infrared light is just past our limit of visibility in the red part of the spectrum. By using a red-transmitting filter and infrared sensitive film (generally available till a few years ago), you could record some interesting effects, that we'll get into in a few minutes.

As digital imaging came into popularity and film use slipped a few years ago, many scientific and special-use films disappeared, including IR. Fortunately, the new digital sensors had some sensitivity in the infrared band which was generally bad for normal imaging, and is filtered out. Some had a "red leak" where the filter still passed some IR - my first digital camera, a Sony, could be used with an IR filter and LONG exposures of .5 second or so. Here is a shot of Melinda in her pre-engagement days in her tie-dye t-shirt and sunglasses. Note the IR image loses the shirt color, as well as the tinting in the sunglasses! My newer Canon cameras didn't have a similar red leak, so were not able to be used for IR.

But where there is a need, there is a solution! There are cottage industries showing up to remove the normal IR blocking filter in the new digital cameras and replacing them with clear or specialty filters. Amateur astronomers for several years have been modifying cameras to better transmit the red hydrogen wavelength to better show gas clouds in space. Similarly there are also folks who can install the IR transmitting filters. Jim Chen, of San Diego was recommended by a friend, and after finding a used, reconditioned Canon 20D camera on E-bay, sent it off to him for modification. It came back the other day and I've been having fun with IR! So what Jim does is change the normal filter in front of the sensor with a deep red (almost black) filter that transmits light just beyond the range of vision. The advantage of this over using a clear filter over the sensor, and an IR filter over the lens is that you would not be able to focus and compose with an IR filter - the field would be pitch black. The viewfinder looks normal, auto focus and auto-exposure still works fine, but you get the full IR effect.

Some of the effects you get with IR - since the clear sky is blue, it is quite dark in the infrared part of the spectrum. So the sky is dark, while live vegetation is brightly reflective, or white. Many dyes used in clothes and paints do not reflect well beyond red, so results can be unexpected. A stop sign will not show the "stop" printing. As above, tie dye and sunglasses some out uncolored. When viewing long distances, small atmospheric particles scatter light in the blue -longer IR wavelengths are less affected, so better visibility results. Check out Jim Chen's site for more results, or Google "IR Photography" - I just got 56.5 million hits! Well, that is good for an intro - here are some IR-visible comparison pictures taken this afternoon near the trail head of Finger Rock at the top of Alvernon.

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