IR conversion and the color filter array

Started Mar 1, 2013 | Questions thread
kenw
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Re: IR conversion and the color filter array
In reply to JLTaylor, Mar 1, 2013

The first thing to realize is how the RGB channels will balance very differently with an unconverted camera and R72 and with a converted camera.

In many unconverted cameras you are barely getting any IR in at all, you are mostly getting a narrow window of around 700nm in the red channel.  This will be most dramatic in a camera that has a very strong blocking IR filter but that starts at a longish wave length.  You'll get a very red response.

In some unconverted cameras the IR blocking filter is rather weak and may start at a shorter wavelength.  In this case you'll be getting some IR response from all three color channels, often you'll see magenta in daylight WB.

In a converted camera the IR blocking filter is gone and the CFA filters typically pass IR quite well for all three channels.  You may see a color at default WB but this has as much to do with the default WB gains in the camera as it does any difference in IR transmission for the different CFA elements.

As to the false color effects they do have a bit to do with the CFA response to IR.  For example, in my converted G1 a daylight WB with a 720nm filter gives a reddish tone but with an 830nm gives a pastel magenta.

As to the sky, no it has almost no near IR emission.  The sky is very dark.  The leaves are a chlorophyll reflection, it is somewhat broad and the peak wavelength varies - but it is very reflective in near IR which is why the leaves are so bright.

Also be aware, for most cameras people doing false color IR start by swapping the red and blue channels.

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Ken W
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