The Foveon and IR 'colors'

Started Jan 31, 2014 | Discussions thread
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xpatUSA Forum Pro • Posts: 12,248
The Foveon and IR 'colors'

It is well known that the so-called 'hot mirror' is easily removable from the Sigma DSLRs starting with the SD10. Each 'layer' in the sensor becomes sensitive to IR (even the 'blue' layer) due to the wide bandwidths of the Foveon method. So it gave little surprise when my IR photographs were not the expected red (which is often suggested in the literature), for example:

Raw data from CEZEO utility converted with 2.2 gamma, default RGB matrix

The lurid purple color is due to the blue channel, after conversion, having about 50% of the red channel. Curiously, there is little or no green in the above image. This certainly runs counter to the Foveon response, where there is a goodly amount of IR response from the 'green' layer:

F7 sensor response without a hot mirror

So, why no green and why a whopping 50% blue?

To get decent RGB pictures, even with the hot mirror in place, the Foveon requires a pretty fierce matrix or matrices to convert from the raw data to an RGB image. I have a utility called 'CEZEOx3fReader' which allows access to such a matrix. Here is the default:

This matrix is the one that gave the purple image

Some pretty big numbers in there and three of them are negative. Note, too, that the high values are pretty much in line with what one might expect with respect to the spectral response posted above.

Out of idle curiosity, I set the values to be a diagonal matrix instead. Values of 1, to give 'what goes in, comes out'. Voila - that's more like it:

CEZEO RGB, 2.2 gamma, 1,1,1 matrix

Here's a RawDigger composite for the same file:

RawDigger RAW composite output, 2.2 gamma, luminance 'stretched'

Comments welcome.

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