Wednesday 19 November 2014

Forensics: Gaussian Analysis - White Balance

The very Gaussian nature of white balance should be recognisable to most.  A Grey Card is like a compass where white is magnetic north and there are three axes instead of two.  Manual white balance correction is much like navigation using ones senses, without the aid of a compass.  Those experienced in navigation without a compass will be able to make do under most conditions but bias can easily throw us off course, so a compass is required for accurate, reliable navigation.  For instance, it has been shown that if a person is blindfolded (or navigating in a snow storm or in fog) and asked to walk a long straight line, they will end up walking in a wide circle.  The other senses, trying to compensate, will lead us astray.

The white balance which we are most familiar with is Kelvin colour temperature - the blue-yellow axis (B-Y).  This is the colour tint introduced by the sun's position in the sky (Rayleigh scattering).  HERE I discussed another familiar scenario where green light transmitted by a foliage canopy bathes a scene in a soft, diffuse, green light.  This can only be corrected for by having access to the green-magenta axis (G-M).  Normal manual white balance tools do not cater for this axis.  In Adobe Photoshop this axis is referred to as Tint and is provided for in Camera Raw.  

Red colour tints are common in nature, in desert areas where the soil and sand has a high iron content.  Coming from a temperate climate myself, it was immediately noticeable to me how the red soil of Australia transforms the colour of everything, even of the blue sky on that continent, such is the volume of red light reflecting from the land.  So, what of the third axis red-cyan (R-C)?  If we have access to the colour temperature axis (B-Y) and the tint axis (G-M) we don't actually need to have access to R-C.  In my experience we are not very good at navigating via the R-C axis anyway.  When I worked in photofinishing I observed that everyone in the lab naturally drifted towards colour correction using G-M and B-Y.  Studies of the eye reveal that we have twice as many green sensors (cones) as we do blue and red.  This might explain why most people seem to be drawn towards correcting the green-magenta axis before any other.  Despite having fewer blue cones I guess we are also quite well tuned in to the B-Y axis due to our daily experience of observing the behavior of sunlight.  For more on the anatomy of the eye (and camera sensor) see HERE

I recently came across this animated gif which I created some years ago.  It neatly shows the compass-like nature of white-balance correction and the trial and error of navigation without a compass.  CLICK on the image to enlarge and run the animation from the start.

In summary, if we are interested in the forensic analysis of white balance we need grey card calibration.  Manual white balance correction can be done with experience but cannot be 100% relied upon.  For more on white balance see HERE.

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