How do I do "white balance" correction in my scans. What is it exactly? How
does it work? thanks in advance.
There is sort of two cases..
If we assume there is always some bit of white in any image (a cloud, an
eye, etc), but perhaps there is a color cast so that it doesnt come out
white on our film or in our scan, then we can correct/force it to white by
adjusting the three RGB channels of the histogram individually, lowering the
White Point in each channel so that the White Point just touches the start
of the brightest data actually in that channel. Then that brightest tone
(assumed white) will become white. This is easily done manually, and I
believe this is how VueScan uses the term. It usually works well, but some
images dont have any white (flower closeups of non-white flowers for
example) and then it fails, and some other method must be used. Similarly,
scanning software often offers a tool to click on an image area that ought
to be a neutral color (gray or white) and any color cast actually there (at
the click) will be neutralized away in a similar way, that is, that color
cast is removed over the entire image of course.
The other case common today is about the color temperature of the lighting
in the original scene, which can prevent white from appearing pure white.
Digital cameras have White Balance settings for sunlight, shade,
incandescent light, florescent light, etc. This setting seeks to make white
come out white in those various colors of lighting, much like using the
proper corrective filter.
