[argyllcms] Re: White Point

  • From: Ben Goren <ben@xxxxxxxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Sat, 7 Nov 2015 21:06:22 -0700

On Nov 7, 2015, at 8:33 PM, Iliah Borg <iliah.i.borg@xxxxxxxxx> wrote:

Teflon tape is better applied to a matt white backing.

Just to expand on this: the easiest solution is a piece of something stiff
(like foamcore) with a piece of top-quality OBA-free fine art paper on top (I'm
very fond of Crane Museo Portfolio Rag these days), and then multiple layers of
teflon tape wrapped around it and fastened to the back. Elmer's glue works just
fine for fastening. You could use a drop to glue it to the back, wrap a few
layers, use another drop, and so on, to work up to however many layers you
want. Maybe in a criss-cross pattern. With a quality fine art paper as backing,
you won't need many layers. The back side can be ugly; no need to make it neat,
so long as the front is smooth.

The surface is going to be delicate, but it's also dirt cheap and it's quick
and easy to make. Consider it disposable.

However...it's also less important / useful than I once thought. With a decent
raw developer like RPP that does linear exposure adjustments before anything
else, especially any sort of tone curve, exposure becomes all about ETTR (don't
clip any RAW pixels you care about) and then a simple combined linear
adjustment for both exposure and white balance so that a known sample (or
average of samples) of any color has the proper RGB values in the camera's
native color space (for the various combinations of scene and profile
illuminant, etc.).

Or...if you know the camera's spectral response, the spectrum of the actual
illuminant in the scene, and the reflective spectrum of a sample, you can do a
bit of math to predict the RGB values the camera is going to create even before
you press the shutter, which gives you the white balance parameters. Compare
predicted with what you actually get and you get your exposure offset plus any
necessary fine-tuning of white balance to account for errors in various


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