What surface should I place test charts on top of when taking readings?


When taking readings using a colorometer (colormunki in my case) does the qualities, e.g. luminance and opacity, of the surface on top of which my test chart paper sits matter?

I have been testing out various matte papers, taking readings with the test chart paper placed flat on my desk. The desk is a middling pine kind of colour and while taking some readings I suddenly became aware of the difference, by eye, in luminance between the test chart paper sat directly on my desk vs the paper sat on top of another sheet of (white) paper.

So I quickly measured the difference using the colormunki. With the test chart printed on fairly thin white paper (118gsm) and placed on top of a sheet of thick off-white card vs. the paper placed on my desk, readings are as follows:
At the black end luminance is 0.5 lower, density 0.02 lower.
At the white end luminance is 1.0 lower, density 0.01 lower.

So having the whiter card surface underneath the test chart paper appears to ‘lighten’ the readings so to speak. Granted it’s not exactly a rigorous test but it is in line with my by eye observation that there’s a difference.

For test charts printed on thicker papers the difference doesn’t appear so pronounced. When I started piezography printing I was using exclusively 300gsm Somerset paper for a while which is probably why I didn’t notice, but now I’ve moved on to much thinner papers, i.e. with far lower opacity, it caught my attention.

Feels like kind of a newbie thing to miss but I can’t find any reference to the issue at all anywhere.

Is there a best practice for dealing with this?

Couple of ideas I had were:

Reading the test chart paper while placed on top of a surface that is as white as possible, e.g. some bright white matte acrylic, wood or card?
Reading the test chart paper while placed on top of a thick stack of the same paper?

Or is it that these differences get accounted for in the clever curve wizardry of the piezo software, as one curve becomes another etc - so long as the test paper is being read on top of the same surface every time?

To start testing this properly and seeing how it affects curves and final prints is probably the right way to go, but thinking about all the various of factors and permutations is really making my head hurt and honestly I would like to get making art for the time being. So I would very gladly run with a solution that someone else has come up with to this specific part of the workflow :smiley:


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