Digital BW, The Print

On Tue, Apr 23, 2002 at 05:45:05PM -0400, John Chervinsky wrote:
> I am attempting to calibrate my printer output using a calibrated X-Rite 810 densitometer.  I am using MIS FS inks through an Epson 1160 onto PhotoRag 308 paper; I am on a Mac computer.  I realize that this question has come up in the past, but practical elements of technique were omitted.  Below you will find the reflective densitometer (visual) output vs a 21 step stepwedge.  I did measurements using both the Randall and Piezography workflows.  If you look at the numbers carefully, you will conclude as to how shockingly non-linear they are.  I would have expected, for example that the 50 % reading would have been close to 1.7/2 = .85.  The Measured value of around 0.6 is WAY off!
> 
> STEPWEDGE  CONE      RANDALL
> 50.00	     0.61     	0.66
> 

The density numbers are just about right if we are distributing the data
according to human perception. If we say that we percieve a middle tone as
18% (as in a standard grey card) to 22% reflectance then this equates to 
densities of 0.74 and 0.66 respectively. This then is simlar to the results 
you are getting when doing your actual measurements.

I think where you (and indeed I until I thought about it more) are going
astray is in thinking of 50% as a middle between maximum and minimum
densities rather than just a standard deviation from minimum density.
Doesn't the eye always seek a white point and adjust from there?

-- 
Tony Terlecki
ajt@...
Running Debian/GNU 2.2 Linux