Digital BW, The Print

John,

You 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. ...

>...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!

>...

You're densitometry reading are correct.  Whether the system is "WAY off" is
a question that I've never found a really good answer to.

In general, I've been told that human visual response is logarithmic --
which is what those densitometry readings are.  However, whether this is an
exact relationship or just a close approximation is not clear to me.

As a practical matter, in designing the variable-tone system, I used Piezo
outputs as the standard, because I wanted to be able to use the same file
for both the Piezo driver and the Epson driver, using the VM curves.  When I
took a number of Piezo ink samples (throwing out the clearly bad inks),
printed them only from newly-filled cartridges (thus avoiding the "greenies"
and other problems that infect a number of CIS systems), and used the latest
profiles, I got an average 50% density of 0.61 -- just what you report.

So, I have asked both here and elsewhere, "What is linear" -- how does one
find the theoretical middle of our visual response.  I have received no
adequate responses.

I assume that Cone and his software designers had some printing industry
background and expertise that went into their system (or not -- who knows).
Again, as a practical matter, that system works and many of us use it.  So,
for me it became more of a practical issue of consistency with a system that
worked well and was widely used by those likely to use the variable-tone
system.

Have you tried printing a "black ink only" 21-step test file?  On the ones I
tried, the "50%" is closer to 50% of the reflection of the paper white --
yet another definition, and even further "off" than the Piezo definition of
the "50%" value.

Paul
http://www.PaulRoark.com