Digital BW, The Print

Ken, comments are below your questions.  

Lou

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, Ken Alexander
<k.alexander@...> wrote:
>
> Hi Everyone,
> 
> I feel funny asking this after successfully printing
> in black and white for so long, but I'm still trying
> to get a firm grasp on what Dmax is.  My (probably
> oversimplified) understanding is that it is the
> maximum darkness (density?) that a given combination
> of paper and ink can achieve.

That is correct.  Each printer/paper/ink combination has a maximum
black it can achieve (often called Dmax).  The paper and ink are
obvious contributors, but the printer can also have an impact. 
Different dithering, resolution, number of passes, etc, can affect the
ability to achieve a solid black.  Too much ink, or incorrect
combinations of ink, can result in pooling, fogginess, and reduced Dmax.  

> 
> What I still don't understand is:
> - how is it measured?
> - what units is Dmax expressed in, or is it a
> dimensionless number?

Print Density is usually measured with a densitometer or a
spectrophotometer.  It is often expressed in density units, but also
in L* units(the L* component of Lab, ie, Lightness).  They are two
different ways of expressing the exact same Dmax.  Since Lab space is
often used as the universal translator for printer profiles, and most
spectros read Lab data, it is often used, but both units are widely
used.  

> 
> I did try googling Dmax a while back but wasn't really
> satisfied with what I found, especially regarding the
> two questions above.

A great place to learn more (and see a translation graph that equates
L* and density) visit http://www.BruceLindbloom.com.  Click the
"Calculators" link, and then "Companding Calculators".  If you click
density as the input and L* as the output (or vice versa) you will see
a graph that converts from one to the other.  You can also input
specific values to convert back and forth.  A density of 2.2, for
example, is equivalent to L* = 5.6994.  The calculator also lets you
see density expressed in Y units, or your choice of two different
gammas.  Pretty flexible.  

Bruce also has a lot of articles, formulas and other information on
his site related to color, density, color management, etc.  It gets a
bit geeky fast, but it is loaded with tons of good information.