Digital BW, The Print

Steve, et al:

I've been following this thread and enjoyed the exchange of information. I 
have a few comments that, I believe, correspond to a few of things Tyler has 
been talking about. I’ll keep this as short as possible.

On the desktop we use software to control the behavior of hardware. With 
displays, for example, the machine is calibrated to known standards and a profile 
is then built that describes the color gamut and gamma of the device. Inkjet 
printers are linearized and although calibration and linearization perform 
similar functions, for their respective devices, they aren't the same thing. 
Displays aren't linearized. The calibration includes a gamma setting.

All image devices used in the digital darkroom—cameras, scanners, displays 
and, most important to this discussion, inkjet printers—include a gamma setting. 
As do analog film and printing paper. Gamma is built into imaging systems to 
account for human vision, which is anything but linear. In fact, you can think 
of gamma as an attempt to objectify our subjective experience of the world 
(which explains, in part, why we have different gamma settings to choose from in 
the digital domain).

Linearization controls the amount of ink used in each ink channel. The basic 
idea is to accurately map the image file information, including whatever gamma 
you've selected, to the printed surface. One of the issues with current OEM 
print drivers, which cannot be linearized, is that the image file gamma has to 
be mapped to the driver gamma bias. People like Paul Roark have done an 
amazing job of working through this issue.

With good software, linearization can be tweaked when necessary or desired 
(it just depends on how far you want to crack open the "black box"). Printer 
driver software, OEM or RIP, doesn't "care" what color space or profile is 
attached to an image file. It does care what gamma the image represents. It exists 
to manage the hardware functions of the machine and translate pixel information 
into droplets of ink.

Years ago, Kodak developed a methodology for mapping image density to code 
values, called the "Jones Plot". It addresses the issue of how image information 
is translated from one form into another. It’s useful to us because there 
are, in fact, significant semantic issues currently at play in the digital 
darkroom. In time, these issues will resolve themselves (talking about the 
translation of image information is a little like talking about resolution, where the 
same terminology is used to describe different things). 

Long story short (Ha!): if possible, it is best to work with a linearized 
inkjet printer. If a system is properly color managed you should be able to work 
with any conventional gamma setting offered through Photoshop's working modes 
(or build your own, including any variation of the LAB model) and produce 
prints that appropriately match the image on the display.


Bill Kennedy
Associate Professor of Photocommunications
St. Edward's University
512/448-8680


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