Digital BW, The Print

Steve,

ICC profiles are based on spectral data - which we may call
the "raw" data of how color originates, the frequency of the
spectrum that a color belongs to. 

Spectrophotometers used to cost in the $10,000 range not
that long ago. Even a couple of years back they cost in the
thousands. It is only now with the eyeOne and other instruments
that it is possible to get spectral reads within "affordable" range.
A "spectral read" means that all frequencies of the visible
light are analyzed (in groups or frequency "bands") and each
is assigned a value of intensity. From that data, various
numbers are derived based on interpretations that the ICC
consortium has determined (such as Lab etc).

Densitometers, on the other hand, have been around for decades
and even though the X-Rites (810 etc) carry a hefty list price, it's
not so much for their reading technology as it is for their stability
and reliability under the daily grind. What they do is break down the
spectrum in 3 bands by simply filtering the light they receive
with red, green and red filters and reading the intensity of light
coming through each filter.
The filters they use are calibrated to give their maximum cutoff
for a frequency (color) that matches that of a specific photographic
process (E-6, C-41) etc. This is usually referred to as their Status
(A, M etc).  Because the filters are fixed, the numbers that come
out can be interpreted as RGB values, where R, G and B are 
predetermined colors based on the fixed filters in the hardware. 
CMY numbers can also be derived as simply the complimentaries of RGB.
A densitometer is fundamentally a process control instrument
geared to a specific use by its internal, fixed construction. 

A spectrophotometer OTOH, can read anything at all. More expensive
instruments have higher resolution and accuracy, but their
inner workings are based on the same principle.  Since ICC
profiles try to negotiate how our human vision relates to
all the display and print possibilities we now have, it needs
data that is as broad as what is humanly visible. In fact, it
goes all the way to UV, just to deal with Optical Brighteners
in papers. None of that is possible through the "narrow 
window" of the 3-filter densitometer.

Of course, for black and white, i.e. density alone, its
possible to use either instrument and, if the math is the same,
arrive at the same log numbers for density. 
In IJC, for example, you can use either log density scales
or the L of Lab. As long as you compensate for the
different linearities of the scales (via gamma), you end up with the
same results - though these are not ICC profiles. 

The only twist to that story is that most spectros
are auto- or semi-auto readers which makes it faster
and easier to enter grayscale data. Though there are
auto readers among densitometers, they seem to be
more money than they are worth and too much trouble for 
just reading grayscales. 

The way things are going with bw printing and toning,
we really should be counting on using the full spectral data
for future profiles or soft proofs (people here have
already done the latter). The spectro has a big
advantage over the densitometer in assessing the
coloration of a grayscale. It makes possible a
more dependable way to tone, and a pretty
scientific way to find out troublesome inksets
that will be prone to metamerism. Those that do
will show suspicious spikes in their spectral data,
a sure sign that they won't look the same under
different lights. 

Of course, these are not just issues involved in
making ICC profiles; I'm describing the broader scope of which
the spectros are useful for. In short, no, you can't make
ICC profiles with an RGB instrument, but you can use
it to linearize a grayscale and 
control various color darkroom processes.
Everything else needs a spectrophotometer.


Antonis


--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Steven Karafyllakis" <steve@s...> 
wrote:
> 
> Hello all;
> 
> In the last exchange about this, I believe Lou Duna stated that the 
> xrite-810 can't be used to create icm profiles, and yet the 810 will 
> read RGB values, which sounds contradictory. Can someone explain 
> further, and if it is the case that you need a spectro, what did 
> they use before spectros? They have only been around a few years 
> yes? ICM has been around much longer? 
> 
> Steve Karafyllakis