Digital BW, The Print

I should add that using the "best fit" gammas puts all the mid-tones much
closer to where they should be (ie value 128 comes a lot closer to L*=50=18%
reflectance middle gray etc).  I imagine this would be of value to the Zone
guys targeting a Zone V middle gray from exposure meter through to print...


> From: Steve Kale <stevekale@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Thu, 16 Jun 2005 23:40:47 +0100
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: Re: [Digital BW] Re: Optimal RIP gamma - was how many shades of grey?
> 
> I did a little more playing around with this and the following might provide
> some food for thought for RIP designers as they think about their
> "linearize" functions.
> 
> I was interested in what value for gamma would produce the least total error
> in print reflectance (XYZ_Y) vs that proposed by the CIELab model of
> vision's L*.  Someone with better stats skills could find a better measure
> of "fit" I am sure but this is a useful first observation.  I simply
> calculated the XYZ_Y value for each 8 bit value (and measured the difference
> between that and the XYZ_Y value that Lab would generate).
> 
> I assumed an average matte paper had a dynamic range of L*=16 to L*=96 and
> an average photo paper had a dynamic range of L*=3.3 to L*=96.  (The
> reference, Lab, of course goes from 0 to 100.)
> 
> For matte paper, if the RIP linearizes printed L* values from ink black to
> paper white (as they do today) the shape of the print reflection function
> (as measured by XYZ_Y) is best approximated (by this measure of fit) by a
> gamma of 2.05.  For photo paper this number is 2.38.  So these are the sorts
> of gamma implicit in the way we do things today.
> 
> I then asked what gamma would be the BEST fit to the benchmark, Lab, given
> the particular black and white points of the two papers.  This produced
> gammas of 2.38 for matte paper and 2.27 for photo paper, ie quite different
> from those produced by linearising L*.
> 
> All this of course assumes that getting as close to CIELab is the goal, ie
> that that model best represents the way we see.
> 
> Food for thought....
> 
> Steve