Digital BW, The Print

FYI this is the response I received from Phil Green at the London College of
Communication - the same Phil Green whose names appears at the top of the
www.color.org International Color Consortium site:

"Two points: the operation may be defined in XYZ but that does not mean
it has to be implemented in XYZ; and a scaling operation will have the
same effect whether it is implemented in XYZ or CIELAB, the only
difference being a slight shift in the low XYZ values close to the
threshold where the cube root function is replaced by a linear scaling."

FYI the points I quoted below were from Bruce Fraser's Real World Color
Management (pg 41 and 42).


> From: Steve Kale <stevekale@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Tue, 18 Oct 2005 19:59:12 +0100
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Conversation: [Digital BW] ICC v. Transfer Function in Epson driver
> Subject: Re: [Digital BW] ICC v. Transfer Function in Epson driver
> 
> Paul
> 
> I just sent you this off-list as well.
> 
> OK here is my attempt to explain luminance scaling with respect to XYZ_Y and
> not L*.  It¹s a simple explanation and represents my understanding.  I have
> asked a guy who should know to give me his explanation and I¹ll make sure
> you hear it.  Here goes:
> 
> People read too much into ³Lab is how the eye sees².  My understanding is
> that humans perceive luminance first and colour second.  Ok so we need a
> luminance component and a hue component.  Lab fits this bill but so does
> XYZ.  To quote Bruce Fraser, Lab originated out of an ³attempt to create a
> space that is perceptually uniform ­ in other words, distances between
> points in the space predict how different the two colours will be to the
> human observer.²  L* is ³approximately the cube root of the luminance value
> Y (which is a rough approximation of our logarithmic response to
> luminance).²  The bit in parentheses is critical.  Again from Bruce ³the
> primary Y doubles as the average luminance function of the [eye¹s] cones.²
> The eye sees luminance as described by XYZ_Y not L*.  It makes sense then to
> scale for white point and black point in XYZ_Y.
> 
> It is this Y that we use when talking about density = -log10(XYZ_Y).  Logs
> are nice because they turn non-linear responses such as the eye¹s
> sensitivity to light into straight lines.
> 
> 
> Cheers
> 
> Steve