Digital BW, The Print

Tony

It's worth understanding this in two steps.  Density is a log function of
luminance.  Log functions are simply nice because they turn curves into
straight lines.  In 1931 the Commission Internationale de l'Eclairage
standards body developed the CIE XYZ colour space is their best attempt to
represent the eye's reaction to light and the Y component/axis models the
average luminance function of the eye's cones.

The formulae you have been presented with do two things in one step.  They
convert CIELab's L* (the Lab colour space is a perceptually uniform colour
space) to CIE YYZ's Y.  From there you get to density by taking the log of
XYX_Y (ie the log of luminance).  The last thing is to multiply by -1
because it is reflective rather than emmissive.

So first you need to get from L* to XYZ_Y:

If L*>7.9996248 (or >8) then:

XYZ_Y = ((L*+16)/116)^3

Else

XYZ_Y = L* x 27/24389

And once you have XYZ_Y then

Density = -LOG(XYZ_Y)

Of course your EyeOne also gives you the XYZ_Y directly depending on the
manner in which it is used.

Unless I am mistaken, John needs to check the second part of the equation he
gave you.

Cheers

Steve




> From: John Moody <moodymz3@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Sun, 30 Oct 2005 08:59:02 -0500
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: RE: [Digital BW] Black Density measurement
> 
> Well, LAB_L is a cell name; you can read about names in the user guide if
> you wish.
> 
> Try this, paste the following into cell B1.
> =IF(A1>=8,LOG(((A1+16)/116)^3)*-1,LOG(A1/903.3)*-1)
> 
> Now if you put 14 in A1, you should see 1.762 show up in B1.
> 
> John
> 
> -----Original Message-----
> From: DigitalBlackandWhiteThePrint@yahoogroups.com
> [mailto:DigitalBlackandWhiteThePrint@yahoogroups.com]On Behalf Of Tony Riley
> Sent: Sunday, October 30, 2005 8:38 AM
> To: digitalblackandwhitetheprint@yahoogroups.com
> Subject: RE: [Digital BW] Black Density measurement
> 
> I'm sure I need to thank you for your prompt response, John,
> But you may not be surprised to know that you've thrown good info at a
> phillistine!
> 
> I do have excel but how do I set up the conversion?
> 
> And just as a taster what would an L* value of 14 equate to?
> 
> Interesting stuff for a Sunday afternoon !
> 
> Tony R
> 
> 
> 
> 
> 
> 
> [Non-text portions of this message have been removed]
> 
> 
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