Re: Infering the Lightness Range of a device
Re: Infering the Lightness Range of a device
- Subject: Re: Infering the Lightness Range of a device
- From: Roger Breton <email@hidden>
- Date: Fri, 7 Dec 2001 12:05:14 -0500
Andre-
I just checked inside Adobe's own "USWebCoated(SWOP)v2" and I found in
the A2B1 Table that 100c/100m/100y/100k maps out L*=11.77, a*=0.76 and
b*=0.32.
If I check off BPC using that Profile in Photoshop, I get that exact
readout in the info palette when I place the Picker over a patch of
100/100/100. If I check BPC, the readout becomes L*=0, a=0 and b=0. BPC
rescales the darkest tone to L=0.
Now, when I look at the B2A1 Tag, opposite L=0, a=0 and b=0, I find
64.7% cyan, 59.2% magenta, 57.2% yellow and 90% black for a total of
270%. If I start from a true L=0, a=0 and b=o value in lab mode, if I
place the picker on that patch, i do not get the above cmyk recipe but
rather 75c68m67y90k for a total of 298%, the stated TIL of TR-001, which
is what USWebCoated(SWOP)v2 is based on. Interesting? Do not know how to
explain this.
But as far as estimating Lightness Range of this device, if
100/100/100/100 maps out to L=12 in the A2B1 tag then if 298% is
actually the darkest tone that the USWebCoated(SWOP)v2 represent, this
means that the true Litghtness value of 298% is certainly above L=12.
But how do you compute it using the information in the profile?
Regards,
Roger Breton, Laval Qc
On Friday, December 7, 2001, at 10:44 AM, Andre Sch|tzenhofer wrote:
That can't be the case in actuality? We all know that the printing
conditions that this profile represent does not have a "black point"
equals to "perfect black" but rather around L* = 18.
I think a normal transformation (perceptual + relcol with BPC on + sat)
always uses the complete dynamic range of L, so the LUTs in a profile
are "clean" - with substrate color calculated out of the measured data
and scaled to L* = 100 (lightning conditions D50) as well as scaled to
L* = 0 regarding the black point. The program creates LUTs dependent on
the situation of the gamut's size of L, but in cases of transformation
lightness is relative. After transformation, "black ink" has to be
switched on - to display the correct black of the device. I assume this
relates to the information given by the measurement of the Field CMYK =
100/100/100/100 in the testform or something like that, in any case a
value representing the blackest black of the device. The B2A1 Tag from
absolute colorimetry depends on the total amount of ink and GCR or UCR
settings, graybalance and so on, defined by the operator who created
the profile. So, what we see on monitor or proof is not the blackest
black possible at all but the blackest black possible regarding this
conditions. Therefore, Lightness is a relative "factor" and there has
to be a "black point tag" or anything else which includes this
information. Black point compensation on extends the dynamic range of
lightness in case of relative colorimetrical rendering from "absolute
black" to "perfect black".
Please somebody correct me if I'm wrong with that.
Regards, Andre Schuetzenhofer