Re: filmscanners: creating correction curves from scannedcalibration chart?

[John Brownlow <john@pinkheadedbug.com>]

on 11/11/01 11:53 AM, Ned Nurk at ned_nurk@hotmail.com wrote:

>> From: "michael shaffer" <rarewolf@roadrunner.nf.net>
>> Because my system is calibrated, I am not assuming.  If my monitor is
>> truely gamma calibrated, and I use a gamma=2.2 working color space, then a
>> 18% gray card's RGB value should be R=G=B=117 (see formulas below, they
>> aren't mine).  I ask you ... under what circumstances would an 18% gray
>> card
>> be anything else? (you may also assume I have ambient illumination under
>> control.)
> 
> your assumption being that 0,0,0 is totally black and 255,255,255 is totally
> white in that 2.2 gamma colour space. Would be a pretty daft colour space as
> you can't get either on a monitor or printer and so you end up wasting a
> whole bunch of values that could never be properly expressed.
> 
> generate an icm profile for you monitor (i.e. with a color sensor) and
> scanner and ignore the rgb values.

this is nuts. loads of people, me included, edit by the numbers. The whole
point of an ICM profile is so that the same RGB values display the same on
different profiled devices.

Incidentally, 18% grey means that the card REFLECTS 18% of the light. So it
is a relative grey (relative to 100% reflectance of the incident light). Not
to 'pure' white, whatever that may be.
--
John Brownlow