----- Original Message -----
From: "Ned Nurk" <ned_nurk@hotmail.com>
To: <filmscanners@halftone.co.uk>
Sent: Sunday, November 11, 2001 11:36 PM
Subject: Re: filmscanners: creating correction curves from scanned
calibration chart?
>
>
>
> >From: John Brownlow <john@pinkheadedbug.com>
> >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.
>
> Sorry to tell you, but you are *so* wrong. RGB values mean diddly squat.
> They are just a measurement of the amount of current in a scanner ccd or
the
> output voltage of a video card. The ICM profile provides a description of
> the transfers that are required to turn the RGB values into proper
> colimetric tristimulus values (i.e. XYZ). That then enables transfers
> between colour spaces to happen.
>
> (extreme example) You could have a scanner that mapped mid grey at
20,20,20
> and a monitor that mapped it at 230,230,230. The ICM Profile describes
this
> and allows colour management systems to convert from one colour space to
> another, and the RGB (device-dependant) values change accordingly,
depending
> on which space you are in, but the device-independant colour space values
> (XYZ, LAB, X'Y'Z' etc) would not change.
>
> >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.
>
> It all gets majorly complex when talking about scanning film, as the CCDs
> are only registering a value that they see between their black and white
> points. The first problem with doing all of this is 'has the film been
> processed exactly the same way as the one the profile was generated from'
> and 'is the exposure totally spot on?
>
> The answer to both of them is generally no :-)
>
>
> _________________________________________________________________
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>
>
Surely for each colour space there is a particular value for each
representable colour of that colour space. When you look at the values in PS
they have a precise translation to a particular colour.
Quite what their equivalent values would be for your monitor, printer or
scanner is irrelevant as that is the whole point of using an ICM profile for
each device - you don't need to know. You get your data from the scanner and
the profile is used to convert to your working space. The data in you
working space is converted by the monitor profile so that you see the
correct colour for the correct RGB values of the working space. Similarly
the printer profile is used to covert from the working space so that the
correct colour is produced on your printer.
Sadly in practice profile conversions can never be entirely accurate as each
device can only represent a particular set of colours. Hence you should use
a colour space with a wide gamut for editting. This gives you the best
chance of least inaccurate colour reproduction on each device.
Steve
