Color managment - Please sticky if worthy

Started Mar 29, 2013 | Discussions thread
Simon Garrett
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Re: Color managment - Please sticky if worthy
In reply to fft81, Mar 30, 2013

Thanks for posting.

Added to what Bob said, few minor points:

  • Technically, a profile is not a correction table.  It is a measurement of the colour characteristics of an input or output device (or colour space).  Profiles do this by describing the mapping between the device colour space and a "profile connection space" (CIELAB or CIEXYZ).  It may be a table, or a parametric description. 
  • For printers it isn't always the driver that does colour management.  Generally the program writing to the printer can do the colour management instead of the driver, and I think this is probably the way most photographers choose. 
  • For monitors, it isn't calibration that corrects the colour (or does most of the correction), it's profiling.  For most monitors, you can calibrate the white point and tone response curve (that is, correct them by means of driver look-up tables) but the program gets the correct colours by means of the colour space information in the profile, which is obtained by profiling.  In general, both profiling and calibration are done at the same time, but they're not the same thing. 

The last point seems to cause a lot of confusion.  When you profile and calibrate a monitor (e.g. with a color munki), two things are done:

  • Calibration: This means creating adjustment tables to allow the driver to correct the white point (e.g. to 6500K or whatever) and the tone response curve (e.g. to a gamma of 2.2).  Once the tables are created, they're loaded into the driver each time the computer boots, and the driver applies them to all image data it sends to the monitor.  This means that all programs get the corrected white point and TRC, except some games and video players, which may bypass the driver and go straight to the hardware. 
  • Profiling: This means measuring the colour space of the monitor (after calibration) and putting that measurement into the profile.  Programs that do colour management (and only those programs) can correct the colour space of image data before sending it to the monitor, to match it to the monitor characteristics. 

Some newer and high-end monitors can have their colour space calibrated, as well.  You can't alter the dyes and phosphors in the screen, but the firmware in the monitor may be able to emulate a colour space, provided that it's narrower than the monitor's native colour space. So, for example, a wide-gamut monitor may be able to emulate the sRGB colour space.  Reviews I've read on tftcentral and prad.de suggest that in-built calibrations of monitor colour space aren't always as accurate as profiles obtained by a colormunki or whatever.

So far as I know, printers are only ever profiled, not calibrated, but I stand to be corrected on this.

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Simon

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