Color managment - Please sticky if worthy

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

Bob Collette wrote:

While your writeup is generally correct, there are some areas that don't agree with my understanding of color management.

Printer: Actually, the printer ICC profile does take into account the viewing illuminant.  By default, the viewing illuminant is assumed to be D50, however some profiling software allows you to change the viewing illuminant to a different standard.  However, you are correct in that the viewing illuminant will affect the print appearance.  For example, if you view a print under incandescent lighting, it will look different than if viewed under daylight.

When a printer is profiled, a set of color patches with known code values (RGB or CMYK) is printed with no color management applied (printer's native response).  The resulting print is measured using a spectrophotometer and the resulting colorimetric data is used to build a mathematical model of the printer.  Once the model is constructed (RGB/CMYK to Colorimetric), the inverse model can be calculated (Colorimetric to RGB/CMYK).  If we assume an RGB system, there are 16.7 million possible color values that can be requested.  Obviously, the printer ICC profile cannot have 16.7 million entries contained within it.  A 3-D lookup table is used with a number of entries ranging from tens of thousand to hundreds of thousands (depending upon the size & quality of the profile).  The other "missing" entries are interpolated from the 3-D table.

In a color managed system, the application software (e.g. Photoshop) reads the RGB code value for an image pixel and maps it through the image color space (sRGB, AdobeRGB, etc.) to get a colorimetric value (L*a*b*) for that pixel.  The colorimetric value is what is then sent to the printer.  The printer ICC profile takes the requested colorimetric value and then calculates the required printer code values (RGB or CMYK) to produce that color value (assuming it's within the printer's color gamut).

Monitor: The monitor luminance value is normally chosen to give a good match to the print, given it's viewing illumination, not to maximize the monitor's color gamut (although it likely does improve it's gamut).  The higher the print viewing illumination, the higher the monitor luminance should be to get a "brightness" match.  If you view your print under relatively dim light, you'll need a very low monitor luminance to give a reasonable print-monitor brightness match.  On the other hand, if you view the print under bright illumination (sunny day), you'll need to set the monitor to a much higher luminance level to match the print.

Everything you mention above is correct. I was trying to avoid writing a 3 tome encyclopedia and just give people functional understanding of what to do to establish color managed workflow and what parameters effect that flow. Lets be honest, most people reading this forum are not going to try to disassemble OEM driver or try to write their own RIP.

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