Inkjet Linearization & Grey Balance

General Color & Graphics Printer Calibration & Profiling
1

I’m working with an Epson 9600 using Bestcolor/EFI ColorProof (v5) as the front end. The 9600 is using Ultrachrome inks (Photo Black). The Ultrachrome Photo Black is not really very black, it’s more of a reddish-brown. Which leads me to the point. ColorProof has a fairly thorough process for linearizing the inkjet prior to profiling. Overall ink limit, individual channel ink limit, etc., are all set. All of this is done colorimetrically (I’m using their Premium option). As the final step in the linearization process, a test file is output for evaluation. The test file consists of color ramps of each of the 4 colors, C, M, Y, & K and also a grey balance ramp that consists of swatches composed of equal amounts of CMY

C M Y K

5 5 5 0
10 10 10 0
15 15 15 0
20 20 20 0
… an so on.

According to their documentation, the CMY ramp should match the K only ramp. I see 2 problems with this. First of all, due to the reddish-brown hue of the K ink, the K only ramp will never give a neutral grey appearance.

Secondly, it seems to me that using a CMY ramp with equal amounts for each color is a flawed approach to checking grey balance. In theory, equal amounts of CMY should give grey. However, in practice, due mainly to impurities in ink, this is not the case. Isn’t that the whole reason why we print in CMYK and not just CMY? Based on that, it seems that a CMY grey ramp needs to consist of swatches with uneqaul amounts of CMY ink. What should those amounts be though? Are the SWOP values
C M Y
75 63 63
50 39 39
25 16 16
a reasonable place to start? BTW, do those numbers accounts for paper and other factors as well?

So utimately, what I’m trying to determine, is how to know when my 9600 is grey balanced. Also, are my above assumptions/statements correct? Or, is there something simple here that I’m just missing?

Thanks for any feedback
Chris M

2

“…there something simple here that I’m just missing…”

Chris,

Based on what you have shared, I think I can help you identify what you are missing.

The color ramps are used as a tool to give the user an indication of the quality of the CMY grey-balance prior to profiling.

As you’ve acknowleged, one will never get the perfect balance through the entire ramp.

Also, it is important for you to understand that the colorimetric ink limiting and linearization process is designed to equalize the dot gain behavior of the inks so that the profiler can make the most of the printer/ink/paper color gamut.

Equalizing the dot gain behavior of the inks through proper ink restriction settings, ink limits and linearization should result in a fairly decent grey-balance ramp.

The profiler will usually complete the job of fine-tuning the gray balance.

3

At 11:09 AM -0800 3/7/05, christ0pher wrote:

“…there something simple here that I’m just missing…”

I must be missing something here too.

I’ve been giving this some thought and I’ve decided that a gray balance ramp of this sort just doesn’t make any sense. A few points:

  1. Equal CMYK values would never be gray if the channels were linearized individually - the ink just aren’t that pure.

  2. If you actually wanted to gray-balance the inkjet you (they, in their software) would have to monkey with each channel’s curve after linearizing, throwing the linearization off. Also - very important - you would have to know your desired black generation at the time of gray balance. Normally gray balance is achieved through CMY balance. But with the UltraChrome blacks being so far off neutral, CMY gray balance would be next to meaningless. As soon as K is added the balance would be thrown off.

  3. I would say just linearize and then let the profiling package work out the gray balance. Use lots of patches too - at least the ECI target.

Regards,

Steve

o Steve Upton CHROMiX www.chromix.com
o (hueman) 866.CHROMiX

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