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Is it metamerism, or constancy?

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  • Is it metamerism, or constancy?

    I have noticed an interesting issue that has appeared since we started proofing with our Epson x800 printers and Epson's k3 inkset. I need to ask this question: Has Epson done an excellent job of improving constancy, or eliminating metamerism, or both? The issue this is causing is that our press sheets and proofs are an excellent metameric match under 5000k lighting, but when you introduce a different light source they are not a very good match (customers office for example.) The Epson's constancy is excellent, the press sheet not so good. We print using paper that has UV brighteners added, is this the cause of the unconstancy in the press sheet? Or is it the inkset we use at press, or both? Does anyone have any suggestions for resources that we could use to research this issue? We would like to make the press sheet more constant in different light sources. Would software such as Alwan with its 'super GCR' help this. Thank you in advance for any thoughts, suggestions, or opinions. We are a registered G7 printer.
    Very best regards,
    Todd

  • #2
    Re: Is it metamerism, or constancy?

    Todd:

    You cannot make the press sheet and proof consistent under different light sources. The best you could do would be to find ink and paper combinations of the press and the proofer that would be a closer spectral match. You would have to get the spectral reflectance curves of the inks and papers, and probably look for papers with no UV brighteners as well. The only way to match under different lighting would be to make the proof by actually printing the sheet on the press.

    The standard illuminant, "D50," is one of several CIE illuminants, and is a mathematically defined theoretical light source that is impossible to perfectly create. Unfortunately, that is the standard adopted for printing, and is the lighting that viewing booths are designed to match closely. It does not closely relate to the lighting that any end-user will see the finished product under, but the best we can do is to pick an illuminant, and make a proof that will closely match the press sheet under that illuminant. If the standard illuminant were changed, the color profiles would have to change, and different color values would have to be used on the proof to match the same press sheet.

    Visible light spans an infinitely divisible spectrum of wavelengths, and therefore the domain of possibilities for a "color" of light is infinitely dimensional. Fortunately, our eyes have three distinct bands of sensitivity, and the final result is that our brain receives what is essentialy a three coordinate simplified perception of the light. For this reason, you can have two "colors" that are very different spectrally, but give your brain the same stimulus. When you see an inkjet proof perfectly match a press sheet under D50 lighting, the spectral reflectance is still likely very different, but you see the same color. If you then change the lighting, the colors will then appear different.

    From my experience, I would say that the reflectance of offset printing inks is very different from that of inket proofing inks. The more they differ, the more dependent the match is on the lighting.

    Comment


    • #3
      Re: Is it metamerism, or constancy?

      Todd,
      Kyle is right on all counts.

      Color matching between two samples with different sprectral responses (your proof and press sheet) is called a metameric match. Press inks have a high degree of color inconstancy, so they can change radically under different lighting. What you've described is typical.

      Now, you CAN set up your viewing conditions to be more similar to "typical" fluorescent office or retail lighting - F2 illuminant. And you can restructure your profiles for this. But, again the color will shift under different lighting conditions - you're only changing the conditions under which a match occurs.

      rich

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      • #4
        Re: Is it metamerism, or constancy?

        Hi Todd,

        I really think that your problem is due less to the printing inks and more to the UV brighteners in paper. Your viewing booth uses fluorescent light to simulate D50. No matter what, most of fluorescent light has spectral power spikes in near UV spectrum.
        I suggest you try to match proof and print under natural daylight. If you print for a very specific light environment you'd better test the match with some previous printed job and its proof.
        Increasing GCR might help with a better constancy of the neutrals but the color saturated areas will still be affected. As I said though, I really doubt your problems are mainly because of press ink.


        Regards,
        Gabriel

        Comment


        • #5
          Re: Is it metamerism, or constancy?

          Constancy-inconstancy, is this a word in scrabble? The press sheet and inks will always look different under different illuminants because of the color cast of the illuminant. Last I heard, the pigmented Epson inks were a spectral match to the typical or SWOP shades of printing ink. That was the idea. I guess when it comes to UV absorbing blue emitting paper brighteners, no one can predict the effect on appearance, just that the paper is brighter with no ink on it.
          John Lind
          Cranberry Township, PA
          724-776-4718

          Comment


          • #6
            Re: Is it metamerism, or constancy?

            color constancy
            noun
            the tendency for a color to look the same under widely different viewing conditions

            It is actually a term with a definition. I think many times the word metamerism is used in the wrong context. I often hear people refer to the press sheet as having a higher level of metamerism than the proof. I really think what they mean is the press sheet has less color constancy, or more color inconstancy? I think if Epson has been able to do what they have done with their printers, someday we may be able to do the same with the press sheet. Imagine the delight of the end user when they view the finished product in their office environment and it matches the proof that they originally ok'd. If somebody wants to earn a million and early retirement come up with a paper/ink combo that improves the color constancy of the press sheet and is GRACol compliant. If I had the smarts and the resources this would be my future endeavor.

            I have built and used profiles using light source readings that we have taken at customers with success, and for the time being I think this may be the best solution for the few customers that require this of us. I don't think it is something that you want to do for every customer (too cumbersome!) The few customers have much higher expectations than the other 97% of our customers, product/paint/ finish matches, etc. I thought it was an interesting question, and if we don't ask solutions may never be presented. I like to think of it being like device independent Lab, light source independent printing?

            Thanks to all,
            Todd

            Comment


            • #7
              Re: Is it metamerism, or constancy?

              Todd:

              What procedure are you using to measure light sources at your customers' offices, what instrument are you using, and how much better have your proofs matched under that lighting? Are you simply getting a CCT (degrees Kelvin) reading and adjusting profiles to compensate, or are you getting spectral readings and generating new press/proofer profiles from spectral reflectance data?

              Regarding the semantics:

              Two different color stimuli are "metamers," or a "metameric match," if they have different spectral power distributions, but appear the same to an observer. "Metameric failure" is when two different color reflectances match under a given lighting (like when you choose a paint to match a swatch at the paint store), and then do not match under a different lighting (like when you paint the last third of your living room and later discover after turning on the incandescent lights that you can see the break between paint strokes). So two color reflectances that have the potential for matching or mismatching depending upon lighting are both a metameric match, and also exhibit metameric failure. The term you would use depends upon wether the final result makes you happy or sad, i.e., wether they match or don't match under the lighting that you want them to match under. "Metamerism" generally describes the concept of different color reflectances having different apparent relationships under different lighting. The term can be equally well applied where an exact match is not concerned. For example, if two "colors" look more similar under one light than another, you would call this metamerism even if the "colors" could never exactly match under any normal "white" light. For this reason, every pair of color reflectances, unless identical, have or exhibit metamerism, and metamerism is basically a property of color vision (the observer) rather than of an object, paper, light source, or printer. I use an Epson 9800, and am not sure of how close to average our offset inks are, but I've been meaning to sample solid and overprint patches from the press and the Epson to satisfy my curiosity, because I've noticed that the match of some colors from proof to press depends greatly upon the illuminant. Keep an eye on this thread - I'll post the results when I get them.

              Even if you found inkjet and offset inks that were a perfect spectral match, everything would probably still be messy because overprints surely do not act the same. Inkjet inks are probably mixing differently than offset inks. For example, lay-down sequence has an effect on color with an offset press, but an inket printer doesn't really have a lay-down sequence, it's just spraying the ink in the same place at the same time. Does anyone know if SWOP defines inks in terms of reflectance?

              "Constancy" is another property of color vision. It refers to our perception of color being relative to a sort of reference frame. If you are in a room illuminated with magenta light, a neutral gray stimulus will be perceived as being green, whereas a neutral gray stimulus in a room illuminated with green light will be perceived as being magenta. A color is perceived relative to its context, especially what is perceived to be the color "white," not in a strictly absolute manner. For this reason, an object that is green will be properly recognized as being green even if viewed under lighting with a strong magenta bias that causes the absolute stimulus to be a neutral gray. Hence the term "constancy."

              Comment


              • #8
                Re: Is it metamerism, or constancy?

                Alright, you can use the words color constancy in scrabble.

                The idea of color viewing booths is just a work around, to make the negotiations go easier. Then there are the viewing boots with multiple illuminants, A, D50, Fluorescent, Sears Fluorescent, etc. General office light will never be consistent enough, not to mention the surround effects.

                Are you calling the press sheet less constant because it's different than the constant proof? The paper changes with every job, while the Epson proofing material is constant. Is that what you are referring to?

                The secret to a constant match is a spectral match. If the spectral curves of two colors are the same, they will match anywhere to anyone. If you printed the vast majority of offset KCMY's on the same paper they would all be very close when printed to the same density. The magenta might vary a little in the blue part of the spectrum, blue shade, yellow shade, but mostly around Pigment Red 57, which is SWOP magenta. Some blacks will have more blue toner than others. This was the basis of the SWOP ink verification program at GATF. Samples of the SWOP standard ink exist and are printed against trial inks on the approved SWOP paper (whatever that is now). The curves were always pretty close, and a visual comparison under 5000K still would show just barely noticeable differences. Very tricky business. To this extent, there are spectral reflectance curves for SWOP. In fact, if you access the data for TR001, you might find curves on the paper Champion Textweb, but that may not exist anymore. Back to ground zero. You may be able to still get the inks from GATF. Were these inks ISO2846? Pretty close, with the Euro magenta a little more blue and the cyan more red. In this case, it was the SWOP inks printed on the APCOII/II paper from UGRA-EMPA (Swiss National Standard Institute).

                If you know your SWOP history, what came next was Off Press Proof Verification Program. Here is where metameric matches under 5000K were legalized. When held to the same standards as ink and paper, the vendors hit a brick wall, due to some serious chemical and physical circumstances, the least of which were spectral mismatch. Then you started to hear about "Appearance Matches."
                It no longer mattered whether the substrate looked like the SWOP paper, whether the ink densities were close to SWOP, whether the tone value increase was similar or balanced, it only mattered that you told the operator how to make it look like a press sheet under 5000K. Welcome to Color Management.

                John Lind
                Cranberry Township, PA
                724-776-4718

                Comment


                • #9
                  Re: Is it metamerism, or constancy?

                  @ John Lind

                  you wrote "If the spectral curves of two colors are the same, they will match anywhere to anyone."

                  Well, that is an interesting statement, but I think we can all agree here that it is very unlikely that one might get even a small 1mm x 1mm portion of an Epson proof and a press sheet to have an identical spectral curve.

                  Related to SWOP -- No one has followed anything close to SWOP guidelines for 20 years !

                  Champion Textweb, a No. 5 coated groundwood with 70 brightness - a very yellowish stock - is not being made anymore, nor to my knowledge is anyone printing at 133 line screen - certainly not Magazine publication printers, "which was who SWOP was designed for - so SWOP is irrelevant, long live Gracol.

                  Of course I abadoned all hopes of the SWOP group back in 1989 when I read this;

                  http://findarticles.com/p/articles/m...v18/ai_7627311

                  Clearly, it was doomed to irrelevance before then - no one prints to SWOP guidelines.

                  @ Todd Miller;

                  I fail to see much anyone can add here beyond the fact that proofs are designed to simulate a press sheet UNDER A VERY SPECIFIC controlled lighting condition.

                  When you print a color document that is intended to simulate how the final press sheet will look like, it is important that ALL PARTIES understand that the "proof" is created with certain assumptions of the viewing condition, one very popular one being in a viewing booth where the light emits a specific color temperature, most often 5000K.

                  Outside of these viewing conditions, metamerism occurs, colors no longer visually match, and you might as well wear color tinted glasses.

                  So, to your questions ;

                  "The Epson's constancy is excellent, the press sheet not so good."

                  - jahn comments --- HA HA HA ! that is really funny - so, why not print thousands of Epson proofs and send that out ? (smile) - yes, I always like to point out how long 1 color epson proof takes to print verses how long a printing press takes to print that same image...

                  "We print using paper that has UV brighteners added, is this the cause of the unconstancy in the press sheet? "

                  - jahn comments -- YES. One of Pantones BIGGEST issues with printing the Pantone guides is paper inconsistency.

                  "Or is it the inkset we use at press, or both?

                  - jahn comments -- YES. You can also ad blankets, humidity, plate wear, temperature, ink viscosity and a long list of things that cause each press sheet to not be perfectly matched to one another - but again, consider that the marking engine in the epson is spraying droplets of water much smaller than a human blood cell at a spining target using inks and paper far different than you are printing with on the press, and you will grasp how tricky it is to make them look the same in ONE ligting condition, never mind SEVERAL lighting conditions !


                  "Does anyone have any suggestions for resources that we could use to research this issue?"

                  Jahn comment - teach eveyone that you must view proofs and press sheets in a 5000k light booth, period.

                  "We would like to make the press sheet more constant in different light sources."

                  Jahn comments - This is technically impossible, and this has been proven by countless studies. Do not waste another nano second persuing this goal.

                  "Would software such as Alwan with its 'super GCR' help this."

                  GCR has nothing to do with overcoming metamerism. When a designer builds objects with 400% density (100% of all 4 CMYK process colors) systems like Alwan can convert these into something more press friendly. Some people report ink savings, but most uses exclaim better grey balance control without compromising saturate colors too much. Again, this will not help your press match your Epsons (which is backwards philosophically) but may help sheet to sheet consistency

                  hope you find this information helpful !





                  For those who want to learn more about metamerism related to monitor and print represnetations of press simutaions, here is a couple of links to a well written blog posts;

                  http://jimraffel.com/2008/01/16/67-m...py-vs-monitor/

                  http://jimraffel.com/2005/10/26/gold...or-management/
                  Michael Jahn - Slightly used PDF Evangelist
                  Simi Valley California

                  Comment


                  • #10
                    Re: Is it metamerism, or constancy?

                    Michael Jahn wrote:
                    GCR has nothing to do with overcoming metamerism. When a designer builds objects with 400% density (100% of all 4 CMYK process colors) systems like Alwan can convert these into something more press friendly. Some people report ink savings, but most uses exclaim better grey balance control without compromising saturate colors too much. Again, this will not help your press match your Epsons (which is backwards philosophically) but may help sheet to sheet consistency

                    If the proofer uses Device Link profiles or 4-D Transforms and reduces the amount of CMY and replaces this with K, you probably will have less problems. GMG created a test suite with a Roman16 test image in gray.
                    With less and more GCR in the proofs.
                    The proofs were more stable and the Color Server files with more GCR were more stable in print.
                    Metamerism is bsaed in color. Less color in both media (proof and print) means less metamerism caused by the pigments. Of course there are other reasons why metamerism will occur.
                    Some (less stable) digital printers had severe color problems. A new front end was equiped with a kind of DVL-technology and the user and owner reported that their printer was far better (less unstable) than before. It was not the performance of this digital printer but the file harmonizing process, I think.

                    Comment


                    • #11
                      Re: Is it metamerism, or constancy?

                      @ Jahn: "GCR has nothing to do with overcoming metamerism. When a designer builds objects with 400% density (100% of all 4 CMYK process colors) systems like Alwan can convert these into something more press friendly..."

                      You are confusing GCR with UCR. Gray component replacement replaces a portion of the third CMY component with black to the same effect. This occurrs accross the entire gamut, not only 400% blacks.

                      There is research suggesting that GCR can help to overcome a certain ammount of proof/print inconsistency, though I agree with you and doubt very much that it would have any affect on metamerism.

                      -Brian

                      Comment


                      • #12
                        Re: Is it metamerism, or constancy?

                        I don't think GCR has anything to do with it. Don't forget about metameric grays and metameric blacks. Try reading some of the old papers by Henry Hemmindinger, a color science giant. The blacks can be more spectrally different than the cmy, especially with ink jet.

                        @ Michael Jahn:
                        Let's get some GATF or RIT lab to print the SWOP or ISO inks on the EPSON paper. Then you would see a spectral match for the pigmented inks.

                        You really should give SWOP a little more credit. While no one ever followed it perfectly, it was a line in the sand, sometimes shifting sand, that helped with process control and process expectations. It progressed similar to Euroscale Inks and FIPPS magazine specifications in the UK. Rightfully, it trained a lot of people so that today they can understand and appreciate ISO12647 and ISO2846. The humble Hi-Lo references created a homogeneous US web and sheetfed industry and led to Status T and the T-Ref. Our shortcomings are forged in brass while our virtues are written in water.

                        John Lind
                        Cranberry Township, PA
                        724-776-4718

                        Comment

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