How Color Print Tricks the Eye

Consider for a moment the marvel that is color printing.

It uses a scientific trick of lightwaves - Metamerism - plus engineering marvels that electrostatically charge a piece of media in very specific places, then release precisely placed, incredibly small grains of colored toner, and a developer fluid that reacts with the fuser's heat to bond those particles, all to trick your eye into thinking that a reflective surface is an emitting surface.

Hundreds of times a minute.

Or spray micro-drops of ink from two directions at once as the media rolls by.

And the controlled combination of colored toners or inks trick our eyes into interpreting a reflective surface as if it were an emitting surface. This is a trick worthy of magic, but really, it's science.

Here is a basic explanation of the color spaces involved with printing.

Simply put, the Cyan, Magenta and Yellow printing inks cancel out the Red, Green & Blue wavelengths of light coming to our eyes from the media's surface, metamerizing the wavelengths, tricking our biological processor into thinking it is the RGB of our human color vision.

Why red, green and blue? You may remember from science lessons in school - visible light is a spectrum of light, described as frequencies, ranging from reds through the colors of the rainbow into blues and purples.

From a scientific point of view, light can be a mixture of any of those monochromes, light of a single frequency.

However, we have light-sensing cells called cones in the retina of our eyes to detect the amount of light in the red, green and blue areas of the spectrum. Because of this, “true” monochromatic yellow light, which lies between red and green on the spectrum, is indistinguishable from a mixture of monochromatic red and green light.

From a design point of view, since we can’t perceive the difference, it simply does not matter, and so we can abstract any color we can see as a mixture of red, green and blue.

The science of digital color, however, describes color using the L*a*b* color model, among others, which is a bit less intuitive but more closely approximates how the human visual system works.

It uses the Lightness values of an image (the grayscale version) as an axis to rotate opposing poles of chromacity described as a* (red vs green) and b* (yellow vs blue) to describe the gamut of human color vision.

“But wait," I hear you cry, “You just told us human eyes sense red, green and blue!” That is true, it’s called the Trichromatic model of vision, and while it describes how the individual cones in the eye work, it doesn’t accurately describe the visual system as a whole.

Separating the human perception of lightness from color leaves the a & b dimensions as measures of chromaticity, brightness independent of color. This is important, as some colors appear brighter or darker, despite being at the same intensity.

For instance, we see a fully saturated yellow as a lot brighter than a fully saturated blue.

Regarding ranges, L is measured from 0 (dark) to 100 (light), a from -120 (red) to +120 (green), and b from +120 (yellow) to -120 (blue).

So let's look at the rods and cones mentioned above.

Human color vision takes place in the retina, and ends deep in the brains 's visual pathways.

The ventral stream (purple) is important in color recognition. The dorsal stream (green) is also shown. They originate from a common source in the visual cortex.

The retina is the innermost of three tissue layers that make up the eye. The outermost layer, called the sclera, is what gives most of the eyeball its white color. The cornea is also a part of the outer layer.

The middle layer between the retina and sclera is called the choroid. The choroid contains blood vessels that supply the retina with nutrients and oxygen and remove its waste products.

Embedded in the retina are millions of light sensitive cells, which come in two main varieties: rods and cones.

Rods are used for monochrome vision in poor light, while cones are used for color and for the detection of fine detail. Cones are packed into a part of the retina directly behind the retina called the fovea, which is responsible for sharp central vision.

When light strikes either the rods or the cones of the retina, it's converted into an electric signal that is relayed to the brain via the optic nerve. The brain then translates the electrical signals into the images a person sees.

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GCR (Gray Component Replacement)

Experts agree that in color reproduction black can be beautiful if used wisely. Black can add details and contrast in the reproduction which is impossible to achieve with the three process colors. However, if the black is not used judiciously, it will do more harm than good. It will make the colors look dirty and create an unnatural contrast.

— Dr. R. K. Molla

Reseparating customer supplied files is rapidly gaing popularity with printers and publishers as a way to lower print manufacturing costs. Reseperating allows for greater control of the final output, including specific filtering for media, color management, and finishing options.

Although the application is not limited to specific print market segments - these are the ones that are most quickly adopting this technology:

• Newspaper publishers
• Insert and flyer printers
• Magazine publishers
• Catalogers
• Directory printers

Within the CMY color space, a range of colors can be achieved by combining the three primaries. This combination in its turn can be thought of as a hue component (which will require a maximum of two primary colors) and a grey component (a mixture of all three, in an appropriate quantity to give the required saturation). If the grey component is replaced by black ink, the same color is being achieved by using two primaries and black. The act of substituting a quantity of black for the grey component is known as "Grey Component Replacement" (GCR).

GCR is also termed "achromatic color removal."

In grey component replacement (GCR), contrary to under color removal (UCR), the CMY values that add to grey all along the tone scale can be replaced with black ink. UCR only adds black to the CMY equivalent of what would have printed as a grey or near-grey.

Although there are many benefits to reseparating customer supplied files in order to use GCR, the most promoted and fairly easy to justify is in regards to reduced ink usage - typically suggested as a savings of around 20% in CMY inks with an increase of about 6% in K ink used while maintaining the same visual appearance in presswork.

Based on that figure, calculating a return on investment seems fairly straight forward. For example - based on the industry average of ink consumption for a sheetfed printer being about 2% of their gross earnings, a $10 million dollar a year printer will spend $200,000 a year on ink. If they reduce their ink usage by 20% they will save about $40,000 a year in ink costs. Theoretically, if the printshop spent $10,000 on a reseparation solution their payback time would be just three months and they will have saved $30,000 in the first year of implementation - a very good investment.

This is the removal of the gray components of the three colors and replacing them with black.

In GCR reproduction, all the primary and secondary colors remain the same as the normal chromatic reproduction, however, the blackening effects by the tertiary colors along with the gray components of the other two colors are removed and replaced with black. 

Various percentages of GCR can be applied to the separation for economical reasons and visually more pleasing results. The black dot sizes are increased to replace the gray component that has been reduced in the process colors. If 100 percent GCR were used, every color area in the reproduction might contain dots of only black and two or three process color inks. 

Commonly used percentages are 50% to 75% GCR. The goal of GCR is more consistent color, increased detail in the shadows, shorter press makeready, and possible ink savings.

Advantage: GCR results in less ink being used, and some of that ink is black which is normally cheaper than the others.

Advantage: The areas where less ink is used are regions of high ink use, so the potential for drying and offset problems is reduced.

Advantage: The resulting output is less susceptible to changes in the printing variables since you are not continually trying to balance as much C, M, and Y.

Because a GCR separation uses a non-chromatic color – black – throughout the tonal range and reduces the proportion of C, M, and Y in the mid- and quarter tones, the color in GCR separated images is more stable as solid C, M, and Y ink densities naturally vary through a press run. Note, however, that the added stability means less ability for the press operator to move color if required. For many printers, the increased color stability is a perfect compliment to the industry trend for a “by the numbers” print manufacturing process.

Other advantages:

• Reduced make-ready times/faster start-ups/less wastage
• Harmonized separations enhance press form printability
• Reduced fan-out or web growth
• Dramatic improvement of image appearance when slight press misregistration occurs
• Reduced drying times
• Higher printing speeds
• Improved repeatability of print jobs
• Grey balance within images is more stable


Disadvantages of GCR include:

GCR may reduce the ability to adjust some colors.

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Paper is the 5th Color

Paper is an analog variable in the world of digital printing. It directly affects the hue in the highlights,  it affects the entire color gamut size for the print job, has a powerful impact on shadows, and is often outside the control of the production workflow.

While many people believe that traditional color management is about managing the CMYK inks and their separations, paper has as much of an influence on the color of the final printed piece as it does on the mechanical and chemical action of the inks or toners. Although a given paper brand’s attributes may be consistent, paper characteristics are not standardized. There is variation in attributes between mills as well as variation around an attribute property from a given mill. Paper is considered a commodity but its properties are a long way from standardized.

A lot has changed in the world of paper. One of the biggest changes in recent years has been the increased use of optical brightening agents (OBAs) in many papers to give the appearance of a very bright white paper. Printers have been known to accuse paper companies of using cheaper goods and “cheating” by using optical brighteners.

The initial choices made on paper selection might not take into consideration the impact later in the production chain, and sometimes those decisions can have unintended consequences.

White papers are not RGB 255,255,255. Paper companies control the shade of their papers by adding dyes and other chemicals to affect appearance. More dyes, less reflection, more color compensation. The paper owns some of the color space so detail that requires paper’s hue cannot be reproduced. The recent trends toward blue white papers have resulted in more and more dyes in the paper and further deviations from neutral. Some of today’s papers are equivalent to a 3 percent cyan screen.

Modern color management solutions do allow you to bias your results to either a strictly neutral result with no consideration of paper color, or to neutrality based on the paper color.

This can be very important, as the human eye will quickly key in on the “white” of the paper and judge other colors on the paper based on that shade.

Paper shade, or white point, is the key attribute of paper and is measured using L*a*b* based on CIE XYZ. Described by these three values, color management applications calculate complex color inter- pretations to characterize paper and predict paper’s effect on color reproduction.

OBAs are used to increase the apparent brightness and whiteness of papers and their use is becoming more prevalent in paper manufacturing. They increase brightness and whiteness by absorbing energy in the ultra violet and emitting (fluoresce) the energy in the blue area of the visible spectrum. Because, to the eye, blue/white looks "whiter" than yellow/white OBAs are not really whiteners, but bluing agents. OBAs are also used in ink to expand gamut or brighten 4/C image printed on poor substrates - e.g. newsprint.


While it is not practical for printers to quantitively measure the OBA content of the materials that they use, it is quite an easy matter to qualitatively see the OBA content. All it takes is an inexpensive (less than $15 USD) "black light."

For example, with the black light it is easy to see that the paper used for the Pantone Goe system swatch book (on the left in the image below) contains more OBAs than the conventional Pantone spot color swatchbook on the right. Also, it's clear that the uncoated paper section in the Pantone spot color swatchbook contains more OBAs than the coated section.

Viewed in light that has an ultraviolet component, the papers appear bright and blue. They have an apparent expanded gamut. However, the printed hues will mutate, or change color depending on the light source. This effect is called metamerism and drives a need for light booths and an understanding of the viewing conditions when color matching or judging color. Simply put, printed hues shift, particularly in the highlight tones, when papers contain optical brighteners.

In reality, the rise in the use of brighteners can be attributed to a host of reasons, including production efficiency for maintaining a consistent look to a paper with changing content and a desire from customers for a brighter sheet at lower cost.

Another, more subtle problem can be the intended colorcast of the sheet. While in some ways we might consider OBAs an unintended colorcast, designers will sometimes purposefully choose a paper that has a colorcast.

The inks that are typically used in four color process printing block, to varying degrees, the fluorescence in papers containing OBAs. Black and magenta block the greatest amount, yellow a lesser amount, and cyan ink least of all. What this means is that when an image is printed using a halftone screen, lighter/pastel tones allow more more of the brightening and color shift of OBAs (towards blue) than the shadows. Color is effectively skewed towards the blue from shadows to highlights – but only when the paper being printed on has a high OBA content.

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Maintaining proper viewing conditions for print evaluations is a key part of color management.

The standards for viewing booths have changed over the past couple of years with the most recent release of ISO 3664 Graphic technology and photography — Viewing conditions.

If paper contains optical brighteners, color matching must be done in reference light conditions. When proofing on job stock that contains optical brighteners, it is important to critically examine both separations and curve effectiveness in the cyan containing highlight areas. The cyan/yellow color balance is hardest to achieve with these papers.

Even proofing papers contain optical brighteners. Creating profiles on these papers requires interpretation and tweaking if the press papers have a different level of optical brighteners or if the press papers have no optical brighteners.

Finally, optically brightened papers lose their fluorescence over time especially if exposed to light. The paper yellows. Print hues shift. Once printed, there is no recovery of the original paper whiteness so this should be kept in mind when a job reprints. Trying to match a first printing several months after completion is almost impossible. New proofs are a minimum requirement.

What additional tools can color management bring to the table to help tame the paper problem?

Traditionally, the way to “solve” OBA problems was to ignore them, primarily by using a filter that cut the UV light to stop it from hitting the paper and thus prevented the brightening effect of the OBAs. This is still a very effective approach to process control, but it is no longer the norm in color management. The other way we ignored it was by doing just that, not acknowledging the problem.

Today, we are much more likely to solve the OBA problem by quantifying the amount of OBA by including the UV in the measurement and then adjusting the ICC profile to compensate for its presence. Some recent color solutions provide Optical Brightener Correction (OBC) technology, which allows you to fine tune the profile results by evaluating specific test charts against a series of Munsell color standards in the target viewing condition. This combination of physical standards and measured results allows for a uniquely precise correction for optical brighteners.

An additional parameter that can be handled in color management is the final viewing environment. Traditionally, a graphic arts workflow targets a daylight illuminant (usually noted as D50/2 – describing the illumination and viewing angle). One additional way to fine-tune the result is to define the viewing condition of the final destination or illumination at the intended point of use if it is not D50.

This can be done by either using CIE defined illuminants or by actually measuring the lighting in the final environment.

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And what about Glossy Paper?

Paper gloss is related to surface roughness and therefore affects color reproduction. Light of all wavelengths is reflected from the surface of paper. How it is reflected defines both its gloss and dot gain characteristics.  

If the paper is glossy and smooth, it scatters less light and there is less dot gain. Light is reflected almost like a mirror (specular reflection). 

Matte, dull and uncoated papers scatter more light resulting in more dot gain. Also, these papers require more ink to achieve a given density further increasing the dot gain. 

Papers from different manufacturers absorb ink and toner/developer solution differently. There is no overall standard for surface roughness, ink absorptivity or developer absorptivity within the paper classification scheme. For the most accurate color, press profiles should be made on the chosen stock for a particular job. 

Matte and uncoated papers are even more variable.

The best color reproduction will occur on:

• Bright papers with uniform spectral reflection;
• Papers that are smooth and glossy;
• Papers that are neutral in shade; and
• Papers that exhibit minimal fluorescence.

One curve for all paper surfaces leads to less than optimum color.

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Color Bars

Color bars are printed in the trim area and are used for quality control purposes by the printer.

Squares of colour are printed on the area of the page to be trimmed off, which the printing press operator uses to check colour density and consistency is maintained. This checking process is automated by some printers, with digital scanners tracking the colour bars to ensure quality and consistency is maintained.

• Unlike the live image area of the press sheet, color bars are consistent job to job. Therefore they are more efficient at providing a benchmark and can be used to track trends in variation over time.
• Color bars can be tailored to meet the needs and measurement capabilities of individual print shops.
• Color bars may be used to measure all aspects of the "print characteristic" - solid ink density, overprinting (ink trapping), dot gain, grey balance, as well as issues such as slur and dot doubling.
• Color bars can reveal issues with ink hue, blanket condition, impression cylinder pressure, etc.
• They can be used forensically to help understand why a specific job did not meet expectations.
• They are efficient since, unlike the live image area, they are a constant made up of well defined elements that continue from proof to press sheet.

An offset printing press is essentially a complex machine for laying down a specific film thickness of a specific color of ink onto a substrate. The digital version uses toner electrically charged then fused onto paper. The ink or toner is metered out in zones across the width of the press sheet according to how much coverage is required for each color in each zone.

The trick is that if you are producing a critical spot color with a build of process inks, those inks and all associated print attributes need to be “spot on.” The best way to control critical press factors like density, dot gain, print contract, etc. is to measure as many color bar patches as possible.

Color bars can serve many purposes. They can be used to determine color accuracy against a given standard or to determine a proof’s accuracy against a final print.

Color bars also can be used to measure consistency for the duration of a long print run, from job to job when a print job is reprinted, or between two similar printers.

So, ranked in order of importance, here are the patches we suggest for a four color press color bar.

1. Unprinted substrate patch (to zero out substrate when necessary)
2. Solid patch for each process (and spot) color (needed for solid ink density (SID))
3. 3/Color Grey patches at multiple tone values
4. Mid-tone patch for each process color (to gather dot gain (TVI) values)
5. Process color over prints (needed to measure trapping efficiency of inks)
6. Additional highlight and shadow tone patches of each process color (for a 75% patch to calculate print contrast)
7. GATF Star targets or microline targets (used to visually evaluate for press slur and doubling issues)

The items listed above are important and truly necessary, but if space is an issue, start with No. 7 and work backwards removing items until the bar fits. Also, items 3 and 4 could be easily reversed, but having one without the other makes diagnosis of grey balance issues difficult at best.

Grey balance targets

Grey balance targets are made up of a patch of three screened process colors that are balanced so as to appear as neutral grey under standard printing conditions. They are typically printed adjacent to a black screen tint of a similar value to allow for a quick visual, or measured, evaluation of how grey balance has shifted.

Grey balance targets can be useful since variation in any of the three process colors because of dot gain, slur, doubling, density, trapping, and registration will be reflected by a shift in hue away from neutrality. The 3/C patch will take on a bluish, reddish, or greenish color cast.

The idea behind this target is that any grey balance color shift away from neutrality suggests a possible color shift in the live image area. However, in production printing the grey balance target may not be a reliable indicator of presswork issues.

Color bars are not a requirement for quality printing, however, they are key to making proofing and printing more efficient and effective while reducing overall production costs.

Color bars are available from a variety of sources, including IDEAlliance (at no charge) and iStockPhoto.

Many imposition packages also include color bars in their imposition templates.

Another option supported by industry standards is the Ugra/FOGRA Media Wedge CMYK, which monitors the quality of digital proofs. It also can serve as a digital control aid to monitor the effect of imaging in CMYK mode and other prepress work.

The CMYK tonal values of the Ugra/FOGRA Media Wedge are based on ISO standards. The Ugra/FOGRA Media Wedge is available for purchase from FOGRA.

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Metamerism

Things That Go Weird in the Light.

Have you ever compared two garments in a store and decided they matched, only to find that when you left the store and went out into daylight they no longer matched and instead looked quite different? Do you recall the blue & black / white & gold dress image that was an internet meme not long ago?

If so, you have seen an optical phenomenon called metamerism failure (muh TAM erizm). Strictly speaking, metamerism occurs when you see two samples match under one light source (illuminant) and not match under another.

How can this be?

Well, it comes down to the difference between how an object affects light, and the color it appears to our eyes. Objects affect light by selectively reflecting or absorbing light of different wavelengths. So an object that absorbs most blue wavelengths and reflects most red wavelengths will usually appear red to our eyes. The actual color it appears to us is dependent on the spectral composition of the light reflecting off the object.

Let's say, for example, we have two objects that each reflect red light in approximately the same way but one reflects blue light while the other absorbs it. If you put both objects under reddish lighting (and most indoor tungsten lighting falls into this range) then they may appear to be very close to the same color. As there is very little blue light falling on our objects, the difference between their blue reflectiveness is almost invisible. The red reflection is about the same so they both reflect similar wavelengths and our eyes see them as the same color!

This would not be a problem if we didn't have many different colors of lighting in everyday life.

So let's take our objects outside into mid-afternoon daylight. Sunlight at that time of day contains considerably more blue light than indoor lighting. As before, our pair of objects will reflect red light similarly but one of them will reflect a significant amount of blue light while the other absorbs it. Our eyes will see the blue light from one object combined with the red light and we would probably call the result magenta. Suddenly what we thought were two reddish objects no longer match at all!

In many ways this very phenomenon is essential to color reproduction, which we discuss below, but when colors "shift" from our expectations, clients stop paying bills, and that is a problem.

The fundamental reason for metamerism is that color is a sensation rather than a property of an object. As a result, the cones in your eyes can register the same sensation from an essentially infinite variety of combinations of different light frequencies.

Color perception basically requires four factors:

Light Source + Object + Observer + Interpreter = Perception.

Where will we see this problem in the business of digital imaging?

• Proofs and press jobs failing to match under different lighting.
• Color builds chosen for normal printing failing to match under unusual lighting. A good example of this is trade show booths and how they are lit with unusual lights in exhibit halls.
• Two prints using different technologies - such as inkjet vs photographic print - failing to match under certain lighting.
• A product shot failing to match the product in all lighting conditions.

Can color management using ICC profiles correct for this problem?

No... and yes. ICC profiles are typically built using readings referenced to D50 (5000K) lighting. That means that prints created using these profiles will look best under D50 lighting. Viewing them under any other lighting can give unpredictable results.

Most printing pigments and dyes have been carefully chosen to not conflict with each other or other pigment sets. One exception that is appearing more and more is pigmented inks for inkjet printers.

Sometimes you can measure printed or scan/camera targets with a different light source such as D65 in the calculations. This should make the print viewable optimally under D65 lighting. This is not always successful and requires the appropriate settings to be available both on the instrument and in the software.

Papers manufactured with optical brighteners are especially susceptible to color changes when lights differ in their short wavelength radiation, which can cause some papers to fluoresce.

One closely-related problem cropping up more and more often in the inkjet printing world is often (incorrectly) called metamerism.

When colorants are mixed carefully in a printer, you can achieve a smooth, neutral gray gradient from black to white. With most inkjet printers, the ink combination will include Cyan, Magenta, and Yellow inks in varying amounts along with Black ink. When properly balanced, pleasing black and white images can be printed. Many users are also experimenting with near-neutral imaging such as adding a slightly blue or sepia tone for effect.

With the fugitive nature of dye-based inks, many users are switching to pigment-based inks for the vastly improved permanence. After all, if you are printing and selling works for display, your customers tend to have the expectation that the work will last beyond 2-3 years. Pigmented inks however, can suffer from a pigment balance problem which rears its head in a similar manner to the two-sample metamerism problem.

It is important to note that this is not an expected color shift but rather a shift that appears strange to the eye.

One would expect that a gray tone viewed under D50 lighting would appear to be a warmer gray when viewed under warmer, tungsten lighting. The color balance failure we are referring to here shows up as a green or magenta cast and is noticeably different than a shift normally attributed to warmer or cooler light.

Many people incorrectly refer to this phenomenon as metamerism.

Metamerism, however, is specifically defined as a phenomenon that occurs between two samples. The ink balancing situation does not involve two samples but rather a balance of pigments in one sample.

Strictly speaking, then, it is not metamerism and the problem is more correctly referred to as Gray Balance Failure or Color Balance Failure.

After all is said and done, it is fair to say that metamerism is the enemy of digital printing, right?

Not really, no.

Metamerism, remember, is when an object matches another under a certain illuminant even though the spectral characteristics of the two objects differ. The act of balancing three or four colorants (such as CMYK inks) so they appear to be the same color as an original object is also based on metamerism.

Because of the 3-channel nature of our eyes, we can get 4 inks to appear to match a real-world object like a person's face without the spectral characteristics of the inks resembling the face much at all. This means that the print and the face affect light differently but appear to be the same color to our eyes!

This is the basis of digital imaging and printing today. It is fair to say, then, that without metamerism we would not be able to do ANY of the imaging we do today! It is only when the balance fails that we call it a problem.

Perhaps a match-failure problem should be called metamerism "failure" rather than metamerism, but this term does not seem to be used at all.

As with anything in the color management world, being aware of the problem is half the battle. Now that you know about metamerism and GBF you can consider it as a contributing factor when things don't look right.

Also, if you have no D50 lighting under which to view your prints it is possible they will never look quite right. Invest in controlled lighting for print viewing. With the many variables in digital color work that can give you problems, nailing down lighting is considered a basic requirement for print viewing as well as monitor to print matching.


Since it may be impossible to completely control the lighting conditions under which colored objects are stored, displayed, or judged, the best way to prevent metamerism is to match the object with pigments with exactly the same reflectance properties. In color matching, this precision is the goal of every colorist. However, sometimes their goal cannot be met because the pigments in a target sample submitted for matching may be inappropriate for the planned application.

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Do you have a color management question, horror story or event to share?
Email me at reilley4color@gmail.com