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

Measuring Color

MEASURING COLOR

While there is an art to designing for and selecting the right colors, there is definitely a color science, and that means color can be measured. Scientific measurement of color output enables greater control in the print production process.

Translating color into mathematical calculations based on data generated by measuring devices eliminates the need for a press operator to “eyeball” the press sheet to see if it looks approximately right.

Color measurement instruments are able to receive color data in the same way our eyes receive color -- by gathering and filtering light that is reflected from an object, whether that object is a flower or a sheet of paper printed with offset inks or toner.

The measurement device; however, transforms the color into a numeric value that allows us to scientifically analyze the quality of a specific color object.

There are three different devices used to measure color characteristics, and each has its role during the color workflow and production process.

These devices are colorimeters, spectrophotometers and densitometers.

Colorimeters

Colorimeters measure colors using filters to determine the nature of the color. In the world of graphic communications, colorimeters are most frequently used to calibrate output devices, including monitors, printers and even LCD projectors.

A colorimeter can sometimes be used as an alternative to a spectrophotometer, but it is not as accurate. In scientific fields the word generally refers to the device that measures the absorbance of particular wavelengths of light by a specific solution.

Colorimters are far more useful in the chemistry of color, such as formulating inks and toners, than they are in the practical color management of your print devices.

Spectrophotometers

A spectrophotometer measures wavelength reflections. A light source shines through or on the item being measured, such as a printed sheet, and a detector detects how much light has been absorbed by the area of the printed sheet being measured. This absorption is then converted into a number, which can be analyzed by a computer.

A spectrophotometer (also called spectroreflectometer or reflectometer), takes measurements in the visible region (and a little beyond) of a given color sample. If the custom of taking readings at 10 nanometers (billionth of a meter) increments is followed, the visible light range of 400-700 nm will yield 31 readings. These readings are typically used to draw the sample's spectral reflectance curve (how much it reflects, as a function of wavelength). Spectrophotometers are considered to be the most accurate technology available for measuring color characteristics.

An example of a spectrophotometer is the EFI ES-1000 spectrophotometer.

Another is the iPublish Pro 2.




Densitometers

A densitometer measures color ink or toner density. Densitometers are usually used in offset printing. Because inks are known standards, a densitometer helps in controlling the amount of ink on a page and the resulting color.

Color standards, such as the standards delivered by Pantone, include ink densities as part of the color specification.


XRite makes a fine densitometer.

BabelColor has a fun online densitometer that you can play with, or use for serious color management purposes.

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

The Role of the RIP in Digital Color

Raster image processors (RIPs) control printing devices.

They translate, in a very direct way, the page description language of PostScript into a bitmap image, either CMYK or grayscale, including trapping, font data, formatting, kerning, color input/output profiles, bleed, imposition, metadata and legal validation. What you send to the printer gets translated into a picture the printer is capable of reproducing, and there you go.

They have become increasingly sophisticated and play a significant role in the Color management because they process files for printing on digital and offset output devices, including proofing and CTP systems as well as digital printing devices.

While you can get adequate results from a simple printer driver, which performs many of the same functions as the RIP, dedicated RIP software can offer a great deal more control, more finely grained tuning of files for fine art or production applications. RIPs can offer print queuing, batch processing, color separations, halftone screening, as well as checking for missing fonts or graphics.

An effective RIP incorporates such things as ICC-compliant color management system and profiles as well as workflow integration to deliver optimum results. It cannot operate as an isolated application with proprietary tools.

Rather, RIPs must integrate with a production environment and facilitate the exchange of color profiles among the various constituencies of the Color Management Workflow, including designers, agencies, prepress operators and print service providers.

Keep in mind that graphics creation packages allow users to create files that can be very difficult to print. Also, many designers have little in-depth knowledge about the printing process and are not aware that their designs create production issues. At a minimum, an effective RIP accommodates these complex constructs, so the final printed product closely matches the design intent.

It also widens the range of file types that can be accepted into the production process. In addition, an effective RIP should be able to handle special or spot colors and correctly process overprints and transparencies.

RIPs also concatenate variable data, pairing the context of a database expressed as a CSV (Comma Separated Value) text file with tags in a PDF file to replace the tags with variable data, whether words or pictures, based upon the values from the database.

Transparencies and variable data do not play well together, since Postscript is a layers page layout description language, and both of these items prefer to be the topmost layer in the stack, and when they have to fight it out, VDP usually wins over transparency. However, a properly tuned RIP can process these files correctly, ensuring the final PDF that goes to print is correct.

Each RIP handles color management the same way. Input color profiles are translated to L*A*B* values, which are then converted to the output color profile assigned to the RIP, and the resulting PDF file matches the output profile directly.

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

What the Heck is Delta-E?

Measuring color on printed output makes sense, but what do you do with the data collected? Also, how do you know when you have a problem? The term Delta-E, (dE) is commonly used in discussing color management and answers these questions.

Delta-E is a single number or metric that represents the “distance” between two colors.

It helps users identify the limits of their workflows and to work within these expectations. The idea is that a dE of 1.0 is the smallest color difference the human eye can see. So any dE less than 1.0 is imperceptible.

Delta-E can help constituents in the Color work flow measure differences between a proof and the final printed product or to monitor whether or not color produced by a specific printing device has drifted. It allows us to measure how far away we are from our ideal. It also helps to remove any subjectivity from the color matching process.

It also can help users determine how effective a particular profile is for printing or proofing, how closely it matches the color original.

If difference is a number showing how 'far apart' two colors are, tolerance is the meaning of the number. Determining a tolerance number defines how much variance is acceptable depending on the printing environment, how color-critical the job is, and other factors.

It should be noted that there are a variety of Delta-E types, including DEab, DE94, DE_CMc and DE2000. It is important to be aware of which Delta-E measurement is being utilized in order to make accurate comparisons. Apples to apples, after all.

DE76 was first created in 1976, the year L*a*b* was created, and is a simple calculation to determine the distance between two colors. While the math is simple, it does suffer from some limitation.

One problem with dE76 is that L*a*b* itself is not perceptually uniform as its creators had intended. So different amounts of visual color shift in different color areas of L*a*b* might have the same dE76 number.

Conversely, the same amount of color shift might result in different dE76 values.

Another issue is that the eye is most sensitive to hue differences, then chroma and finally lightness and dE76 does not take this into account

DE2000 takes into account hue, lightness and chroma factors instead of being simply a raw calculation. As a result it is much more difficult to calculate. But it is also much more accurate and a better representation of how your engine is performing.

It should be noted that the human eye cannot detect differences in Delta-E below a measurement of approximately 2.2.

In environments such as laser printers, a Delta-E of 6 to 8 is perfectly satisfactory and is the level that is often achievable on laser based devices.

Most commercial printers consider a Delta-E range of 2 to 4 acceptable.

A few important points about delta-E calculations in general:

Remember, dE calculations are based on colorimetry which means they are illuminant-dependent. Don't try comparing numbers calculated from colors viewed / measured under different illuminants.

Differing dE due to illuminant is metamerism. If colors are 'adapted' to the same white point then you have a metamerism index.

Always remember that nobody accepts or rejects color because of numbers - it's the way it looks that counts.

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