Fade to black - ink permanence

Unfortunately, ink, like most things in life, is not permanent. The inks used in print production will all fade over time. The main cause is exposure to light (especially UV), which causes the ink/paper system to oxidize. When ink is oxidized it fades. Fading is much more complicated than is usually realized as it depends on environmental factors (light, heat, humidity,), the particular pigments being used, and the substrate the ink was applied to.

Ink manufacturers use fadeometers, along with known ink pigment characteristics, to test fade resistance by exposing the print to light radiation produced from a carbon arc or xenon tube. The arc emits an intense actinic light which in a matter of hours approximates the destructive effect of a much longer period of ordinary daylight. Although it does not exactly duplicate the effect of prolonged exposure to natural light, it is still an effective indicator of the degree of light stability and of the comparative resistance to fading. The results are interpreted with the aid of a chart that correlates the number of hours a printed sample lasts in the fadeometer to the equivalent exposure to direct sunlight taking into account the amount of UV light that different regions receive based on their latitude.

If resistance to fading is an important criteria for a print project, the best source of information is the vendor supplying the inks. They will know the characteristics of the pigments in their ink formulations and can suggest alternatives that may provide better fade resistance - though often at the expense of some other attribute like rub resistance, color vibrancy, or cost.

You can also do a simple test yourself. Simply take a presswork sample and cover half (front and back) with heavy black card. Then attach the sheet to a window.The covered section of the press sheet is the control against which you can compare the fading of the exposed part.
Original image
After 1 month
After 12 months
After 18 months
After 24 months
Some typical fade resistance numbers for sheetfed 4/C process inks based on outdoor exposure near the equator:

• Black (pigment black 7)
- Tint - Max tolerance: greater than 12 months.
- Fadeometer, Max tolerance: greater than 240 hrs.

• Process Cyan (pigment blue 15:3)
- Tint - Max tolerance: 24 months with fading
- Fadeometer, Max tolerance: 120-160 hours shows fading and loss of gloss

• Process Magenta (pigment red 57)
- Tint - Max tolerance: less than 1 month with fading & loss of gloss
- Fadeometer, Max Tolerance: 5-25 hours shows fading and loss of gloss

• Process Yellow (pigment yellow 12)
- Tint - Max Tolerance: less than 1 month with fading
- Fadeometer, Max Tolerance: 2-30 hrs shows fading and loss of gloss

How lightfastness is measured


The "Blue Wool Scale" is the internationally recognized method of quantifying lightfastness, defined under the British Standard BS1006. The scale consists of 8 different strips of wool, each dyed with a blue dye of differing lightfastness. The scale ranges from 8 (excellent – very low rate of fading) to 1 (very poor – extremely fast fading). The blue wool scale is not a linear scale but is rather logarithmic, so that each increase in level is greater than the previous.

When exposed in the same manner as the print for the same length of time, the level of fading of the printed solid is compared to that of the equivalent strip of wool to provide a value indicating the lightfastness of the print.

The "Wool Scale" lightfastness classifications are:
WS 1 - very poor
WS 2 - poor
WS 3 - moderate
WS 4 - fairly good
WS 5 - good
WS 6 - very good
WS 7 - excellent
WS 8 - maximum lightfastness

Tips for the printer:

• When interpreting degrees of lightfastness into production requirements - real-world conditions need to be taken into account. For example a paper that contains a high degree of wood fiber will soon yellow and therefore effect the color of ink that overprints it - even if the ink has a high WS rating.

• A higher ink film thickness than indicated in the standard specifications will result in an increase of the lightfastness of the print because there will be more pigment particles in a given area to withstand the destructive influence of light. The same applies to a higher pigment load ink. On the other hand a thinner ink film thickness will reduce lightfastness. Also, lightfastness in halftone screened areas is generally lower than in the solids.

• Varnishing and lamination of presswork will generally improve its lightfastness. Specific over varnishes exist which contain UV inhibitors and high-grade resins. These can increase the lightfastness of presswork, however, they will not prevent ‘weak’ pigments from fading, merely slightly increase the time they take to fade.

• If two or more printing inks of different lightfastness properties are mixed, the low WS rated one is not improved by the high WS rated one. Instead the low WS rated one lowers lightfastness of the high WS rated ink. Effectively the ink of the lowest lightfastness determines the lightfastness of the mixed Ink.

• Lightening of an ink with transparent white will, in most cases, diminish its lightfastness. Since white pigment does not fade easily, pastel colors with a small concentration of colored pigment and a large concentration of white will fade rapidly to white. For similar reasons, large quantities of varnish should be avoided in colour matches for presswork exposed to sunlight.

As a general guide, lightening with transparent white at a ratio of 1:1 will reduce lightfastness by 1 grade. Lightening with transparent white at a ratio of 1:3 will reduce lightfastness by 2 grades.

• Lightfastness of presswork exposed to sunlight will be lower near the Equator and higher as one move towards the Poles.

• In the northern hemisphere lightfastness of presswork exposed to sunlight will be lower in the summer and higher in the winter.

Top reasons why color instruments don't agree

The increased use of instruments like spectrophotometers in the print industry has created an apparent increase in the level of precision in the measurement and description of color. However, the objective accuracy may not be as it seems - when comparing the measurement results from different instruments - even when coming from the same vendor.

Even when properly calibrated instruments can deliver different measurement values (>DeltaE 7 according to a PIA/GATF study) simply because of how the various instruments respond to the gloss on coated paper, aqueous coatings, UV coatings, and lamination. The use of UV cut filters (as is popular in Europe) can also increase the disagreement between instruments.

The top reasons why color instruments don't agree

• Variations in ambient conditions including instrument Induced sample heating resulting in "thermochromism" where Ink changes color due to a change in temperature and "hygrochromism" because humidity changes the way ink interacts with paper and hence its color.It's a good idea to record temperature and humidity levels whenever measurements are taken.

• Noise introduced by reflectometer instability, instrument and environment induced noise and dark current drift.

• Fluorescence in the substrate coupled with variation in the spectral power distribution of the instrument's illumination - too little or too much UV light.

• Instrument Geometry. There are typically no geometric tolerances on low end instruments. Fiber optic instruments tend to have wide geometric tolerances.

• Spectral bandwidth function may be too narrow or too broad and be too variable from wavelength to wavelength.
• No, or inadequate, black level adjustment. Non-black light trap or directionally sensitive light trap.

• Poor instrument maintenance.

• Infrequent or lack of recertification by factory. Lack of periodic verification

Tolerancing color in presswork - CIE L*a*b* and DeltaE

This method attempts to bring an objective, system independent, instrument-based method to color tolerancing. Because this method uses instruments to define colors, the range of tolerance and deviation from the target it is considered to be objective and unambiguous. It is much more sophisticated than the more subjective methods so far described in my other posts. As a result, a bit of background knowledge about color science is needed in order to understand how this system works and to understand its value and potential pitfalls.

A scientific approach to describing color
From a color science point of view, any color can be described by three basic attributes:

1) Lightness. This is the attribute of a color by virtue of which it is discernible as bright, dark, or somewhere between those extremes.
2) Chroma.This is the attribute of a color by virtue of which it is discernible as purity or intensity of color relative to a neutral color like grey. Also referred to as "saturation."
3) Hue. This is the attribute of a color by virtue of which it is discernible as red, green, etc., and which is dependent on its dominant wavelength, and independent of intensity or lightness.
So, from a scientific point of view, describing a color requires three values/numbers. One for Hue, one for Lightness, and one for Chroma.

Describing a specific color this way can be visualized as finding the location of a specific room in a building.One goes up a central elevator representing the range from neutral dark to light. Then one gets out of the elevator at a specific floor/specific lightness level and travels outward from neutral grey to an increasing amount of chroma/saturation as they move toward the outside edge of the building. Once they reach the desired amount of chroma/saturation one moves to the left or right to find the specific room/hue. So, directions to the specific room/color can be expressed as a recipe: Up X levels (lightness/floor level), Move X Distance (Chroma/Down hallway), Move X degrees (Hue/Along perimeter) = Room/Color.

This three coordinate method of describing a color can be visualized in cut-away form as in this graphic:In reality this 3D color space map is more complicated (you can see a movie of a real 3D color space HERE). However it should be good enough to explain this complex subject.

This three coordinate system (LCh) can then be used to map the location of a specific color.
Unfortunately, LCh has not been widely adopted to describe a color's location within a color space. Instead, the less intuitive L*a*b* notation is most commonly used. L*a*b*, more properly written CIE L*a*b* uses the same 3D model but identifies the color according to it's "L" lightness, "a*" axis value (+a* = more red, -a* = more green compared to neutral grey) and "b*" axis value (+b* = more yellow. -b* = more blue compared to neutral grey).

Defining a color location using CIE L*a*b* coordinates
Using the three coordinate CIE L*a*b* system allows us to numerically identify any color within a color space. In this example, I'll use a print color space and identify the desired color within that color space:
Tolerancing a color using CIE L*a*b*
Color tolerancing using CIE L*a*b* involves comparing the measurements, taken with a spectrophotometer, of a color sample (the output) to the data of a known color (the specification or input value). Then the "closeness" of the sample to the specification is determined. If the sample's measured data is not close enough to the requested color values, it is deemed to be unacceptable and adjustments to the process may be required.

The amount of "closeness" between two colors can be caluculated using a variety of methods. These methods calculate the distance between the two sets of measurement coordinates (e.g. CIE L*a*b* values) within the three dimensional color space. The size of the distance is defined by the size of the tolerance and is expressed as a "DeltaE" value (Delta Error).

To calculate the "closeness" of the specified color and the sampled color, the specified color is pinpointed by its position in CIEL*a*b* color space. Then a theoretical "tolerance sphere" is plotted around the color.The sphere, with the specified color at its center, represents the acceptable amount of difference between the specified color and other measured samples (the color output). The actual size of the tolerance sphere is determined by the customer's specification's for acceptable color difference. The tolerance value is expressed in delta (∆) units such as ∆E usually written as DeltaE (delta error). Measured data that falls within the tolerance sphere represents acceptable color.Measured data that falls outside the tolerance sphere represents unacceptable color.

Typical customer tolerances in the graphic arts industry usually range between 2 and 6 ∆E. This means, for example, that samples outside the tolerance sphere lie more than 6 ∆ units away from the specified color. Tolerances less than 2 ∆ units are typically unachievable given normal process variation. Differences between two colors that are up to 4 ∆ units away from each other are usually not visible to most viewers.

Issues, concerns, and caveats when using CIE L*a*b* DeltaE tolerancing
While this method can bring an objective and potentially unambiguous method to color tolerancing there are several issues to be aware of that can cause misunderstanding and error.

1) CIE L*a*b* DeltaE tolerancing is instrument dependent, however, different instruments can deliver different values from the same color sample.
Some of the reasons include: poor maintenance of instrumentation, infrequent recertification by the factory, lack of periodic verification, spectral bandwith differences, lack of geometric tolerances, variations in fluorescence in the substrate and instrument illuminant, instrument and environment induced noise, dark current drift, variations in ambient conditions, thermochromism (ink changes color due to a change in temperature), hygrochromism (humidity changes the way ink interacts with paper and hence its color).

2) CIE L*a*b* DeltaE values are dependent on the formula used - and there is no universally agreed standard for the formula that should be used.
Some formulas are: DeltaE 76 (sometimes referred simply as DeltaE), DeltaE 94, DeltaE 2000, and DeltaE CMC. In general, DeltaE 76 values are highest, DeltaE CMC values the lowest especially for saturated colors, DeltaE 94 and 2000 are lower than DeltaE 76 but higher than DeltaE CMC.

For example, these two color patches are made up with the indicated CIE L*a*b* values:The DeltaE difference between these two colors as reported by the different color difference formulas:
CIE 76: 7.10 (a large difference - unacceptable)
CIE 94: 1.51 (well within typically acceptable variation)
CIE 2000: 1.57 (well within typically acceptable variation)
CMC: 2.26 (within typically acceptable variation)

So, depending on the formula used to calculate the difference in color a measured sample may, or may not, be within acceptable tolerance.

3) It is harder to see the differences when colors are very saturated. It is easy to see a difference when colors are near neutral.
Formulas like CIE 94 attempt to compensate for this difference in visual color acuity, however, it is not the predominantly used formula. That honor goes to CIE 76. It's therefore important when discussing color variation to specify which formula is being used to calculate DeltaE values so that the numbers can be better interpreted.

4) The color performance of a system or press sheet is sometimes reduced to a single DeltaE value as a statement of being within tolerance. This can be very misleading since the single DeltaE value is an average of all sampled colors and will likely not reflect the performance of specific critical colors.
Statements such as "This press sheet is within 2 DeltaE of the proof" are virtually meaningless.

5) There are no CIE L*a*b* controls on a press.
If a color on a press sheet is out of DeltaE tolerance - the press operator effectively has to guess at what should be done to correct the problem using tools not designed for this function like solid ink density, water, impression pressure, etc. to effect a change in color.

Tolerancing color in presswork by eye

Once the press operator has achieved their solid ink density targets during make ready, they typically will do a visual examination to compare presswork color to proof to evaluate the closeness of the match. They will also do visual comparisons during the run to check the consistency of the match through the press run. Print buyers typically also do the same thing - relying on their eye for color to verify the match and consistency.
For most people in the graphic arts, the eye is the final arbitrator on the quality and consistency of presswork - however, this method has some limitations caused by the fact that the eye is part of a very tricky instrument: the human brain. For example, look carefully at this graphic containing light green and light blue swirls:
Light green and light blue?

Actually there are no light blue swirls, What you see as light blue is actually the same color as the green ones. They are both R 0, G 255, B 151. Cutting out a section of the "light blue" swirls and lining them up with the light green ones proves they are indeed the same color.This illusion is so strong that you might have to down load the image into PhotoShop and confirm it for yourself.

There are several characteristics of our eye/brains that can play tricks on our perception. Being aware of them will help you more clearly understand how your color perception can be mislead and hopefully provide a clearer view as to what you are actually seeing.

1) The eye/brain auto-white balances. The eye/brain selects an area that it "knows" is white - forces it to appear white and balances other colors accordingly. This is the trick that allows us to see a white paper as white with surrounding colors being natural under a variety of different colored lighting situations. This often causes problems with monitor proofs where an image should be "white" i.e. should result in no halftone dots in the presswork, but in fact has grey or even a color cast in it that results in dots being printed. The eye/brain sees the area as white when in fact it is not.

2) The eye/brain has no color memory. Not only does the eye/brain auto-white balance, it also rebalances color whenever you look from one object to another. This makes comparing two colors that are separated, by even a small distance, impossible. For example, in the below image, the press operator cannot effectively compare color between the image on his soft-proof with the color on his press sheet.
The only way to compare two colors is by cutting through one sample and overlaying it on a reference (e.g. cut press sheet over proof) like this:If the color aligns across the cut then you have a match.

3) The eye/brain cannot judge variation consistently. In order to tolerance acceptable color variation, for example custom/brand colors, you need to have a reference high/low density guide that provides an example of the two extremes that the color must fall within. Providing a swatch guide with holes through it as in this example:allows users to place the swatch over the press sheet to more easily confirm the color match as well as whether it falls between the two acceptable extremes.

4) The eye/brain's perception of color is influenced by the size of the area of color. This is one of the reasons that the paint color selected from a small paint chip seldom appears the same as the color once it's painted on the wall. The same issue happens when selecting a spot or brand color from a swatchbook. Always try to get a reference chip, draw down, or previously printed sample, that is as large as possible.

5) The eye/brain's perception of color difference is not uniform for all colors. It is difficult for the eye/brain to see differences in highly saturated colors. However, a small degree of variation is easily seen when colors are near neutral. Variations in the green part of the spectrum are more easily noticed than the same degree of variation in the red part of the spectrum.

Tolerancing color in presswork using solid ink density

Background information - ink film thickness & solid ink density

Offset printing presses are designed to lay down a film of ink, in the presence of water, onto a substrate - usually paper. The ink forms the image while the "water," more accurately fountain solution," prevents the non-image area on the printing plate from accepting ink. For the process to work, there needs to be a critical ink/water balance with the goal of having an ink film thickness between one micron.

If the ink film thickness is too great, the result can be "ink tailing/misting." In addition, the non-image background may take on ink resulting in "catch-up" (sometimes mistaken for "scumming"):
On the other hand, if the ink film thickness is too thin, the result can be a breakdown of the ink on the sheet causing low contrast, loss of sharpness, and mottle:
So, from a color tolerancing point of view, because the function of ink is to filter light and allow us to see color and because its thickness also effects the integrity of the printing process - ink film thickness on the sheet becomes an important metric to measure and tolerance in presswork.Top: CMYK at high ink film thickness/solid ink density.
Bottom: CMYK at low ink film thickness/solid ink density.
There is no practical way to directly measure the ink film thickness on a press sheet. However, there is an indirect way and that is to measure the solid ink density (SID) using an instrument called a densitometer.

Color tolerancing through densitometry

Measuring SIDs in the solid ink patches in the color bar with a densitometer does not actually provide information about the color being printed. However, because it indirectly provides information about ink film thickness (which impacts color and tone reproduction) SID values are valuable for process control and defining variation during a press run where the instrument, ink, and substrate remain the same.North American (Status-T) high-low specifications for acceptable SID variations measured with ink dry.
Top: Commercial sheetfed, Middle: Magazine/heatset web,
Bottom: Newsprint/coldset web.
From a color point of view, the assumption is that all three chromatic colors vary in the same direction and therefore remain in relative balance. When that happens there is a shift in color saturation (higher SIDs = higher saturation) as well as tone reproduction (higher SIDs = higher dot gain/TVI). If one color, e.g. Cyan, is at the maximum low point while another color, e.g. Magenta, is at the maximum high then the result may be a visible color bias in the presswork.
Typical SID variation in presswork graphed by measuring color bar patches every 10 sheets through the press run.
While a densitometer can also be used to monitor variations in non-process, i.e. spot/Pantone colors, usually a printed sample of the target color, including a high/low density tolerance reference, is used instead since this helps both print specifier and supplier visualize the acceptable range of color change as SIDs naturally vary during the press run.Checking for spot color variation
Addendum: Densitometer set up - "Status" condition

Densitometers are set by their manufacturers to an industry defined "Status" which defines the total response of the instrument including light source, optics, filtering, and receptor for given wavelength. The primary responses for the print business are "Status E" and "Status T" (ANSI PH2.18 and DIN 16536). In addition, densitometers are available with or without polarizing filters. Dry ink density readings from polarizing and unpolarized densitometers as well as those set to Status E vs Status T will not agree. Typically European instruments are set to Status E and use polarizing filters while North American instruments are set to Status T and do not use polarizing filters.

The important thing to be aware of is that if SID information is shared outside of the printshop - then the Status of the instruments that were used to determine SID values must be known. In addition, it is critical that all instruments within the printshop are set to the same Status. In North America, where many of the presses and their closed-loop color control systems are from Europe, it is not unusual to find the press set to Status E polarized while the handhelds are set to Status T unpolarized which can easily result in quite a bit of confusion in production.

Conducting The Press Check

Printing is a completely different imaging process than proofing. As a result, it is not always possible for the presswork to perfectly "match" the proof. Unfortunately, there is no objective, practical, way of defining what is an acceptable variation from the proof that still constitutes an acceptable pressrun. The role of the press check is to enable the customer, or their representative, to directly communicate their presswork concerns and acceptance, with the press operator, so that a successful pressrun can be achieved.

The customer should be able to go to the press and speak to the press operator directly. Working through an intermediary like the pressroom manager only slows down the process and causes communication errors especially when if they are managing multiple press checks at the same time.

Press check basics for the printer

• The customer should not be press side during initial make ready. Ideally you would have a comfortable holding area/lounge where customers can wait until the press operator has a sheet ready for inspection.

• An initial make ready sheet marked clearly "For Content Only" could be given to the customer so that they can check the sheet for content issues like substituted fonts, low resolution images, missing graphics, etc. This reduces time wasted at the press when doing the color approval. Also, if content errors are discovered then the job can be halted soon enough to avoid excessive make ready wastage.

• The press operator should be told ahead of time, by the CSR or sales representative about issues, concerns, and critical success goals the print buyer has for the job. For example, if a particular blue in a product image is important to the customer - the press operator should know that fact. Having this knowledge shows the customer that the shop is personally involved in the success of both the press run and the customer. It also helps establish better communications, if any color adjustments need to be made.

• When the press operator is satisfied with the color then a press sheet should be pulled and filed as "first press operator OK." Press operator OK'd color sheets can be used later by the printshop to identify and evaluate print manufacturing issues from prepress through the pressroom.

• When the customer arrives at the press, the press operator should introduce themselves.

• When the customer is at the press the press operator should stand aside and allow the customer "breathing room" to examine the press sheet against the proof.

• Tools such as Pantone swatchbooks, ink draw downs, loupes, note paper and pens for customer use should be readily available.

• The press operator should try and use correct terms and use them consistently. For example, do not use the term "blue" if you mean "cyan."

• Encourage the customer to explain what concerns they have rather than tell you how to fix them. Their role is to identify the problem - your role is to know whether, and how the problem can be fixed.

• If you make a press adjustment to fix an issue, tell the customer what you will be doing and how you feel it will fix the problem. This helps confirm to the customer that you correctly understood their issue and it also helps educate them which, in turn, will make future press checks go quicker.

• If there are any issues that you have with the press to proof match - let the customer know that right away rather than have them discover it themselves. Doing so tells the customer that you are not trying to hide anything from them. It also helps establish a mutually respectful environment.

• Be aware of time - every minute the press is idle the company is losing money. Remember, you can get press sheet sign-off with notes to cover some issues (e.g. "OK for color - but must remove all circled spots and hickies). Don't rush the customer, but don't let them dally at the press either.

• Some shops will have a "light room" where customers can take the sheet to view it under "standard" office lighting. This helps mitigate metamerism and substrate fluorescence issues.

• Make sure that the OK press sheet is signed and dated by the customer, the sales representative/CSR, and the press operator. Any continuing issues to be dealt with later should be noted on the sheet.

• Have a cardboard tube ready so that OK'd press sheets can be taken away by the customer.

• Thank the customer for attending the press check.

Press check basics for the print buyer
The role of the print buyer at the press check is to directly communicate their presswork concerns and acceptance with the press operator in the presence of the sales person to help ensure that any color issues are dealt with according to the customer's requirements.

• Once you get the call from the printer, gather up any material related to the job, samples, proofs, spot color draw downs, paper samples, mock-ups, folding dummies, etc. Also, make sure to bring loupes, color swatchbooks, scissors/x-acto knife, pens and notebooks.

• It's a good idea to get an imposition proof from the printer so that you can check three things; are there any potential inline color issues, are pages on the press form imposed the same way as they were on the imposition proof, and finally when the sheet is backed up on press do the pages back up correctly.

• Arrive at the printshop on time. Identify yourself at reception and explain why you are there. Then wait for the sales rep or CSR to be escorted to the waiting lounge or press floor.

• If you are part of a group attending the press check, identify which single individual will be the lead. That is the one person who consolidates the opinions of the folks attending the press approval and therefore the one who speaks for the group to the press operator.

• While waiting to go out to the press floor, ask for an initial make ready sheet that can be checked for content issues like substituted fonts, low resolution images, missing graphics, etc. This will reduce time wasted at the press when doing the color approval.

• At the press, introduce yourself and your team to the press operator. Wait to be invited anywhere near the press console.

• Do not touch or use any equipment at the press unless you specifically ask permission first.

• When you are offered the sheet for examination, ask the press operator if they are happy with the sheet and if they have any concerns/issues with it.

• Engage your sales representative for input and guidance with any thoughts/concerns you have. Tap into their experience.

• Recognize that time equals cost so be focused on the task at hand. If you are working with a team, assign checking roles to each. For example, someone checking registration, another checking for hickeys/specs, low resolution photos, swapped or dropped fonts, etc.

• If you are alone, have a written, organized, step by step yes/no, pass/fail procedure to checking the press sheet . Typically the process goes like this:
1) Is it printed on the correct paper? If a specific paper grain direction was required ensure that it is running in the correct direction.
2) Is it in register?
3) Over all, does the press work color align with the proof? Are there any obvious color issues?

4) When critical color alignment is required, cut the press sheet through the important color and overlay that section of the press sheet on the proof. Colors that may appear correct when compared side by side may appear different when directly overlaid.If you cannot clearly see where the press sheet ends and the proof begins you know you have a critical match.

5) Use your reference material to confirm the correctness of special/spot/brand colors.
• If you have any color concerns/issues, try to describe them clearly and unambiguously. Then describe just as clearly and unambiguously what you want to see. Do not tell them how to fix the issues. Your role is to identify any problems - it is the press operator's role is to know whether, and how any problems can be fixed. You can ask whether a solution you thought of might solve the problem. For example, you could say: "I think this area is too red. Would reducing the Magenta a touch fix it?" Phrasing a suggestion as a question can also help your press operator better understand your meaning according to how you describe the problem.

• Try and use correct and unambiguous terms and use them consistently. For example, do not use the term "blue" if you mean "cyan." Try to avoid terms like: "This area is too hot" or "Can you punch it up a notch?"

• If a press adjustment is made to fix an issue, ask the press operator what they are doing and how they feel it will fix the problem. This helps you to better understand the print production process, and its limitations, better.

• Most press operators will try very hard to achieve what you’re looking for, however, once they've made their press moves and you are still not satisfied it will be up to the sales rep to authorize trying anything else. If it's a really serious issue, the sales rep may stop the press and pull the job.

• Keep in mind that the start/stop/start/stop press cycling during a press check means that the press is not yet running in a stable fashion. Once the press OK is complete and the press is running at optimal speed, some small color issues will clear up by themselves.

• Be aware of time. Respect the printshop's need to maintain their production schedules. Remember, you don’t need to remain until the sheet is absolutely perfect. Just mark it as “OK with changes as noted.” (e.g. "OK for color - but must remove all circled spots and hickies).

• Some shops will have a "light room" where you can take the sheet to view it under "standard" office lighting. This helps mitigate metamerism and substrate fluorescence issues.

• Make sure that the OK press sheet is signed and dated by the person in your team who has authority to take responsibility. Any continuing issues that are to be dealt with later should be so noted on the sheet.

• Ask for a few copies of the OK'd sheet to take away with you for your records.

• Thank the press operator and crew for their performance during the press check. They really appreciate it and will remember you in a positive light during your next press check.

Fashion in the pressroom

In well run printshops the owners will usually provide prepress and pressroom workers with clothing, in part to present a clean unified look to visitors and customers. However, there are some outfits that should never be seen near the press.

Here are two prime examples of pressroom-fashion faux pas:

From North America:
From Japan:
Apart from their difference in weight, what is their sartorial sin? In a word, color. The color of what is worn gets reflected into the press sheet color. It can be a difficult problem to be aware of since our eye/brains instantly auto-white balance - so white paper will still look white. Unfortunately, the red or blue outfits in the above examples will distort the hues of the color in the presswork and may lead to incorrect color adjustments.

However, the below fashionistas are more in keeping with the discipline of the press room:Grey shirts, grey pants, and optionally, grey hair. Grey balance on press and grey balance in the closet.

This rule also applies to print buyers/specifiers when conducting a press check. To evaluate color effectively wear the neutrals, black, white, grey.

The principle of dot gain compensation plate curves

In a film workflow the industry standard was to create film output that was linear. This meant that a 25% tone request in the original Postscript file would create a 25% dot on the film, a 50% request would create a 50% dot, and so one for all requested tone values. However, in a CtP workflow controlling tonality in the print reproduction process, allows you achieve the presswork quality you want without adjusting the press. It also provides the flexibility to tailor the print characteristic to meet different customer expectations.

Dot gain, or tone value increase (TVI), is a normal part of the print reproduction process. Controlling tones using calibration means that you can manipulate the exact size of the dots on the printing plates so that tone saturation and gray balance are controlled on the press sheet.

Tonal calibration can account for:
• type of plate or film used
• type of paper stock used for printing
• type of dot shape used
• type of screening used—for example, FM/Stochastic or AM/XM conventional, and frequency (lines per inch (lpi))

(Note: Adjusting CtP laser exposure is not tonal calibration and will affect the run length and performance of the plate.)

You cannot use tonal calibration as a substitute for stable operating conditions. Operating conditions must be controlled as a separate process. In fact, without a stable operating environment, you cannot achieve accurate tonal calibration let alone reliable press output.

What Is Tonality?

Printers are used to being concerned with dot gain/TVI. Indeed dot gain values are often included in printing specifications. However, for the purposes of calibration - tonality or dot area, rather than dot gain, is the key metric. It does not matter what dot gain you have. What matters is whether you achieve the required final tone values or dot areas at each originally requested tone.On the left is the desired "correct" tone reproduction and on the right is incorrect tone reproduction.
Tonality in printing is the progression of tints from blank paper to solid ink for each requested tone value in a printing job. It is measured with a densitometer, and reported as either dot gain/TVI or dot area:Dot area and dot gain - two ways of charting the same data.
The target print characteristic tone curve

Building dot gain compensation plate curves always begins with a target print characteristic, i.e. what you want to achieve on press. This is called the target curve - the current tone reproduction that you wish to achieve. It could be your current press work, a proof, or it could be an industry supplied set of tone values. You measure the target sample and enter the dot area (tonal value) for the tints achieved on the target curve graph. If the target is a press sheet, for example, your current 150 lpi AM/XM presswork, the graph will represent your current tone print characteristic:Target print characteristic tone curve - what we want our presswork to look like.
If you change your screening, for example going to FM screening, higher solid ink densities, or higher lpi AM/XM screening, etc. then, if nothing else changes, the tonal response on press will change due to the difference in dot gain:New print characteristic tone curve caused by a change in screening method being used - what the presswork now looks like after changing the halftone screening.
The goal of implementing dot gain compensation plate curves is to make the new press work mimic the original target press tone response. In the above example, the boy's face should appear the same as the original image despite the dot gain caused by changing the halftone screening.

Creating the dot gain compensation plate curve

Building a dot gain compensation plate curve starts with comparing the current target tone response with the tone response of the new presswork. In this case run to the same solid ink densities, on the same paper and press - only the screening has been changed:On the left is the current target tone curve and on the right is the new tone response resulting from the change in screening.
The graphs are then examined by looking at the original requested Postscript tone and the target response (left chart) and comparing it with the new tone response (right chart):In the current target tone curve a 50% tone request resulted in a 68% tone in the presswork. That same target 68% was delivered in the new presswork from a requested tone value of 30%.
Put another way, we are looking for what requested tone value in our new presswork delivered the same final tone value in the target presswork. In this example a 30% tone request in the new presswork delivered the same tone value as a 50% request in the old while a 50% request in the new gave the same tone as a 70% request in the old.

Here's another way to visualize it:Target 150 lpi compared with FM tone response.
Remapping the tones is simply doing this:Find the tone in the new presswork that delivers the required tone response in the old target presswork.

The comparison between target curve and new current curve is made for each 10% change in tone.

The idea is then to map these values so that a tone request in the original file gets changed to a new value that produces the same final tone as the same tone request did in the old target presswork. The result is a lookup table for tone swapping.

In this example:

The requested 10% tone is remapped to request for a 4% tone
The requested 20% tone is remapped to request for a 10% tone
The requested 30% tone is remapped to request for a 18% tone
The requested 40% tone is remapped to request for a 24% tone
The requested 50% tone is remapped to request for a 30% tone
The requested 60% tone is remapped to request for a 40% tone
The requested 70% tone is remapped to request for a 50% tone
The requested 80% tone is remapped to request for a 65% tone
The requested 90% tone is remapped to request for a 80% tone

The lookup table creates the dot gain compensation plate curve.
The lookup table is applied in the workflow to remap the requested tones to the actual tones on plate that will deliver the desired final tones in the presswork. The result is tonal alignment of the presswork despite differences in dot gain.On the left is the original target 150 lpi tone response. On the right is the "normalized" tone response of the FM screen.
Some points to keep in mind

1 - It does not matter if the plates are initially run "uncalibrated" or linear for the target presswork.
2 - A dot gain compensation plate curve is not usually applied to the tone range from 0%-5% and 95% to 100%.
3 - One dot gain compensation plate curve is usually applied to all process colors.
4 - There may be a need to apply a specific dot gain compensation plate curve to one of the process colors to maintain gray balance.
5 - Dot gain compensation plate curves cannot compensate for differences in gamut between FM/Stochastic screens and conventional AM/XM screens.