Print Measurements & Standards · The Complete Guide

Every press operator has an eye for colour. But the eye is not a reliable measuring instrument. Two operators looking at the same sheet will reach different conclusions about whether the blue is slightly too purple. The same operator will reach a different conclusion in the morning versus the afternoon as eye fatigue sets in. And no eye can detect a 0.02 density unit drift in cyan ink density, which is a measurable, real change that affects colour balance across the sheet.

Measurement replaces these subjective judgements with objective numbers. When a densitometer says the cyan solid density is 1.38 and the target is 1.40, the deviation is 0.02 units, a small but real discrepancy that the press operator adjusts with precision. Without measurement, the operator makes an impression-based adjustment that may overcorrect or undercorrect. With measurement, adjustments are exact. This is why press rooms that measure consistently produce more consistent colour than those that rely on visual judgement, not because their operators are more skilled, but because their data is more reliable.

A densitometer measures the optical density of a printed ink film, how much light the ink absorbs. It shines a defined light source at the printed surface and measures how much light is reflected back. A solid ink area absorbs most of the light (high density); an unprinted paper area reflects most of the light (low density, close to zero). The density value is expressed on a logarithmic scale where 0.0 is perfect white and higher numbers indicate darker, denser ink coverage.

What density tells you about the press

Ink density is directly related to ink film thickness on the printed sheet. When density is high, more ink was delivered. When density is low, less ink was delivered. By measuring the solid ink density of each CMYK colour from the colour bar on every production sheet, the press operator can:

• Verify that all four inks are printing at their target densities
• Identify which ink zone (across the press width) needs adjustment
• Track density drift during a long run and correct before it becomes visible
• Confirm that the OK sheet can be reproduced on the press before each run begins

ISO 12647-2 target densities for offset printing

Densitometer types

• Reflection densitometer, the standard press room instrument. Measures the density of opaque printed surfaces. Most modern densitometers can also function as spectrophotometers, measuring colour in L*a*b* as well as density.
• Transmission densitometer, measures the density of transparent materials, films, negatives, and plates. Used in pre-press and plate room environments, not on the press floor.
• Spot measurement vs scanning, traditional densitometers measure one patch at a time. Modern scanning spectrophotometers can measure an entire colour bar in a single pass across the sheet in a few seconds, producing a complete density and ΔE profile across the full press width.

A spectrophotometer measures the full spectral reflectance of a colour, how much light of each wavelength (from approximately 380nm to 730nm) is reflected by the measured surface. From this spectral data, it calculates the CIE L*a*b* colour values that represent how the colour appears to a standard human observer. This is a more complete and more accurate description of colour than density, two inks with the same density can look very different in colour, and a spectrophotometer captures that difference while a densitometer does not.

L*a*b* · the device-independent colour reference

• L* (lightness), 0 is perfect black, 100 is perfect white. A neutral grey at 50% has an L* of approximately 50.
• a* (red-green axis), positive values are red, negative values are green. A neutral colour has a* near zero.
• b* (yellow-blue axis), positive values are yellow, negative values are blue. A neutral colour has b* near zero.

The advantage of L*a*b* is that it is device-independent, it describes what the colour looks like to a human observer, regardless of what device produced it. A printed cyan measured at L*=55, a*=−37, b*=−50 can be directly compared to the ISO 12647-2 target L*a*b* for cyan on coated paper, and the ΔE (colour difference) calculated. This comparison is objective and reproducible, the same result regardless of who makes the measurement or what densitometer they use.

When to use spectrophotometry vs densitometry

Dot gain (technically called Tonal Value Increase, TVI) is the difference between the dot percentage specified in the digital file and the dot percentage that actually prints on the sheet. A 50% dot in the file prints as a larger dot, typically 65–72% on standard coated paper in sheetfed offset. The dot has "gained" 15–22 percentage points. This is the ISO 12647-2 target dot gain for paper type 1 (coated), and it is expected, normal, and compensated for in the plate curve.

The Murray-Davies formula

Dot gain is calculated from density measurements using the Murray-Davies formula:

Interpreting dot gain · what the numbers mean in practice

• TVI below target (e.g. 10% instead of 18%), dots are printing smaller than expected. Midtones appear lighter and less dense than the standard. Possible causes: insufficient impression pressure, ink too tacky, blanket not properly seated. Also caused by over-compensation in the plate curve.
• TVI at target (18% ±4%), press is performing within ISO tolerance. This is the normal, expected condition for a well-maintained and correctly calibrated press on coated paper.
• TVI above target (e.g. 28% instead of 18%), dots are printing larger than expected. Midtones appear heavier and shadows fill in. Possible causes: excessive impression pressure, ink too fluid, blanket swollen or worn, high humidity causing ink emulsification. This is the most common condition in Indian press rooms where pressure management and humidity control are variable.

Grey balance is the ability of the press to reproduce a neutral grey, a colour with no visible colour cast, from a combination of cyan, magenta, and yellow inks. Equal percentages of CMY do not produce a neutral grey in offset printing. Due to ink impurities and the specific spectral characteristics of each ink, a visually neutral grey requires more cyan than magenta or yellow. The exact balance depends on the specific ink set, substrate, and printing conditions.

Why grey balance is so important

The human visual system is extremely sensitive to colour casts in neutral areas. A grey that is slightly warm (yellow-red) or slightly cool (blue-green) is immediately perceived as wrong, even by non-experts who cannot articulate what is wrong. This makes grey balance the most sensitive single indicator of press colour accuracy, if the grey balance is neutral, the entire colour system is likely in balance. If the grey has a cast, something in the ink density balance is wrong.

The grey balance patch in the colour bar

A standard colour bar includes a grey balance patch, typically built from C50 M40 Y40 (not equal percentages of CMY, because equal CMY does not produce neutral grey). This patch should appear visually neutral grey when printed correctly. It is measured with a spectrophotometer: a neutral grey has a* ≈ 0 and b* ≈ 0. Any deviation from zero indicates a colour cast:

• a* positive (red cast): magenta or yellow density too high, or cyan too low
• a* negative (green cast): cyan too high, or magenta too low
• b* positive (yellow cast): yellow density too high or cyan/magenta too low
• b* negative (blue cast): cyan or magenta too high, or yellow too low

Ink trapping is the ability of a subsequent ink colour to adhere to a previously printed wet ink film. In offset printing, every colour after the first prints onto a surface that already carries wet ink. If trapping is poor, the second ink sits on top of the first without bonding, producing flat, incorrect secondary colours and inaccurate hues in overprint areas.

The Preucil trap formula

Trapping is measured by comparing the density of an overprint patch (two inks printed together) to the density of the second ink printed alone. The Preucil formula calculates a trap percentage:

What causes poor trapping

• Incorrect ink tack sequence, the second ink must have lower tack than the first. If the second ink has higher tack than the first, it will pull the first ink off rather than laying down on it.
• Excessive dampening, too much fountain solution emulsified into the ink reduces ink tack, causing the second ink to trap poorly onto the first.
• Ink temperature too high, heat reduces ink tack. On high-speed presses running for long periods, ink temperature can rise enough to cause trapping deterioration.
• Incompatible ink series, inks from different manufacturers may have incompatible tack curves that produce poor trapping even with correct sequence.

A colour bar is a strip of standardised measurement patches printed in the waste margin of every production sheet, outside the trimmed image area. It provides the measurement points that allow the press room to verify and control every critical quality parameter without disturbing the image area. A sheet without a colour bar cannot be properly measured or verified.

Standard colour bar patches and what each measures

ISO 12647-2 is the international standard for offset lithographic printing. It defines the target L*a*b* colour values that CMYK primary and secondary colours should achieve when printed correctly on specified paper types, plus the tolerances within which deviations are acceptable. Meeting ISO 12647-2 targets means the press is producing colour that matches the defined international reference, enabling reliable colour communication between design studios, press rooms, and clients across different locations and production facilities.

Paper type classification in ISO 12647-2

ISO 12647-2 primary colour targets (Paper Type 1 · coated)

A press verification routine is the systematic schedule of measurements taken during production to confirm that the press is performing within acceptable tolerances. It converts measurement from a reactive activity (checking when something looks wrong) to a proactive one (tracking trends before they produce visible problems).

Recommended measurement schedule

The measurement log · tracking trends over time

Individual measurements catch immediate problems. A measurement log tracks trends. When cyan density is recorded at 1.45, 1.43, 1.41, 1.40, 1.38 across five consecutive measurement points during a run, the trend is clear, cyan is drifting low, even though no single reading has yet crossed the action threshold. The press operator can correct ahead of the threshold rather than after it. A simple spreadsheet or press room logbook recording each measurement with timestamp and sheet count is sufficient. The data compounds in value over weeks and months, revealing systematic press drift, substrate batch differences, and seasonal variation that individual readings never show.