K Mean Black

• Name: Printing Technology
• Branch: Printing Technology Diploma 6th Sem
• Published: May 25, 2025

Quality Management Part - 2

2) Dot gain

• When halftone dots print larger on the press than what they originally were on the plate or film, resulting in a loss of detail and lower contrast in the image.
• Dot gain occurs on every job to some degree.
• It is predictable to a point and can be compensated for when film and plates are produced Dot gain often occurs in long press runs, due to plates and/or pressure settings wearing or changing throughout the run.

• Dot gain is the difference between the halftone values in the screen film and in print.
• The dot gain Z (%) is obtained from the difference between the measured halftone value in print FD and the known halftone value in film FF.

Z (%) = FD – FF

• The major component of dot gain is Optical Dot Gain, by which light scatters in the printing substrate.
• Dot gain may also be contributed by dot spreading, press gain, slurring, or doubling.
• Mechanical Dot Gain - An aspect of Dot Gain associated with physical increase in the area coverage of a halftone dot when printed, compared to the area of the dot on the image-setter film.
• Optical Dot Gain - Contribution to dot gain caused by light scattering and absorption in the substrate, and absorption of light on the underside of a printed dot, thereby increasing the density in those areas.

Here are some other factors that cause dot gain:

1. Ink and water balance.

2. Blanket construction or other properties.

3. Blanket height.

4. Roller settings.

5. Bearer pressure settings.

6. Plate wear.

7. Ink temperature.

8. Piling.

 

3) Print Contrast

• A method of evaluating and optimizing the density of the ink deposited on the substrate during printing.
• Print contrast is calculated by measuring the ink density of a solid area and the ink density in a 75% tint.
• The relative print contrast is also calculated from the readings of the solid ink density Ds and the screen (or tint) ink density Dt.
• The Dt value here is best measured in the three-quarter tone (ie 75% of tint). The print contrast is calculated according to the formula:

K(%) = Ds – Dt x 100

Ds

• A print should have a contrast as high as possible. This means that the solids should have a high ink density, but the screen should still print open (optimum halftone value difference).
• When the inking is increased and the ink density of the dots rises, the contrast is increased.

 

• However, the increase in ink feed is only practicable up to a certain limit. Above that limit the dots tend to exhibit gain and, especially in three-quarter tone, to fill in.
• This reduces the portion of paper white, and the contrast decreases again.
• If the contrast value deteriorates during a production run in spite of constant ink value in solid DV, this may be a sign that the blankets need washing.
• If the solid density is correct, the contrast value can be used to assess various factors which influence the print result such as

1.  Rolling and printing pressure,
2.  Blankets and underlays,
3. Dampening Solution,
4. Printing inks and additives.

 

 

4) Ink Trapping

• It indicates how well an ink is accepted when printed onto another ink as compared to when it is printed onto the printing stock.
• The term trapping refers to the action of printing an ink film on top of another ink film, as in process color printing.

• Proper trapping results in well printed materials, while poor trapping results in successive inks that do not adhere properly and bead or rub off readily.
• A distinction has to be made between wet-on-dry and wet-on-wet printing.
• Wet trapping refers to trapping performed in wet multi-color printing, where one ink is laid down on top of a previously printed, still-wet ink. If the second ink has greater tack than the first ink, poor trapping will occur.
• Dry trapping is a multi-color printing process in which one ink is laid down on top of a dry ink.

• The ink tack affects wet-on-wet ink trapping.
• The color of the overprint changes as a function of ink sequence.
• When ink sequences are altered between black ink (high tack) and yellow ink (low tack), the ‘black over yellow’ overprint results in less darkness than the ‘yellow over black’ overprint.

 

5) Dot Area

• The size of a halftone dot, expressed as a percentage of the total surface area, which can range from 0% (no dot) in highlights to 100% (solid ink density) in shadows.

 

• By carefully measuring the dot area in various regions of an image at various stages in the reproduction of an image, dot densities can remain consistent.
• In digital halftoning, dots (in this case called cells) comprise much smaller printer spots which, depending on the resolution of the output device, can be used to create dots of various sizes and densities.
• Varying the number of spots that make up a halftone cell can work to fine-tune the dot densities by increasing the number of shades of gray available as the number of spots in a cell is increased.
• The amount of ink layered on a page (colors printed on top of each other as in 4- color process printing) is the Total Ink Coverage (TIC) or Total Area Coverage (TAC) for a document.

• The printing method and type of paper are two key factors in determining the maximum ink coverage that is acceptable.
• TIC is the sum of the cyan, magenta, yellow, and black (CMYK) values for any part of an image. For example,
• C40 M30 Y20 K15 is 40+30+20+15 = 105% TIC.
• C50 M50 Y50 K100 would be 250% TIC.
• If the maximum TIC for a specific printing method and type of paper is exceeded the layers of ink added last in the print run may not adhere properly to the underlying layers of ink resulting in inaccurate colors, slow ink-drying with possible rub-off of ink, wrinkling of the paper, and ink bleeding through the paper.
• Too much ink can result in photos with muddy colors or dirty brown instead of a nice neutral black or a loss of detail in the shadows from the excessive ink. CIE Lab
• CIE - International Commission of Illumination. This is the organization responsible for setting the world-wide color measurement standards.
• An organization called CIE (Commission Internationale de l'Eclairage) determined standard values that are used worldwide to measure color.
• CIELab is the color space that ICC Profiles and CMMs often use as an intermediary space when converting colors.
• So a monitor to printer match translates colors from the monitor's space (RGB) into Lab and then into the printer's color space
• The values used by CIE are called L*, a* and b* and the color measurement method is called CIELAB.
• L* represents the difference between light (where L*=100) and dark (where L*=0).
• a* represents the difference between green (-a*) and red (+a*), and
• b* represents the difference between yellow (+b*) and blue (-b*).
• Each colour is then a measure of their a*, b* and L* values, giving a definitive point of reference on the colour space.
• This space gives a precise value with each value having a specific point on the space, creating that particular hue of colour.
• CIE LAB is standard measurement tool to measure the difference (Delta E) between the colours represented (displayed) and the deviation from the true colour location on the CIELAB space.

 

Colour difference - Delta E

• Even if two colors look the same to one person, slight differences may be found when evaluated with a color measurement instrument.
• If the color of a sample does not match the standard, customer satisfaction is compromised and the amount of rework and costs increase.
• Color difference can be defined as the numerical comparison of a sample's color to the standard.
• It indicates the differences in absolute color coordinates and is referred to as Delta (Δ).
• These formulas calculate the difference between two colors to identify inconsistencies and help users control the color of their products more effectively.

• To begin, the sample color and the standard color should be measured and the values for each measurement saved.
• The color differences between the sample and standard are calculated using the resulting colorimetric values.

 

ΔE between 0 and 1	In general, this deviation cannot be perceived.
ΔE between 1 and 2	Very small deviation; only perceivable by an experienced eye.
ΔE between 2 and 3.5	Medium deviation; perceivable even by an unexperienced eye.
ΔE between 3.5 and 5	Large deviation
ΔE exceeding 5	Massive deviation