All about Dot Gain/TVI (Tone Value Increase)

When a tone value is requested, for example, in a page layout application:it becomes represented by a halftone dot pattern generated in prepress by the workflow RIP (Raster Image Processor)
which is then imaged onto a printing plate
which is then inked
and, in the case of offset printing, transferred under pressure to the blanket
from which, again under pressure, the inked dots are transferred to the substrate – paper in this example:
As the halftone dots move through each stage of the process they are altered slightly. "Dot gain" is the term that is used to describe the difference between the requested tone value in the original application file and the resulting apparent final tone value on the substrate as measured with a densitometer. In a film-to-plate workflow, the requested tone value is usually considered to be the tone measured on the film, rather than the tone requested in the application.
Dot gain is inherently neither good nor bad, it is simply a characteristic of a process that uses pressure to transfer an ink to a substrate.
By convention, total apparent dot gain is an incremental, or add-on, increase in apparent dot area – not a multiplier. For example, 18% dot gain means that the tone value, e.g. 50% on the plate has resulted in a 68% tone value in the final presswork (50% + 18%) rather than 59% (50% x 1.18).
Traditionally, dot gain is used as a process control metric, i.e. for a given tone request there is an expected, target, apparent dot gain value – e.g. for a requested 50% tone the expectation is the presswork will gain 18% resulting in the 50% request measuring 68% on the press sheet. If the target dot gain is not achieved then the print process is examined to determine the cause for failure.

There are two primary components of dot gain; mechanical (the physical spreading of the ink under pressure), and optical (the effect of light scatter within the substrate around the perimeter of the dot) - effectively the shadow of the dot within the substrate.The physical and optical dot gain combined are loosely referred to as "dot gain" and, although a reasonably appropriate term, it can be misunderstood and misinterpreted. "Tone value increase," or simply TVI, is a more appropriate term that better describes the over-all effect and is a term gaining in popularity. From a measurement point of view dot gain and TVI are identical.
Because of the optical gain component, it is not possible to directly measure dot gain/TVI. The method most commonly used involves using a densitometer to measure and compare a patch of solid ink (100%) to a specified tone patch (e.g. 50%) with the application of a formula to calculate the total apparent dot gain/TVI.Because of the complexity of dealing with the non-linear mechanical and optical components of dot gain, many formulas have been proposed to calculate total dot gain/TVI, including; the Demichel/Neugebauer equations, the Murray-Davies equation, the Yule-Nielsen equation, the Clapper-Yule equation, the Huntsman model, etc. Currently, the most popular equation, although faulty, none the less is the one that is built into most densitometers - the Murray-Davies equation. Since different formulas give different results, the important thing is to make sure that when discussing dot gain/TVI values, the formula that was used is also communicated.

Historically dot gain/TVI target values for presswork were provided for the 50% tone only. These values, for a 175 lpi halftone screen were: C 20%, M 20%, Y 18%, K 22% at their appropriate solid ink densities. Unfortunately, defining a single tone value as the target for dot gain/TVI does not indicate the actual appearance of the actual presswork.
For example, here is the theoretical print characteristic plotted using only the published 50% dot gain/TVI target values:
However, measuring and plotting dot gain/TVI at several points through the tone scale defines the "print characteristic" and provides a much more effective appearance-based target for the presswork. Here, based on an actual press run using a 175 lpi AM screen, is the print characteristic targeting the same published 50% dot gain/TVI target values:
In order to achieve better alignment between proofs and presswork as well as presswork from different locations, industry standards and specifications are increasingly adopting appearance-based targets (i.e. a specified print characteristic) for press work. The goal is not to achieve a specific dot gain/TVI at a single tone, but to target the tonal print characteristic irrespective of what dot gains/TVI values are needed to achieve it.

Making the print characteristic the target also enables different screening technologies, such as FM/Stochastic screens that have a different inherent print characteristic, e.g.:to make use of dot gain/TVI curves applied in plate making to align their presswork appearance to the industry target/standard/specification.

Dot gain/TVI can provide more than just the print characteristic, it can also reveal issues with ink transfer. In this example, plotting the dot gains through the tone scale:shows that although the 50% gains are within specification and produce correct grey balance, there are issues with ink transfer in other parts of the tone scale, and hence this print condition should not be characterized/profiled until the on-press issues are resolved. One of the characteristics of proper ink transfer in CMYK presswork is when all four dot gain/TVI curves have a similar contour, are clustered together, and are smooth as in this example:
Having a documented standard for dot gain/TVI also helps print production to forensically determine whether a color shift (as in the right side of this image):is caused by incorrect curves applied to plates, too high SIDs, slur, doubling, pressure/squeeze, etc.

Some of the factors that cause dot variations (dot gain/TVI):

Film: mounting, exposure time, vacuum in plate exposure, development time, development chemical condition
CtP: laser exposure integrity, development time, development chemical condition
Printing plate: material wear and tear, dampening solution quantity, pH value, water hardness, temperature, incorrect tone curves
Inking: ink film thickness, consistency, temperature
Printing plate/blanket: packing, type of blanket
Printing substrate: coated, uncoated, surface texture
Presswork: ink transfer, pressure/squeeze, slur, doubling, offsetting, over/under emulsification


Below are the historical dot gain/TVI value (and SID) targets for different classes of presswork. These values are based on densitometers set to Status "T" black backing, measured dry.

Sheetfed offset:
Grade 1 & 2 premium gloss coated @ 175 lpi:
C: 1.40/20%, M: 1.50/20, Y: 1.05/18, K: 1.70/22
Grade 1 & 2 premium matte coated @ 175 lpi:
C: 1.30/22%, M: 1.40/22, Y: 1.00/20, K: 1.60/24

Web offset (SWOP):
Grade 3 & 5 coated @ 133 lpi:
C: 1.40/20%, M: 1.50/20, Y: 1.05/18, K: 1.70/22

Newsprint (SNAP):
Coldset @ 85 lpi:
C: .90/30%, M: .90/30, Y: .85/28, K: 1.05/32
Heatset @ 100 lpi:
C: 1.08/32%, M: 1.15/32, Y: .95/30, K: 1.20/35

Creating Custom Halftone Dots

When we think about halftone dots we're usually thinking in the traditional terms of Round, Elliptical, Square, etc., however, halftone dots don't have to be restricted to such simple shapes. For specialty projects you might consider using a custom halftone dot (click on images to enlarge).

Perhaps a "Star" dot to reflect the iconic status of the subject:
Or you could even use the subject itself as the dot shape:
How to create a custom halftone dot

A halftone screen is built using several components. The two that are needed to create a custom halftone are the "spot function" which defines the shape of the dot and the "threshold array" which determines how each dot is created.
The spot function can be any graphic, including a photographic image. It should be visually simple, made up of 256 levels of grey and fit into a square shape.
For this example we'll use the Apple logo for the spot function - making it our custom halftone dot.To convert it into a threshold array we'll use the blend tool in Adobe Illustrator:On the left is the original logo in Illustrator. Next is the logo at two sizes - the small black apple in front of the larger white logo. The third graphic is a 256 level Illustrator blend of the small black apple and large white one. The graphic is then imported into PhotoShop and cropped to minimize the amount of white in the graphic. This will become our threshold array: The Apple logo threshold array.
To apply the custom halftone dot:

1) In Photoshop, reduce the apple image to make a "dot" the size desired for the final image. For example: an image that is 75 pixels wide would make 8 dots per inch for a 600 pixel wide image. Use "Image"--> Image Size to change the entire image as required.

2) Select the apple image with the Rectangle Selection Tool. Then choose Edit--> Define Pattern. Give it a name (in this case "Apple logo") and press OK.
3) Halftone the original image by choosing Image--> Mode--> Bitmap. For "Method", choose "Custom Pattern" and then choose the "Apple logo" pattern. Then enter an Output Resolution. The amount entered will determine how small the halftone dot will be in the final image. The higher the number, the smaller the dots. Choose a resolution that is a multiple of your target output device's resolution. For example, if your output device has a resolution of 2,400 dpi, choose 2,400, 1200, 600, or 300 dpi for the bitmap.
Click OK.

Voila! Steve Jobs is now rendered with a custom halftone using the Apple logo as the dot shape.
While black and white images are the easiest to do as well as the most effective, it is possible, with a bit more experimentation to do a 4/C image. This one uses the Star dot. Click on image to enlarge:

Because you can't rotate the halftone screens for each of the process colors - the trick is to take each process color channel and rotate it to the correct angle, screen it and then rotate back so that they overlay correctly and recompose the image. Here is the step by step process: Open each channel as a new document. Rotate each channel to the correct angle: C +15º, M+75º, Y 0º, K +45º.
Convert each channel to a bitmap using the pattern/threshold array. Convert each channel back to greyscale. Rotate each one back to its original 0º state, C -15º, M-75º, Y 0º, K -45º. Finally, return each bitmapped channel into a composite CMYK image and align the channels.

The Wayback View – A visit to Sun Chemical Ink - 1951

Preview images from the video
The high technology used in ink manufacturing is revealed in this two minute long film. Of course, black ink only – after all, who needs color in their newspapers? Some flag waving provides a rousing ending.


Please press the play arrow to view the video. Note that it may stop for a moment while the video buffers in the background.

Cross-Train

Often, the only time that employees interact between departments is when there is a problem. This can lead to departments becoming isolated and teams being less effective. One way to help break down the walls is through employee cross-training. This is not to say, for example, that a press operator should become a prepress expert. Rather, the approach is for an individual to first learn the basic job functions of the people who work on either side of them – essentially the supplier and the customer side in the production sequence. Job “function” in this sense means how employees on either side measure, subjectively and objectively, their success (as well as their failure). It also includes understanding the issues and barriers they face in achieving that success. This helps the employee better understand the role they play in the context of other departments, and the part they play in helping the company to achieve its goals.
Cross-training emphasizes the notion that everyone has to be involved in getting the job done right and ultimately making their company successful. Cross-training can include informal technical learning sessions as well as “field trips” to other departments, for example, having a prepress operator spend time on the pressroom floor to better understand press operator issues. Cross-training can also be extended beyond the company walls, for example, having Sales take a press operator on a customer visit to help them connect with and better understand customer issues.

How to reveal DSLR dust bunnies

This is a tip you may want to pass on to your customers to help them provide better originals for you to work with.

Many of the original images for print are now created with digital cameras rather than scans of transparencies. Unfortunately, pro DSLR cameras with their interchangeable lenses are susceptible to "dust bunnies" - particles of dust that settle on the glass plate in front of the sensor and show up as shadows in the final digital image. Often, these shadows are not noticed until the press is running. In this sample, a promotional still shot from the upcoming "Transformers: Revenge of the Fallen" movie, several dust bunnies have made their way into the image. Here is the original:Click on images to enlargeTo reveal the dust bunnies using PhotoShop, create a duplicate layer of the image then go to the "Menu", select "Image"/"Adjustments" and the "Equalize" option.
This exaggerates the visibility of the dust bunnies. Making them more visible makes it easier to clone 'em out of the original image. Click on the image below to see the dust bunnies (where the arrows are pointing) more clearly.The original image was downloaded from HERE

Printshop Evils

Overheard at various printshops:

Halftone screen angles

With the exception of FM (stochastic screening), all screens consist of dots arranged in a regular pattern or matrix. The vertical and horizontal distance between successive dot centers is constant and is a function of the screen frequency. When the screen is aligned parallel to the paper edges, the screen angle is said to be 0° or 90°. The rotation angle away from the vertical axis is known as the screen angle. The screen can only be rotated up to 90° before it repeats itself. For example, a screen rotated 15° is at the same angle as 105°, 195°, and 285°.
A black and white halftone image consists of a single screen. The screen pattern is very noticeable when positioned at 0° and is least visible when rotated 45° as illustrated below.For that reason, black and white halftones are usually printed with 45° angled screens – particularly with coarser screens.

When two (or more) screens are printed on top of each another, a visually objectionable pattern known as moiré may occur. The most serious moiré patterns occur at very small angles between screens. Below are two overlaid halftone grids angled at 5 degrees and 10 degrees apart with the resulting moiré pattern:
The best angle between two screens that is least likely to cause moiré, and is most forgiving to small degrees of error, is 45°. However, in four color process printing, four different screens must be superimposed and all four screens must be angled within the 90° limitation.

A set of standard screen angles has been established that is based on a combination of theory and experience. First the least visible color, yellow, is placed at the most visible angle 0° (90°). Then the most visible color, black, is placed at 45°. The cyan and magenta are then placed between these two. Cyan at 15° (105°) and magenta at 75°. These angles represent a best all around compromise for most pictures and represent the standard, most commonly used screen angles. They also form the least objectionable moiré – the rosette pattern (more on rosettes here).
Because the Yellow printer is only 15° from the Cyan printer it produces moiré. The visibility of the moiré can be exacerbated if the Yellow becomes contaminated by ink traveling into it from previous press units. To help reduce the visibility of the Y/C moiré, most screening systems run the Yellow at a slightly higher frequency (lpi) – typically 108% of the frequency of the C, M, and K printers.Left: Yellow at the same frequency as Cyan. Right: Yellow at a higher frequency to help reduce visible moiré.

These standard screen angles are based on analog photomechanical screens and do not work best with electronic screens. At angles other than 0° and 45° a type of moiré patterning within one screen "single channel moiré" may occur. To avoid this problem, some vendors utilize shifted angles of 7.5° to introduce "noise" around the edges of the dots in order to break up and eliminate the visibility of single channel moiré.

Most printers will have a standard screen angle set that is used for all their jobs. However, if certain jobs have images where two of the process colors predominate and where those two colors are less than 30 degrees apart, then that screen set should be avoided and a different one used instead.

The following screen angle sets are all valid and are in common use. The sequence for the screen sets listed below is C, M, Y, K (i.e. the first screen set on the list is: 15C, 45M, 0Y, 75K). Remember that screen angles have quadratic symmetry so 0 degrees is the same as 90, 180, and 270 degrees.

Standard 4/C U.S. screen angle set:
15, 75, 0, 45 (possible moiré in greens since C and Y are only 15º apart)

Standard 4/C European screen angle set:
15, 45, 0, 75 (possible moiré in greens since C and Y are only 15º apart)

Other usable screen angle sets: Keep in mind that when two colors are less than 30º apart there is a risk of moiré
15, 45, 0, 75
15, 75, 0, 45
15, 45, 30, 45
45, 15, 0, 75
45, 75, 0, 15
75, 15, 0, 45
75, 45, 0, 15
75, 15, 60, 45

For 2/C jobs (e.g. duotones): Other angles can be used, however, the guiding principle is that the angles should be 30º or 45º apart and that the darkest color should be at 45º to reduce its visibility and lessen "sawtoothing" (see below)
Dark color at 45
Light color at 75

For 3/C jobs (e.g. tritones):
Darkest color at 45
Medium color at 75
Lightest color at 15

For 5, 6, or 7/C jobs (e.g. Hi-Fi color):
Use the angle of the unused color
Violet/Blue uses Yellow or Black angle
Green uses Magenta angle
Red/Orange uses the Cyan angle
Note that, depending on the original CMYK separation, the Black screen angle may be available to be used for one of the extra colors - V/B, G, or R/O.

Dealing with the Yellow printer moiré issue
Interscreen moiré becomes more visible when the angles of any two screens are less than 30 degrees apart. Yellow is usually allowed to be less than 30 degrees because it is such a light color that the moiré is usually not visible. Also, the frequency of the yellow printer is usually made higher than the other three colors (typically around 108% higher) to further minimize the visibility of the moiré. However, the moiré can become more visible if the yellow printer becomes contaminated/dirtied by the preceding process colors, or if its density is too high.

So, when skin color predominates:
15, 45, 0, 75 (avoids M/Y conflict/moiré but introduces C/Y conflict)

Or when light greens predominate:
45, 75, 0, 15 (avoids C/Y conflict/moiré but introduces M/K conflict)

Some printers use a coarse FM screen instead of a conventional AM screen for the yellow printer.This eliminates the moiré issue completely since FM screens do not have a fixed frequency or angle. For a 175-200 lpi AM screen an FM screen of about 35 microns would be used since that dot size will have a dot gain similar to the AM screened colors.

Other screen angle considerations
In certain circumstances, depending on the size of the graphic and the frequency of the halftone, the selected screen angle can distort the accurate rendering of images.

In the below graphic, the halftone screen angle is the same (45º) but the angle of the gray lines have been changed.Note how the screen has affected the rendering of the gray lines at different angles. The artifact at 1, 2, 3, and 4 is referred to as "ribboning" and is fairly common in automobile images.

In the below graphic, the halftone screen angles have been changed to the standard 4/C process angles (K 45º, C 15º, M 75º, Y 0º) but the angle of the three gray lines have been kept the same (0º).Ribboning has appeared in the Cyan and Magenta angles while the Black and Yellow angles have caused the appearance of different dotted line effects.

In the below graphic, the halftone screen angle is the same (45º) but the angle of the gray box has been changed in 10º increments.Note how the smoothness of the edges of the box changes as its angle relative to the halftone screen angle changes. The ragged appearance of edge of the last box is referred to as "sawtoothing."

Screen angles for more than four color - i.e. "Hi-Fi" printing (5, 6, or 7 colors)

Four color CMYK process printing is a good compromise that achieves a wide enough color gamut for most applications while using the minimum number of inks to achieve it. However, sometimes the printer needs to go beyond 4/C in order to achieve a satisfactory rendition of the image. Typically the gamut deficiency will be in the overprint colors - Red/Orange, Blue/Violet, Green.

Here is an original RGB image:And here is the CMYK version of it:To restore some of the original color impact, the printer may choose to use "bump" or "touch" plates to boost color back into areas where it was lost. However, adding extra colors causes problems since all possible screen angles have already been used by the C, M, Y, and K printers.
In this example, these are the four channels that make up the image:Note that there is virtually no Cyan in the Red/Orange areas, or Yellow in the Blue/Violet areas, or Magenta in the Green areas. Therefore, those screen angles become available for the extra bump inks. So the trick is to use the screen angles of these unused colors.
In this example Violet, Green, and Red:In short, the Violet ink would take the unused Yellow angle, the Green ink would take the unused Magenta angle, and the Red ink would take the unused Cyan angle. Note also that, depending on the original CMYK separation, the Black screen angle may be available to be used for one of the extra colors - V, G, or R.