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Screening is the technique that is used in printing to simulate tinits or continuous-tone images such as photographs using dots. Allmost all printing technologies such as offset, gravure or inkjet printing simulate shades of colors using dots.
To illustrate the principle, I have created the grayscale image below. It consists of 256 different tints of gray.
Suppose you want to print the above image. With most of the printing technologies that exist nowadays, you either have ink on the paper or you don’t. To simulate the different tints of gray, there are two possibilities:
- Print the image using dozens of different inks, each a bit darker than the previous one. This would be extremely time consuming and expensive.
- Simulate the gray tints by mixing white areas with printed areas. If the black and white areas are small enough, the human eye cannot distinguish between them and perceive a tint of gray. By changing the ratio of white to black areas, various tints of gray can be simulated.
It is obviously the second technique that is used nowadays. Let’s look in more detail at different algorithms that are used.
If the above drawing with all its tones has to be reproduced using only black dots, a techni
Dispersed dot dithering
to be added
dot diffusion dither
to be added
Clustered dot dithering
The above techniques are well suited for relatively low resolution devices like monitors, inkjet printers or 300-dpi laserprinters. High resolution output devices like imagesetters are easily capable of outputting 2400 dots per inch. Such dots are too small to be copied to printing plates and to be printed on presses. Instead of outputting lots of individual dots, they are grouped (clustered) together to form larger halftone dots. This technique is used throughout the printing industry.
You may have noticed that these dots are much coarser than the two examples of dispersed dot dithering. This is only the case on the monitor because it is a low resolution device. In reality, the clustered dots are small enough to remain invisible to the human eye.
And what about color?
The above examples show you how different levels of gray can be reproduced using dot dithering. The same techniques can also be used for colour printing. Full colour printing can be done by mixing dots that are made up of 3 colours: cyan ( a blue-ish colour, often abbreviated to ‘C’), magenta (reddisch, M) and yellow (Y). These inks are transparent so mixing them creates new colours. This drawing shows which colours you get when you mix them. Unfortunately inks contain impurities and equal portions of all 3 do not create a nice neutral gray but you get a brownish gray. Therefore, a fourth colour is added: black. This has 3 main advantages: – it improves the gray balance – It reduces cost since black ink is cheaper – Registration problems are avoided. Printing text in black is much easier than printing the same text in cyan, magenta and yellow exactly on top of each other. The example below show a full colour image that has been screened using these CMYK colours.
The master angle
Below are 3 rectangles with a 20 percent raster in them. The angle of the raster is set to 0, 22 and 45 degrees respectively. Look at the rectangles from a certain distance or close your eyes a bit. Try to decide which of the 3 angles is the least obtrusive.
The human eye recognizes patterns based upon repetition, orientation and accentuation. The 45 degrees angle could be referred to as the ‘master angle’ since it presents the flattest appearance to our eye – regardless of tile size – at both the centre and edge of the rectangle. The 45 degree angle is exactly halfway between the vertical and horizontal planes. As such, the eye “gives up” on trying to perceive it in these most fundamental alignments.
At the edge of a square cut image (which is still the most common case in graphic arts) the 45° angle trims perfectly in line with the cut and offers no odd edge effects. You can see this in the above images as well. The 0 degree angle leaves a small white line at the bottom. The 22 degree raster leaves some holes near the edges where there are no raster data. The 45 degree raster has none of these effects.
Printing more than 1 color
Even if a screen angle of 45° is ideal in most cases, this does not mean you can print color job using the same screen angle for all colors. Below is a 4 color job that contains just cyan and magenta to keep things simple. Both colours use the same screen angle and are printed exactly on top of each other.
In a perfect world, this may work once but the next day or on the next page, the same tint may get printed with magenta slightly offset. This leads to a completely different colour as you can see below.
Colour shifts are less likely to happen if each colour is printed using a different screen angle. This can be seen below where I put cyan at 45° and magenta at 30°. In the second picture, magenta was shifted. Look at both rectangles from a distance. The overall colour shifts far less than on the previous example.
Colour shift is not the only reason why different colours are not printed at the same screen angle.
Suppose we take the top example and then show what happens if one of the colours is shifted a bit and at the same time changes size a bit as well (which may happen because the paper absorbs ink and stretches a bit.
There is not just a shift of colour but a new thing is happening as well: both screens interact with each other and a new pattern emerges which shows itself as a kind of blocky street pattern. Such an interference is called moiré (pronounce moray).
Books have been written on moiré and the mathematics behind it but I am not going to bore you with that stuff. Studies have shown that moiré can be avoided or minimized when all colours offset 30° from each other. For this reason, the most standard angle combinations are 15°, 45° and 75° — 15° may also be expressed as 105°.
So, we place our most dominant colours at these three good angles and set the weakest plate (in terms of perceptual definition), Yellow, at a 15° offset from one of the main angles – often at 0°.
Note: Angle sets essentially repeat every 90°. The 0° angle is very easy for the eye to see, even in a fairly tight screen ruling. This makes it easy to judge which is the Yellow plate.
Deciding which colour to place at the 45° master angle may be determined by personal preference, or by the colour composition of the images at hand. Many operators will place the Magenta at 45° for scanned separations to reduce the potential for moiré to occur in skin tones. In grayscale or duotone imaging, where Black is used, the Black will almost always be set to the 45° angle. The edge effects caused by angles that are not equal to either 45° or 0° is minimized when scans are keylined, as this will mask the problem.
Specialty applications, like fashion printing and flexography, may require special angles geared toward enhancing the print result.
In fashion printing, screen rotations may be customized on an image-by-image basis to optimize the reproduction of each scan.
In flexography, where anilox rollers are used, screens will often be rotated (usually by 7.5°) to avoid clashing with the pattern of the anilox rollers; which are usually cut at 45°. Rotations may vary depending upon the fineness of the anilox rollers used.
In screen printing on cloth, or other pre-patterned materials, rotations are often used to minimize moiré effects resulting from the coincidence of screening angles and fabric patterns. Again, 7.5° is a popular offset for these applications.
In high GCR separations for offset printing, where black becomes very dominant, the black will mostly be placed at the 45° mark instead of Magenta. Combinations that I like… Low-Med GCR: C>15° K>75° M>45° Y>0° High GCR: C>15° K>45° M>75° Y>0° Fashion: C>0° K>60° M>30° Y>75° Flexo/Screen using HT K: C>22.5° K>82.5° M>52.5° Y>7.5° *Halftone Black may be pushed harder if there’s Black linework. Flexo/Screen using Line-K: C>22.5° K>52.5° M>82.5° Y>7.5° *CMYK plus Line-Black allows you better image control and higher K. Of course, you should exhaustively test new screens before using them in “live” production. We haven’t covered other specialty screening techniques, tightening screens on Yellow (common in web offset work, etc.) or the possible use of FM screening to resolve these issues entirely. Also, some manufacturers spend significant amounts of time in generating high quality screens for a number of press conditions. Even supplied screen sets should be tested for fitness – just because they appear in a PPD doesn’t guarantee great results under real-world conditions.