Halftone
and CMYK.
Offset prints ink or nothing. There is no light ink, no dark ink and no grey ink. Tones are created by halftone screening: dots that grow in the dark areas and shrink in the light ones. The four CMYK inks are each printed at a different angle and mix optically at reading distance.
Tone = dot size
A 10 % area has dots covering 10 % of the surface; a 90 % area, 90 %. The ink is always the same. At reading distance the eye cannot resolve the dots and sees a continuous tone. Under a loupe, the dots come back.
From 10 % to 90 %: the same ink, a larger dot.
The basic quantities
Tone value
The share of the surface covered by ink. 100 % is solid, 0 % is bare paper.
Screen ruling
How many rows of dots per inch. Typically 150 – 175 lpi on coated paper and 133 – 150 lpi on uncoated, where the dot spreads more.
Screen angle
Each ink is printed at its own angle, so the dots do not fall on top of one another.
Stochastic screening
Very small dots of equal size in random positions. No rosette, no moiré, finer detail. It needs a tightly controlled process.
Four inks, four angles
In conventional (AM) screening each ink is set at its own angle — cyan, magenta and black 30° apart and yellow 15° from its neighbours: typically cyan 15°, magenta 75°, black 45° and yellow at 0°, the lightest ink and the least visible. The dots then form the characteristic rosette instead of clashing. When the angles are not kept, or when printed material is scanned and reprinted, moiré appears: a wavy pattern that does not exist in the file.
The typical angles of the four inks.
Dot gain: the dot spreads
On paper, a dot always prints slightly larger than on the plate, because the ink spreads and is absorbed. This is dot gain (tone value increase in ISO 12647-2). A 50 % dot in the file prints at roughly 65 % on coated paper and more on uncoated. It is not a defect but a physical property of the process, and it is compensated in prepress with the profile and curves for each paper, so that the result matches the proof.
What it means for your artwork
Text in solid black. Small type made of four inks, or of a black tint, is built from dots and looks soft. Use 100 % K.
Very light and very dark tints have limits. Below 3 – 5 % the dot may not hold on the plate; above 95 % it closes up to solid.
Gradients need range. A blend from 10 % to 20 % across a large area will show banding. Give it a wider tonal range or a shorter length.
Dark backgrounds benefit from rich black. Black with other inks added gives a deeper result than K alone. The percentages depend on the paper; ask us before you send.
Critical colours need a proof. The screen shows RGB, paper shows CMYK. See CMYK or Pantone and soft proof and contract proof.
Frequently asked questions
What is moiré?
A wavy or checked pattern that appears when two screens coincide at the wrong angle, for example when an already printed piece is scanned and printed again. The remedy: original images, not scans of print.
Why is grey text not crisp?
Because grey is not an ink: it is black dots with white gaps. On small type the dots become visible along the edges. For crisp text use 100 % black or a larger size.
What is the difference between lpi, dpi and ppi?
ppi is the resolution of a digital image (pixels per inch), dpi the resolution of an output device (dots per inch), lpi the screen ruling (rows of halftone dots per inch). Rule of thumb: image resolution about twice the screen ruling, i.e. 300 ppi for a 150 lpi screen.
Send us the specifications of your job.
Size, quantity, material and number of colours. We reply with a technical proposal and a quote.