Formation of print
Contents
• Formation of print
– Colour formation
– Halftone formation
– Screening and detail rendering
Picture consist of millions of different
colours and tones
In printing colours
are created by
halftones (dots)
In photography
colours are created
by continuous tone
All colours can be created using three
process colours (in theory)
=
CMY Yellow Magenta Cyan
Use of black ink
• Black is added to increase contrast
+ =
CMY Black CMYK
Use of black ink
• Example 2
+ =
CMY Black CMYK
Print density
• Print density (darkness) is created by
increasing the inked area (halftone
percentage)
Black 20% Black 40% Black 60% Black 80%
Print colour
• Print colour (tone) is created by mixing halftone percentages
of process colours
M80%/C20% M60%/C40% M40%/C60% M20%/C80%
Conventional and stochastic screening
Conventional screening
– tone is formed by altering
dot size
– distance of dots constant
– dot size depends on
screen ruling
Stochastic screening
– tone is formed by altering
dot position and spacing,
number of dots
– no screen angles (in
theory)
Examples of screening methods
Staccato 25µm Conventional 60 l/cm
Examples of halftone dots and
text
Flexo
(20%, 60 l/cm)
• Ink is pressed to
edges, print density
inside dot varies
• Edges are smooth
• Shadow in the edge
of text
Gravure
(~20%, 100 l/cm)
• Broken shape of dots and
missing dots are typical for
light tones
• Doughnut shaped dots
• Text is screened, serrated
edge
Offset
(20%, 75 l/cm)
• Dots are uniformly
covered with ink
• Edges are ragged
(uncoated papers)
• Uniform text
Examples of halftone dots and text
Inkjet
(20%)
• Dots are quite uniformly
covered with ink
• Dot is formed by using
several droplets
• Edges are ragged or
even depending on paper
• Satellites, escaped
droplets
Electrophotography
(~20%)
• Dots are quite uniformly
covered with toner
• Edges are ragged
• Separate toner particles
(dry toner methods)
• Uniform text
Screen angles
•Screen angle usually
differs from 0° and 90°
– horizontal and vertical
lines are most visible to
eye
– yellow is usually at 90°
because it is so weak
colour
•Correct screen angles
minimize moiré-effect
45°
15°
75°
90°
Moiré and Rosette patterns
• Moiré and Rosette are both caused by regular halftone
structure
Moiré and Rosette patterns
• Moiré and Rosette are both caused by regular halftone
structure
Moiré
Moiré and Rosette patterns
• Moiré and Rosette are both caused by regular halftone
structure
Moiré and Rosette patterns
• Moiré and Rosette are both caused by regular halftone
structure Moiré is minimized
=> Rosette pattern
Screen ruling vs. detail rendering
• Higher ruling is needed for finer details
60 l/cm 70 l/cm 80 l/cm
Screen ruling
• Typical screen rulings
– newspaper printing 30-48 l/cm
– SC-paper in heatset 48-54 l/cm
– coated paper in heatset 54-60 l/cm
– fine paper in heatset 60-70 l/cm
– fine paper and board in sheet-fed 60-80 l/cm
– gravure printing 60-100 l/cm
40 l/cm 60 l/cm 80 l/cm
Examples
Coated Fine,
70l/cm SC,
54l/cm
LWC,
60l/cm
Newsprint,
48l/cm
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