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