1.
Raster Image Processing
(raster Graphics)
(2)
Dr. Ahmed Attia
2019
Technical Concepts For
Graphic Design Production

2.
Screen Ruling Theory

3.
 As you can see in the image before, a photograph can create a
smooth gradation of values from black to white and all shades of
gray in between and that is called a continuous tone image .
 This is not the case, however, with most printing methods,
including offset lithography and desktop digital printing.
 These technologies can only print areas of solid ink. The ink is
never diluted, nor is white ink added to the mix to make shades
of gray.
 The only way to reproduce shades of gray in print is to break the
image up into tiny dots that appear to blend into a continuous
tone when viewed with the naked eye.
What is the halftone?.
(a historical introduction)

4.
Left – a smooth gradation from black to white with essentially
infinite shades of intermediate gray. Such a gradation cannot be
reproduced using typical printing methods. Instead, the image is
converted to a halftone, a pattern of tiny dots that vary in size.
Light gray areas consist of very small dots, allowing the white of the
paper to show through. Dark areas have larger dots, covering most
of the paper. Normally the dots are too small to be seen with the
naked eye. Right – a printed halftone with an enlarged area
showing the dots.

5.
 Such an image, composed of a pattern of tiny dots, is called a halftone.
 The dots themselves are known as halftone dots.
 The darker portions of the image have larger spots with less space
between them; the lighter areas have smaller spots with more space to
reveal the paper underneath.
 At the right viewing distance, our brains then merge all the spots together
to give us the impression that what we’re seeing is one smooth image.
 Halftone process, in printing, a technique of breaking up an image into a
series of dots so as to reproduce the full tone range of a photograph or
tone art work.
 Halftoning is the process used to convert a continuous tone photograph
into a pattern of tiny dots which may be easily printed on a lithographic
press.
What is the halftone?.
(a historical introduction)

6.
 The process begins with a negative film of the original image.
 Light passes through the negative and then through a screen,
usually a plate of glass with a grid of horizontal and vertical lines
etched onto its surface.
 After passing through the screen, the light exposes another piece
of film.
 The screen functions as a diffraction grating, breaking the light into
tiny discreet rays, which create the pattern of dots.
 The result is a duplicate film negative with a pattern of solid dots
instead of continuous shades of gray.
 The duplicate negative is then used to create a plate for the offset
printing process.
Producing halftone
by analogue method

7.
Producing halftone by analogue method

8.
Producing halftone by analogue method
called Photomechanical

9.
Converting a continuous tone image to
halftone image

10.
Converting a continuous tone image to
halftone image

11.
Converting a continuous tone image to
halftone image

12.
Converting a continuous tone image to
halftone image

13.
Converting a continuous tone image to
halftone image

14.
An analogue dots which produced by screen

15.
Screen Ruling theory
 The halftone process introduces another bit of printing
terminology that often gets confused with the others.
 If you look at Figure 1, you’ll see that halftone dots are
arranged in orderly rows or lines, usually oriented at an angle
to the paper.
 In the conventional halftone process, the spacing of these
lines of dots remains constant throughout the image; only the
size of the dots varies to create different shades of gray.
 The spacing of lines of halftone dots is known as the screen
frequency or line screen or screen ruling and is expressed in
lines-per-inch (LPI), i.e., the number of lines (rows) of dots in
an inch.

16.
Screen Ruling definition
 the total number of lines or dots per inch.
 Screen ruling is one of the main printing
characteristics (by analogy with mosaics): the
higher the screen ruling is, the smaller are the
“pixels” of mosaics, and, accordingly, the more
details of the original image can be reproduced (is
measured in LPI – lines per inch).

17.
Screen Ruling Concepts
Higher screen ruling equal more details and vice versa

18.
Different values for screen ruling the lowest
quality and details by 25 lpi – the highest
quality and details by 85 lpi

19.
Different values for screen ruling

20.
Screen Ruling is 65 LPI

21.
Screen Ruling is 133 LPI

22.
Screen Ruling is 175 LPI

23.
Dot Gain
(Increasing the dot size)

24.
Example for dot gain

25.
DOT GAIN – Dot increese

26.
Before dot gain - 1

27.
After dot gain - 2

28.
The relation between screen
ruling and the kind of paper

29.
The relation between screen ruling
and the kind of paper
 Smoothness and porosity of the paper surface affect the
printing ink performance. The ink is wetting the surface,
spreading and setting in different ways. The rougher and
more (macro) porous the paper surface, the more the
printed ink spreads and penetrates into the paper.
 Similarly, the heavier the paper coating is, the sharper the
image dot.
 This has an influence in attaining an achievable print density
and dot gain during printing. Dot gain is the average
spreading of a single color dot on the paper.

30.
The more even and dense the surface, the better
the dot coherence.
Coated Paper
Uncoated Paper
News Paper

31.
Compensation of dot gain
 The quality potential of different papers should be taken into
consideration and compensated for during pre-press.
 Dot gain can cause blurring and loss of printable tones due to
excessive ink coverage.
 In this case, the screen ruling should be lowered to achieve
discrete dots and a wider tonal range.
Finally, The choice of screen Ruling depends
almost entirely on the type of paper being
printed on

32.
The relation between the grade
paper and screen ruling -1
Lowering the screen ruling helps
separate individual dots.

33.
The relation between the grade
paper and screen ruling - 2
 Printing conditions have some effect on screen rulings, but the
main factor is the paper grade and its impact upon dot gain.
 Dot gain, screen rulings and the target print density have an
overall impact upon print quality.
LPI
LPCM
85
34
100
40
120
48
133
54 68
170150
60
200
80
Newsprint
Uncoated
Coated matt
Coated silk
Coated gloss
254
100
65
26

34.
Examples of Paper’s grades
and their line screens – more specific.
WFC = woodfree coated
WFU = woodfree uncoated
MWC = medium weight coated
LW C = light weight coated
MFC = machine finished coated
SC = super calendered
MFS = machine finished special
News
News
MFS
SC
MFC
LWC
MWC
WFU
WFC
Relativeprice
Relative quality
M
echanicaluncoated
M
echanicalcoated
W
oodfree

35.
News paper: 65 -
100 lpi
White Paper: 133 -
150 lpi
Coated Paper: 150
– 175 -200 lpi
Examples of Paper’s grades
and their line screens in the market

36.
The Relation Between The
Paper grade & Screen Ruling &
The Resolution

37.
We choose the kind of paper
for our printing product.
Then, We choose the right
screen ruling.
Finally, We choose the
raster’s graphic resolution.
Introduction

38.
The relation between Screen Ruling
and Raster graphic resolution
1.5
to 2
Quality Factor

39.
The Resolution For Newspaper Paper
65 2 130
100 2 200

40.
The Resolution For White Paper
133 2 266

41.
The Resolution For Coated Paper
175 2 350
150 2 300

42.
 Ahmed Attia - Doctor – Training course for color separation
– 2017.
 Dusan C. Stulik | Art Kaplan – HALFTONE – J.Paul Getty TRUST
– 2013.
 Haralod Johnson – C. Divid Tobie – Mastering Digital Printing
– Second Edition – THOMSON Course technology – 2005.
 Gavin Ambrose – Paul Harris – The Fundamental of Graphic
Design – AVA Publishing SA – First edition – 2009.
 Kalliopi Monoyios - Dots, Spots, and Pixels: What s In A
Name?
https://blogs.scientificamerican.com/symbiartic/dots-spots-
and-pixels-whats-in-a-name/. May 14, 2012
References