Visual control strip for imageable media

A visual control strip for imageable media such as a printing plate, a photographic film or a printable substrate is described wherein the visual control strip is to be applied to the imageable medium. The control strip includes a plurality of first control fields having differing grey tone values which are relatively insensitive to the processing variables which influence the size of the spots or dots produced on the imaged medium. The control strip also includes at least one second control field which is relatively sensitive to these process variables. The second control field is located immediately adjacent to the first insensitive control fields for ease of comparison. The second sensitive control field has a target grey tone value which may typically be 25%, 50% or 75%. To improve assessment of exposure parameters, the insensitive fields are completely surrounded by a sensitive field or vice versa. Methods of using the strip are also described for both traditional or photomechanical and digital reproduction methods.

FIELD OF THE INVENTION 
The present invention relates to devices and methods for imaging substrates 
as well as the substrates themselves. In particular the present invention 
relates to methods and devices for exposure and development of imageable 
media including photographic film and printing plates. The present 
invention also includes devices and methods for printing and the printed 
substrate. More specifically the present invention relates to a visual 
control strip suitable for quality control of imaging devices and imaged 
media. 
BACKGROUND OF THE INVENTION 
Several types of control strips are known. As described in U.S. Pat. No. 
4,852,485, analogue control strips have been used, which consist of a 
piece of patterned film which could be attached onto a lithographic film 
before contacting it to a printing plate. 
CH-A-0 681 929 describes a test "wedge" or control strip which is stored as 
a digital quantity on a storage medium such as a "floppy disk" or in a 
computer and is incorporated herein by reference. The control strip 
consists of a variety of control fields. Each control field contains a 
pattern, e.g. a star target, types, a line series, which may contain 
elements, e.g. checkerboard squares, lines or dots. 
EP-A-0 518 559 describes a method and apparatus for creating a control 
strip. A digital representation of the control strip is printed to form a 
visible, analogue representation of the control strip. The control strip 
may be printed at the same time as a main coloured image to be reproduced. 
The control strip consists of control fields and the elements of the 
control fields can be user defined. 
Postscript.TM. is a programming language created by Adobe Systems Inc., 
California, USA for defining page, lettering, colour and graphics 
parameters of images to be output by a raster imaging device such as a 
printer, an imagesetter or a platesetter. PostScript.TM. is described in 
the "PostScript Language Reference Manual", second edition, 
Addison-Wesley, 1990 (hereinafter referred to as "AdobeRef") and 
incorporated herein by reference. PostScript.TM. files may be incorporated 
in the file for a main image as an encapsulated PostScript file (EPS 
file), as described in Appendix H, pages 709-736 in AdobeRef. 
The general techniques of colour reproduction, e.g. printing, photographic 
films, display devices, are described in "The Reproduction of Colour in 
Photography, Printing and Television", by Dr. W. G. Hunt, Fountain Press, 
UK, 1987 (referred to in the following as "KodakRef"). The Kodak 
"Three-Aim-Point" method is known from this book for producing printing 
plates that will yield results having consistent tone reproduction and 
colour balance despite Image Spot Size Deviation Variables (ISSDV) 
inherent in any system of image production or reproduction. One such 
variable or parameter is the development time for photographic film. 
So-called "hard dot" photographic films, such as Kodak Imageset 2000 
IHN.TM. and imageset 2000 ILD.TM., are very sensitive to the development 
time period. If this period is short, then small spots are formed. If this 
period is longer, then larger spots or spots having a bigger area are 
formed. In several systems, such as in direct thermal systems, development 
is not required. In direct thermal systems, the size or the area of each 
spot may be influenced by the amount of energy applied to a specific 
microdot. Thus in such a system, one ISSDV is the amount of thermal energy 
locally applied. 
Standard "originals" are provided in the form of three neutral density 
(grey) patches : original "A" represents a minimum reproducible density in 
an average transparency or reflection print, patch "B" is a similar 
maximum density patch, and patch "M" is a similar medium density patch. 
When the photographic material is processed, the patches are located next 
to the useful image and processed with it. As a result of experience, 
standard values and tolerances have been determined for the densities 
produced by the patches A, B and M on typical masks and separation 
negatives. 
DE-A-19 507 665 discloses a control strip for visual control consisting of 
two adjacent longitudinal fields. The first field has large elements, the 
size of which is substantially independent from illumination variations. 
The target density of the large elements is position dependent. The second 
field has fine elements, having substantially the same tone value. The 
effective tone value depends strongly on the illumination conditions. 
Although it is possible to assess important illumination variations by 
this method, it is--due to the arrangement of the different fields--rather 
difficult to assess accurately small variations in process parameters and 
to get a quantitative measure for curing the effect. 
OBJECTS OF THE INVENTION 
It is an object of the present invention to provide a method and apparatus 
to control marking of an imageable medium and/or to control the imaging 
device to produce images of the correct density. 
It is a further object of the present invention to simplify calibration or 
routine control of the image quality of imaging devices and imaged media. 
In particular, it is an object of the present invention to provide a means 
to assess and control the exposure of imaging devices and imaged media. 
It is still a further object of the present invention to reduce waste of 
imaged materials in the set-up/proofing stage of the reproduction of 
colour and black and white images. 
It is yet another object of the present invention to improve the 
reliability of calibration or routine control of the image quality of 
imaging devices and imaged media and to make this control less operator 
sensitive. 
It is a further object of the present invention to optimise the size of a 
visual control strip while maintaining functionality as a quality control 
instrument. 
SUMMARY OF THE INVENTION 
The above objects are realised by the specific features according to claim 
1. Specific features of preferred embodiments are set out in the dependent 
claims. 
The visual control strip according to the present invention is preferably 
described as a set of PostScript.TM. commands but the invention is not 
limited thereto. This gives the freedom to incorporate the strip in many 
printing applications such as computer-to-plate and computer-to-film 
applications. This strip may even be used in the last stage of making a 
printing plate or film. 
In a preferred embodiment the visual control strip in accordance with the 
present invention is placed at a location on the imageable medium which is 
imagewise functionally irrelevant. For instance, it may be placed on a 
printing plate at a location which remains ink-free during the subsequent 
printing process. 
The present invention also includes a set of visual control strips wherein 
the target values of the ISSDV (Image Spot Size Deviation Variables) 
sensitive control fields of the respective strips differ from each other. 
Advantageously, three strips may be used with target values for the 
sensitive fields of 25%, 50% and 75%, respectively. 
The present invention also includes an imaged medium and a method of 
assessing imaging quality. 
Further objects, advantages and embodiments of the present invention will 
become apparent from the following description and drawings.

DEFINITIONS 
"Useful image" in accordance with the present invention refers to the image 
which is to be recorded on an imageable medium excluding any other images 
which may be used for control of the imaging process. 
A "microdot" in accordance with the present invention describes the 
smallest addressable spatial unit, i.e. portion, of a substrate or medium 
which can be addressed by an imaging device in order to cause a density 
change in that unit. A microdot may have any suitable shape such as 
square, round elliptical etc. 
In this description, often "density" is referred to. In the context of 
material for visual control, "optical density" is meant. This may be 
optical density for transmission or reflection of light. The light may be 
white light or monochrome or monochromatic light. The "light" may also be 
UV-light or infrared light. Alternatively, by "density" may be meant 
"lithographic effective" or "ineffective". By this is meant ink accepting 
or ink repellant. 
A "spot" in accordance with the present invention is the smallest actual 
image which can be produced on an imageable medium by an imaging device. A 
spot is the result of "rendering" or "marking" a microdot on the medium. 
In a photographic system, a spot is marked by illuminating a portion of 
the photographic substrate by a suitable amount of light. A spot produced 
on a photographic substrate is a developed spot. Depending on the type of 
the photographic material, a black or white spot is formed, or, more 
generally, a spot having a high or low optical density (for reflection or 
for transmission of light) is formed and/or an ink accepting or an ink 
repellant spot is formed. The spot produced on a printed substrate is a 
printed spot. A spot may have any suitable shape such as square, round or 
elliptical. Usually the centre of a spot has the same location as the 
centre of a microdot. Each microdot may contain usually one spot. A spot 
may be larger than its corresponding microdot, i.e. fully cover it, along 
with portions of neighbouring microdots. A spot may be smaller than its 
corresponding microdot, i.e. cover only a portion of the microdot. A spot 
may also partly overlap the corresponding microdot and partly overlap 
neighbouring microdots. This is for example the case if the microdots are 
square and the spots are circular, where the area of the spot equals the 
area of the microdot and the centre of the spot coincides with the centre 
of the microdot. A microdot may be left "empty", i.e. no spot is placed in 
the microdot. Whereas the microdot is a fictive area, a spot has a lower 
density, with respect to its neighbourhood. Alternatively, a spot may have 
a high density, with respect to its neighbourhood. On a lithographic 
printing plate precursor, a spot may give a lithographic effective change. 
Alternatively, a spot may give a lithographic ineffective change. On a 
lithographic printing plate, a spot may ink accetable. Alternatively, a 
spot is ink repellant. A spot may also have a specific "UV-density" with 
respect to its neighbourhood. In fact, in contact systems, using UV light 
sources, it is the "UV-density"--and not the traditional "visual 
density"--of the spots on the contact film which is important. In a 
positive working system, a "light spot" is formed when the corresponding 
location has been irradiated. In a negative working system, a "black spot" 
is formed when the corresponding location has been irradiated. Light and 
dark may be replaced by "lithographic effective" or "lithographic 
ineffective" respectively. 
A "dot" in accordance with the present invention is just one spot or a 
cluster of spots on the imaged medium. A particular type of dot is the 
"halftone" dot which is a dot including a variable number of spots. The 
variation of size of the halftone dots is used to reproduce "halftones", 
i.e. intermediate grey tones between white and black. 
A halftone dot is formed by just one or a cluster of spots, or by a cluster 
of absent spots. If the spots are black and the density of the area is 
low, black spots are clustered in small halftone dots or sparsely 
distributed halftone dots on a "white" background. For black spots and an 
area having a high density, the black background is formed by black spots. 
The halftone dots are the small or sparsely distributed white areas. In 
that case, the halftone dots are formed by clusters of "non-spots". 
The size of the halftone dots and/or the distance between neighbouring 
halftone dots depends upon the grey tone value for the area in which these 
halftone dots appear. For dot-size modulation screening techniques using a 
periodic grid of halftone dots, the size of the halftone dots depends on 
the required grey tone value. For stochastic screening or 
frequency-modulated halftoning, wherein the distance between halftone dots 
is varied, rather than their size, the distance between halftone dots 
depends on the required grey tone value. 
Since usually each microdot may contain at most one spot, it is clear that 
a halftone dot may also be defined as a cluster of microdots. 
"Image Spot Size Deviation Variables" (ISSDV) in accordance with the 
present invention refers to those imaging process variables which 
influence the size of the spots of the image formed on the imaged medium 
and which make the spot size deviate from the desired size. It will be 
understood that the ISSDV also affect dot sizes as each dot may be made up 
of one or a plurality of spots. However, the influence of the ISSDV on dot 
size may depend upon dot design and may be considerably different than the 
influence on a single spot. 
DETAILED DESCRIPTION OF THE INVENTION 
In the following the present invention will be described with respect to 
certain specific embodiments but the present invention is not limited 
thereto but only by the claims. In particular, the present invention will 
be described for convenience with reference to the Adobe PostScript.TM. 
programming language and to the drawings but the invention is not limited 
thereto. 
General methods of imaging suitable media may be either analogue, e.g. 
contact film, or digital. One type of digital method and apparatus is 
described in EP-A-0 518 559 but the present invention is not limited 
thereto. FIG. 1 is a schematic block diagram of such a system 1. A 
computer 2 or similar device is used to create a digital representation of 
an image optionally using a scanner 3 to scan in an image or a store 4 of 
pre-recorded images which may be accessible optionally via a network 5 
such as the Internet. Digital representations of images may be created by 
graphics software such as Quark XPress.TM., Adobe PhotoShop.TM., Adobe 
Illustrator.TM., Aldus.TM. Pagemaker.TM., Corel Draw.TM. or similar. The 
digital representation is preferably stored as an output file 6 in a 
graphics software and output device independent programming language such 
as PostScript.TM.. The present invention is not limited thereto. The 
output file 6 is transferred to a raster imaging device 10 such as an 
imagesetter, optionally via a LAN or network 7. In a raster imaging device 
the image is created line by line, i.e. in a raster. Either in the raster 
imaging device 10 or elsewhere, an interpreter 8 is provided for 
conversion (raster image processing) of the output file 6 into an imaging 
device specific raster data file 9 which can be processed by the raster 
imaging device 10 by scan conversion. It is understood that the raster 
imaging device 10 may include further separate or integrated devices 
necessary for the development of the imageable medium, e.g. the processor 
may contain developing and fixing compartments for the photographic film 
or printing plate. The output of the raster imaging device 10 is an imaged 
medium 11, e.g. a photographic film or a printing plate. The imaging 
system 1 may be a computer-to-plate system. 
An example of a suitable raster imaging device 10 may be a Creo PlateMaster 
controller linked to a Creo 3244 platesetter with plate conveyor, plate 
processor and plate stacker all supplied by Creo Products Inc. Burnaby, 
B.C., Canada. The interpreter 8 may include a Creo Allegro RIP station 
supplied by the same company compatible with PostScript.TM. Level 2. 
Suitable imageable media may be N90A printing plates provided by 
Agfa-Gevaert AG, Wiesbaden, Germany or Lithostar LAP-0 printing plates 
supplied by Agfa-Gevaert N.V., Mortsel, Belgium. The printing plates may 
be based on a thin metal sheet such as electrochemically roughened and 
anodized aluminium (most common plate thicknesses are 6 mil, 8 mil and 12 
mil, i.e. respectively 0.15 mm, 0.20 mm, 0.30 mm) or have a polymeric base 
such as polyester. For colour printing it is usual to provide a set of 
colour separated printing plates, e.g. cyan, yellow, magenta or cyan, 
yellow, magenta and black. In accordance with the present invention the 
same control strip may be used independent of the colour to be used with 
the printing plate. 
A further suitable raster imaging device 10 may be a printer such as an ink 
jet, thermal transfer or electrostatic printer. Examples are: a 
DesignJet.TM. 750C supplied by Hewlett-Packard Corp., USA ; a 
Summachrome.TM. Imaging system supplied by Summagraphics Inc., USA ; or a 
Chromapress.TM. system supplied by Agfa-Gevaert, N.V., Mortsel, Belgium. 
A suitable raster imaging device 10 also may be an imager for photographic 
film such as the SelectSet Avantra 44.TM. supplied by Bayer Inc., Agfa 
Division, Wilmington, USA. 
Imaging devices 10 may be calibrated in accordance with the relevant 
manufacturer's instructions at regular intervals. Such complex procedures 
are not suitable for routine production control of imaging quality. The 
visual control strip in accordance with the present invention has been 
designed to provide rapid, direct and reliable routine control of imaging 
quality which can be carried out by untrained personnel. In the case of 
printing plates, the control in accordance with the present invention may 
be carried out preferably before the actual printing using the plate 
starts, while taking up the minimum of space on the imaged substrate. 
Continuous tone images cannot be represented or reproduced easily by 
digital devices. Conventionally an image is "screened", i.e. converted 
into an array of dots of variable size or variable spatial frequency. 
These dots are small enough that the human eye sees them not as individual 
dots but as areas of different tones. A typical conventional digital 
raster imaging device 10 records an image on the imageable medium 11 in 
accordance with a Cartesian array of elements of the image. In accordance 
with the present invention a microdot or pixel is the smallest addressable 
spatial unit on the imageable medium 11 of the Cartesian addressing system 
of the imaging device 10. For a printer, imagesetter or platesetter, it is 
the fundamental spatial unit which makes up all other graphical structures 
such as lines or coloured areas. For output devices, microdots are also 
called device pixels or RELs (Recorder Elements). Wren the digital raster 
imaging device 10 is an imagesetter, it creates an image on medium 11 
which after development consists of an array of black, white or coloured 
spots. If the imageable medium 11 forms a lithographic printing plate, the 
developed spot on the imageable medium 11 is either ink receptive or ink 
repellent. When the printing plate is used to print the final image, this 
final image consists of printed spots, each printed spot corresponding to 
a developed spot on the medium 11. 
With negative working imageable media 11, such as computer to film, the 
developed spot on the imaged medium 11 representing one microdot has a 
high optical density when illuminated by light, i.e. typically black spots 
for white light as is well known for so-called "negatives" produced by a 
conventional camera. In the case where contact films are produced by 
making use of UV-light (Ultra-Violet) the density is a density for 
spectral UV-light. For a negative working computer-to-plate system, 
illuminated zones usually become ink-accepting after optional development 
and fixing. 
For positive working material, the optical density of the spot is low when 
illuminated by light. For a positive working computer-to-plate system, the 
illuminated regions become usually ink-repellant. 
A typical dimension for a microdot on a 400 dpi (dots per inch) printer is 
63.5 .mu.m, and 7 .mu.m on a 3,600 dpi imagesetter. This means that for a 
resolution of 3,600 dpi, the imagesetter 10 addresses about 20,000 
microdots per mm.sup.2 on the image medium. The present invention is not 
limited to a certain type of imageable substrate. The spots may be 
produced by any suitable means, e.g. by heat onto thermally sensitive 
substrates, by UV or visible or infra-red light onto photosensitive 
substrates, by application of powders, liquids, inks, pigments or other 
substances to an appropriate substrate. 
For example, in the art both heat mode and photo mode systems are known. In 
photo mode materials, the image forming reaction is initiated directly by 
photons having a specific wave length. In heat mode materials, the image 
forming reaction is initiated by heat. This heat may be applied directly 
like in direct thermal printing systems. Alternatively, the heat is 
applied indirectly via transformation of photons to heat, e.g. via 
infrared absorbing dyes. This may be achieved by imagesetters having high 
power infrared laser sources, e.g. from 830 to 1064 nanometer. 
Usually, the imaging device is capable to form one spot on each microdot. 
The actual size of a spot on a microdot on the imaged medium 11 may vary 
from the specified size of the microdot depending upon the settings of the 
imaging device 10 and the nature of the imageable medium. For instance, 
where the imageable medium 11 is a photographic film, the actual size of 
the developed spot may depend not only on the exposure and development 
times but also on the sensitivity and properties of the imageable medium 
11. For convenience, the invention will be described in the following with 
reference to an imageable medium 11 on which are produced black spots but 
the invention is not limited thereto. In particular the present invention 
will be described with reference to an imageable medium 11 for use in an 
imagesetter 10. 
As shown in FIGS. 2A to 2C schematically, the light intensity of the laser 
beam used in imager 10 varies across its diameter. The intensity of the 
laser beam reduces towards the outer parts of the laser beam. Typically 
the light intensity distribution is Gaussian across a circular beam. Each 
photographic film or plate has a minimum light intensity or threshold 
value required to create a spot or an image. In FIGS. 2A to 2C, the 
developed spot 14 produced by the laser beam of an imagesetter 10 on an 
imageable medium 11 is shown black but the invention is not limited 
thereto, the actual developed spot may be in the complementary colour 
(negative working material) or in the same colour (positive working 
material) as the colour of the illuminating light. The spot may also be 
either lithographic effective or ineffective, e.g. ink accepting or ink 
repellant. 
If the energy of the laser is varied keeping the laser beam diameter 
constant, the actual spot size 14 on the imaged medium after development 
alters. With less beam energy the spot size is smaller: compare FIG. 2B 
with FIG. 2A; with more energy the spot size increases: compare FIG. 2C 
with FIG. 2A. The size of the actual spot 14 on the developed medium and 
hence the grey tone value of each part of the printed image produced there 
from is dependent upon the exposure parameters, e.g. The sensitivity of 
the photographic material, exposure intensity and time duration of the 
laser beam and the developing process used. 
Where the imaging device 10 is a printer, the printed spot size may differ 
from the desired microdot size and may depend upon the type of inks used 
as well as the characteristics of the printed substrate. For example, the 
printed spot size may depend upon dot gain. Dot-gain is related to 
spreading of the ink when a dot is printed onto a printing substrate. If 
this spreading is more than anticipated, the effect of the resulting 
oversize printed dots is to produce an image with a greater image density 
than would be expected from the size of the developed spots on the 
printing substrate. The major factors affecting dot-gain are the thickness 
of the ink layer on the printing substrate, the physical properties of the 
ink such as its viscosity and the nature of the substrate surface, e.g. 
whether it is glossy or matt. 
Summarising the above, there are a large number of factors which may 
influence the appearance, in particular the depth of colour, grey tone 
value or density of the final image on the imageable medium 11. In 
accordance with the present invention, the set of variables which cause 
the actual spot size generated on the imaged medium 11 to deviate from the 
specified spot size will be called the Image Spot Size Deviation Variables 
(ISSDV). It should be understood that, in accordance with this invention, 
the term Image Spot Size Deviation Variables include the factors mentioned 
above which may cause variations in image density or spot size because of 
changes in exposure conditions in an imagesetter, changes in the printing 
substrate or printing ink in a printer or the different exposure 
conditions, photographic substrates and developing methods used in 
photographic reproduction as well as any other variables which affect the 
image density of a final or intermediate image. 
One conventional screening or halftoning method consists of grouping the 
developed or printed spots 14 into halftone dots. A halftone dot is 
provided by the presence or absence of an array or cluster of developed or 
printed spots contained within a halftone cell. The halftone cells may 
themselves be part of a larger organisational unit called a "supercell". 
It is sufficient to describe a simple halftone dot with reference to FIGS. 
3A to 3E. FIG. 3A shows a Cartesian array 16 of microdots representing a 
part of an image to be recorded on imageable medium 11. Where the imaging 
device 10 is an imagesetter, it is pre-programmed to illuminate each of 
the individual square elements 12 (i.e. to produce spots 14) in the array 
16 or to leave it unexposed in accordance with the requirements of the 
image to be produced. The imager traverses the array 16 line-by-line or 
column-by-column. The direction of traverse is known as the fast-scan 
direction and the direction perpendicular thereto the cross-scan 
direction. If the imaging device is a printer, it is adapted to print a 
printed spot 14 in each of the square elements 12 or to leave it blank 
depending upon the image to be printed. In the following we will describe 
the invention with respect to an imagesetter but the invention is not 
limited thereto. Similar principles also apply when the imaging device 10 
is a printer or other digital imaging device. 
Each 8.times.8 matrix of microdots 12 shown in FIGS. 3B to 3E is organised 
as a halftone cell 13. The portion of the original image that is 
represented by a given halftone cell 13 has a certain spatially integrated 
grey tone value. To achieve the required grey scale value in the final 
print, the relevant microdots 12 of the corresponding cell 13 on the 
imageable medium 11 are illuminated with the laser light so as to create 
the right number of spots 14 to produce the right grey tone value, e.g. a 
light tone such as shown in FIG. 3C, a darker tone such as in FIG. 3D or 
nearly black as in FIG. 3E. The "dot percentage" is given by the ratio of: 
the area of the microdots 14 to be illuminated to form spots, 
to 
the complete area of the halftone cell 13. 
After development, a medium 11 illuminated with a given dot percentage will 
produce an image of a certain grey tone value which may also be 
represented by a percentage grey tone value between 0% (white) and 100% 
(black). If the imaged medium 11 is a printing plate, the plate will print 
a grey tone value which is related to the dot percentage but will vary in 
absolute grey tone value depending upon the printing technique used and 
the printing conditions. Printing may be done by lithography, gravure, 
flexography and screen printing. The spots 14 in the halftone cells 13 of 
FIGS. 3C to 3E are clustered together to form a halftone dot 15. The 
apparent variation in the size of halftone dots 15 is achieved by forming 
clusters of fixed size spots 14, the size of the clusters increasing with 
increasing grey tone value. The size of the halftone dot 15 is therefore 
spatially modulated, i.e. The dot 15 is "amplitude modulated" (AM). This 
type of screening is referred to as autotypical if adjacent halftone dots 
15 are arranged linearly having a screen angle and the mid-points of the 
halftone dots 15 are spaced by a fixed period. Typical AM screening 
methods are the Agfa Balanced Screening (ABS) technology supplied by 
Agfa-Gevaert, N.V., Mortsel, Belgium disclosed in U.S. Pat. No. 5,155,599 
and HQS Screening.TM. and RT Screening.TM. licensed to Adobe Systems Inc. 
USA by Linotype-Hell AG, Germany. 
If ABS is used with a 45.degree. screen angle, an imaginary line joining 
two corresponding spots in two neighbouring cells lies at 45.degree. to 
the vertical axis of the cell, this angle being known as the screen angle. 
If the cell size is 11.times.11=121 microdots, the distance in output 
device space between two such imaginary lines is given by 11.sqroot.2=15.6 
microdots. 
The frequency of such lines, expressed in lines per inch, is called the 
screen ruling. The screen ruling is dependent upon the resolution of the 
output device 10. For an output device 10 with a microdot resolution of 
2400 dpi, the screen ruling achieved by using such cells is 2,400/15.6=154 
lines per inch. By varying the number of microdots within each halftone 
cell 13, the screen ruling may be altered. 
An alternative screening or halftoning method is the stochastic or 
frequency modulated (FM) method for representing grey tones by binary 
output systems. In this method it is the number of fixed-sized halftone 
dots in a particular area which determines the grey value, i.e. The 
spatial frequency of the halftone dots determines the grey value. The 
distribution of dots is random or quasi-random as shown in FIG. 4A and 
they are not organised into touching clusters except by chance when the 
grey tone value approaches mid-grey to black values. The number of 
fixed-sized dots in a particular area may determine the grey tone value, 
wherein each dot may include several spots. An example of a frequency 
modulated screening method is the CristalRaster.TM. technology provided by 
Agfa-Gevaert, N.V., Mortsel, Belgium. 
A suitable FM screening method in accordance with the present invention may 
be quasi-random. For example, as suggested by B. E. Bayer in the article 
"An optimum method for two-level rendition of continuous tone pictures", 
Proc. IEEE, Int. Communication Conference, Vol. 26, pp 11-15, 1973, the 
sequence of filling the arrays of microdots forming the halftone cell 13 
is regular but is designed to achieve the same effect as a random 
distribution of spots, i.e. regularly growing clusters are not formed. 
Such a sequence of filling the halftone dot 15 is shown in FIG. 4B which 
represents an 8.times.4 cell 13 having 32 microdots. The numbers refer to 
the filling sequence of the halftone cell 13. When no element of the 
8.times.4 array is black, the halftone cell 13 is purely white. When all 
32 elements of the halftone cell 13 are black, the result is purely black. 
The intermediate tones are produced by making the relevant number of 
elements black. There is no growth of regularly sized dots with increasing 
grey tone value. Instead, the clusters of spots remain small and separated 
from each other and their number rather than their size increases as the 
grey tone value increases. In mixed mode screening techniques such as 
disclosed in EP-A-0 740 457, the halftone dot size may be fixed for low 
densities and the mean distance between halftone dots may be variable to 
increase the density or grey tone value, whereas the dot size may be 
increased to further increase the grey tone value. 
The FM screening methods are more sensitive to the Image Spot Size 
Deviation Variables (ISSDV) than the AM screening methods because a change 
in laser intensity in the imagesetter 10 has less effect on the close 
clusters of spots in the halftone dots of AM screening. Where a 
considerable number of spots are clustered together, the overlapping 
created by oversize spots in the middle of the cluster does not change the 
outer contours of the cluster nor the grey tone value. Only the spots on 
the periphery contribute to the increase in size of the cluster and 
therefore to the change in image density. If the dimensions of a spot were 
to change in size by 20%, the area of the spot changes by 44% 
(1.2.times.1.2=1.44). The change in diameter of a cluster of 20.times.20 
spots would only be caused by the peripheral spots. The change in diameter 
would therefore be the same quantitative amount as for a single spot. The 
resulting change in area would only be 2% 
(20.2.times.20.2=408.4=1.02.times.original area). Hence, a 20.times.20 
cluster of spots is substantially more insensitive to the Image Spot Size 
Deviation Variables (ISSDV). The area of a 4.times.4 cluster would change 
by 10% (4.2.times.4.2=17.64=1.1.times.original area). A 4.times.4 cluster 
is therefore more sensitive to the Image Spot Size Deviation Variables 
(ISSDV) than a 20.times.20 cluster. 
FM screening preferentially only uses individual fixed size spots or small 
clusters of spots, so that a change in spot size on the imaged medium 11 
has a considerable effect on the grey tone value of the final image. It is 
particularly important for FM screening methods to be able to set up the 
imager 10 correctly and also to monitor the quality of imager, 
photographic film or printing plate performance regularly, accurately and 
easily. The visual control strip according to the present invention 
achieves this aim. 
A visual control strip 20 in accordance with the present invention is shown 
schematically in FIGS. 5A to 5C at different exposure levels when the 
visual control strip has been formed on an imaged medium 11. The strip 20 
may be imaged onto a photographic printing plate or photographic film 11 
by imager 10. Alternatively, strip 20 may be a strip recorded on 
photographic film which is imaged onto the medium 11 by contact exposure 
as is conventional in the manufacture of lithographic plates. The visual 
control strip 20 comprises a plurality of control fields 30 to 38 
relatively insensitive to the Image Spot Size Deviation Variables (ISSDV) 
and a background field 39 relatively sensitive to the Image Spot Size 
Deviation Variables (ISSDV). Preferably, an alpha-numerical field 40 is 
also provided above or below the control fields 30 to 39. The ISSDV 
insensitive control fields 30-38 are arranged in such a way as to ease the 
visual comparison with the ISSDV sensitive background field 39. That 
advantage is achieved by improving the contact between the sensitive and 
the insensitive zones. One embodiment is shown in FIGS. 5A, 5B and 5C. 
There, the insensitive fields 30-38 are completely surrounded by the 
sensitive field 39. In FIG. 6, the same configuration is used. In FIG. 7, 
the insensitive fields 30-38 have a shape of a circle segment, whereas the 
sensitive field 39 is surrounding completely the insensitive fields. 
According to FIG. 8A, the circular sensitive fields 39 are surrounded by 
the insensitive fields, having stepped dot percentage values. FIG. 8B is 
very similar to the prior art control strip according to DE-A-19 507 665. 
FIG. 8C shows also a preferred arrangement, where the sensitive fields 39 
are pair-wise enclosed by insensitive fields 30-38. The outer dimensions 
of the digital control strip 20 (defined by the bounding box, 41) are 
typically 12 mm or more preferably 10 mm or less in width. Each ISSDV 
insensitive control field 30 to 38 has a different grey tone value. The 
grey tone value of each control field 30 to 38 is also substantially 
independent of, or at least only marginally dependent upon the Image Spot 
Size Deviation Variables within wide limits. This independence can be 
achieved by correct choice of the elements which make up fields 30 to 38, 
e.g. they may be certain types of coarse checkerboard patterns or patterns 
created with an AM screening method of a low screen ruling. 
On the other hand, the background 39 is provided by a field whose grey tone 
value is more sensitive to the Image Spot Size Deviation Variables 
(ISSDV). For example, ISSDV sensitive background field 39 may be a field 
created using a stochastic or frequency modulated screening method built 
up by small halftone dots, each halftone dot formed by one spot or a few 
spots. Alternatively, field 39 may include a fine checkerboard pattern. In 
accordance with the present invention the ISSDV sensitivity of the 
background 39 is normally (default value) set to at least the ISSDV 
sensitivity of the screening method used for the useful image to be placed 
on the medium 11. Thus, if the useful image is to be produced in Agfa 
CristalRaster.TM. technology, the sensitive field 39 would be preferably a 
50% raster field of this type. On the other hand if the Agfa Balanced 
Screening method is used the default sensitive field 39 could be, for 
example, a 4.times.4 checkerboard field or a 50% raster field of the ABS 
type having the same screen ruling as the screen ruling used for the 
useful image. 
The skilled person will appreciate that the ratio of sensitivities of the 
ISSDV insensitive control fields 30-38 and the ISSDV sensitive background 
field 39 is relevant to the correct functioning of the visual control 
strip 20 in accordance with the present invention. Absolute values of 
sensitivity are less relevant provided the sensitive and insensitive 
fields behave in a consistent fashion with respect to the Image Spot Size 
Deviation Variables. For small deviations in spot size, the change in area 
"A" of a spot is proportional to its perimeter "P" (see FIG. 2D). Hence, 
the ratio of 
sensitivities of the ISSDV insensitive field 35 
and 
the ISSDV sensitive background 39 
is given approximately by the ratio of: 
the total perimeter of the elements or halftone dots (clustered spots) 
which make up a unit area (e.g. 1 mm.sup.2 or within one halftone cell or 
one supercell) of an ISSDV insensitive control field such as 34; 
and, 
the total perimeter of the elements or halftone dots (clustered spots) 
which make up a unit area (e.g. again 1 mm.sup.2 or within one halftone 
cell or one supercell having the same size) of the ISSDV sensitive field 
such as background 39. 
For example, if field 34 is designed to have a 50% dot percentage and 
includes a 4.times.4 spot checkerboard pattern and field 39 has single 
(1.times.1) spots, the perimeter of the spots in background 39 is four 
times larger than the perimeter of the 4.times.4 checkerboard pattern, as 
measured in the same area. Hence, the ratio of sensitivities is 0.25. In 
accordance with the present invention this ratio of sensitivity is 
preferably less than 0.35, more preferably less than 0.25 and most 
preferably less than 0.125. High sensitive fields may be formed by: 
a dot-size modulated periodic halftone screen having a high line ruling; 
a frequency modulated halftone screen using small sized halftone dots, 
composed of one or only a few spots or microdots; 
a checkerboard pattern having small square patterns, each square pattern 
composed of 1, 2.times.2, 3.times.3, etc. spots or halftone dots; or, 
a line pattern, wherein the thickness of each line corresponds to one or a 
few microdots. 
Low sensitivity fields may be formed by: 
a dot-size modulated periodic halftone screen having a low line ruling; 
a frequency modulated halftone screen using large sized halftone dots, 
composed of many spots or microdots 
a checkerboard pattern having big square patterns, each square pattern 
composed of e.g. 16.times.16 spots or halftone dots; or, 
a line pattern, wherein the thickness of each line corresponds to many 
microdots. 
In low sensitivity fields, the size of the elements much be such that the 
eye still integrates the microdensities to uniform densities. 
The ISSDV insensitive control fields 30 to 38 may have regularly spaced dot 
percentages or grey tone values, e.g. 30%, 35%, 40%, 45%, 50%, 55%, 60%, 
65%, 70% which are associated with the numerical reference values of -4 to 
+4 in the alpha-numerical field 40. Field 39 may be designed to have a 
target grey tone value of 50%. Thus, as shown in FIG. 5A, under ideal 
conditions, with the imager 10 perfectly calibrated to the film 11, the 
field 34 annotated with "101" is indistinguishable from the background 39. 
FIGS. 5B and 5C show the situation where the medium exposure is not 
properly set. In FIG. 5B the background 39 has been produced so much 
darker than a tone value of 50%, that the field 36, annotated with "+2", 
is indistinguishable from the background 39 rather than the field 34 
annotated with "0". In FIG. 5C, the background field 39 is lighter than a 
grey tone value of 50% so that the field 32, annotated with "-2", is 
indistinguishable from the background 39. To increase the accuracy of the 
control strip 20, more ISSDV insensitive control fields 30 to 38 may be 
provided with a corresponding smaller grey tone value difference between 
neighbouring fields. 
In a preferred embodiment the grey tone values of the ISSDV insensitive 
control fields 30 to 38 are not simply a linear or regular scale of grey 
tones but rather they are linked to the differentiated well-defined sizes 
of the actual spots on the imaged medium after development which are 
responsible for the generation of the different grey tone values. For 
instance, the numerical values of the alpha-numerical field 40 may be 
directly related to the effect of a specific change in size of the spot. 
For example field 32, which has the numerical value "-2", may have a grey 
tone value equal to the grey tone value produced on field 39 when the 
actual spot size on the imaged medium after development is 2 micron 
smaller than the spot size required to produce the grey tone of the field 
34 annotated with the numeral "0". Similarly, "+4", which corresponds to 
the ISSDV insensitive field 38, may indicate that field 38 has a grey tone 
value equal to the grey tone value produced on field 39 when the spot size 
on the medium after development is 4 micron larger than the spot required 
to produce the grey tone value of the field 34 annotated with the numeral 
"0". By linking the numerical field 40 to the effect of the spot size 
variation, the adjustments to the imager 10 may be carried out more 
easily. In a preferred embodiment, inputs to the imager 10 of the number 
in field 40 corresponding to the control field 30-38 which is 
indistinguishable over the background field 39 may be processed by 
suitable logic circuits in imager 10 and result in an automated exposure 
adjustment. 
The present invention is not limited to a grey tone target value of 50% for 
the background field 39, but other values may be chosen. In particular it 
is advantageous to use a sequence of strips 21 to 23 as shown in FIGS. 6, 
wherein the background field 39 of each strip 21-23 may have a different 
grey tone target value, e.g. strip 21 with 25%, strip 22 with 50%, and 
strip 23 with 75%. The ISSDV insensitive fields 30 to 38 are determined 
for each strip 21 to 23 so that each field 34 of the relevant strip 21-23 
annotated with the numeral "0" has the respective grey tone target value, 
i.e. for strip 21, the respective field 34 annotated with the numeral "0" 
has a grey tone value of 25%; for strip 22, it has the value 50% and 75% 
for strip 23. This set of strips may allow the user to adjust and control 
more easily the exposure of imager 10 for non-linearly behaving systems, 
e.g. when recording with FM screening methods on photographic film. It may 
also allow for selecting other exposure criteria. For example, the 
exposure setting may be chosen which results in a visual match of field 34 
annotated with the numeral "0" with the background field 39 on the strip 
21 (target=25%) instead of on the strip 22 (target=50%). 
In accordance with the present invention the ISSDV insensitive fields 30-38 
and the sensitive field 39 are not limited to a linear array as shown in 
FIGS. 5 and 6. The fields may be arranged in any suitable two dimensional 
array. For instance, as shown FIG. 7, the ISSDV insensitive fields 30 to 
38 may be arranged radially on an ISSDV sensitive background 39. Due to 
the familiarity among operators with analogue clock faces, such an 
arrangement can be used easily when formed into twelve equally spaced 
radial fields around 360.degree. thus forming a "grey tone clock". Such a 
clock may have particularly small dimensions, e.g. the dimension of a 
small wrist watch such as 15 mm.times.15 mm. In such cases the numerical 
field 40 can be dispensed with or left away as the operator can read the 
"hours" 1 to 12 of the "clock" without numbers to help. The operator does 
not have to be capable to read the--often very small--arabic figures of 
the "hours". 
The present invention is not limited to foreground fields 30 to 38 (FIG. 
5A-5C) being the ISSDV insensitive fields. A further embodiment of the 
present invention is shown in FIG. 8A. Control strip 24 has a series of 
background fields 30 to 38 which have differing grey tone values, the 
pattern of each field 30-38 being substantially insensitive to the Image 
Spot Size Deviation Variables (ISSDV). Fields 39 which are sensitive to 
the Image Spot Size Deviation Variables are each located as a foreground 
field in one of the background fields 30 to 38. Each field 39 has a target 
grey tone value as described previously, e.g. 25%, 50% or 75% or similar. 
Alternatively, the ISSDV sensitive fields 30 to 38 and insensitive fields 
39, are arranged one above the other as shown in FIG. 8B or alternating 
with each other as shown in FIG. 8C. In all the embodiments of the present 
invention the insensitive fields may take discrete (as shown in FIG. 8A, 
8B and 8C) or continuously varying values. 
The ISSDV sensitive field 39 (FIGS. 5 to 8) may be a checkerboard field, a 
pixel line field, or a raster field. The ISSDV insensitive fields 30-38 of 
the control strip 20 in accordance with the present invention may also 
each be a checkerboard field, a pixel line field, or a raster field, 
however, it is preferred if the raster field is not produced with an FM 
screening method having small halftone dots made up of one or only a few 
spots. 
These three basic types of field in accordance with the present invention 
are shown in FIGS. 9 to 11. FIG. 9 shows a checkerboard, FIGS. 10A and B 
show pixel line fields, and FIGS. 11A and B show raster fields. Each of 
these fields includes one or more microdots. Within the fields dots, lines 
or fill are made up of an array of microdots. Each dot, line or fill 
constitutes an element. A plurality of elements makes a pattern. The 
elements may make up a pattern with a repetitive pattern cell which is 
tiled in order to fill up the area of the relevant control field 30-39. 
Alternatively, the control field 30-39 may include a series of lines or 
larger dots arranged in a predetermined pattern. One larger dot preferably 
comprises an integer number of microdots or spots. 
Where the visual control strip 20-24 in accordance with the present 
invention is a digital control strip, it is preferably scalable. 
Scalability refers to the ability of the digital control strip to be 
transformed, i.e. resized to a different physical size, e.g. smaller or 
larger in one or two dimensions. According to the present invention, the 
strip is designed such that the integrity of the pattern within the fields 
is maintained. This means that scaling has no influence on the number of 
spots in an element of a pattern, nor on the relative location of the 
spots. Thus, when the outer dimensions of a field are altered differently 
in two orthogonal directions, the field is deformed but the elements 
within the field are not deformed. The undeformed elements fill the 
deformed field--if the field has become smaller, the number of elements in 
the field reduces. This property of the digital strip in accordance with 
the present invention may be achieved by defining the elements of a field 
in device space and the size of the fields themselves in user space. 
Device space is the internal co-ordinate system used by the raster imaging 
device 10 for scan conversion of the raster data file 9 and is usually 
expressed or "measured" in "pixel" units. User space is the internal 
co-ordinate system used to create the output file 6 in the device 
independent language such as PostScript.TM. and is usually expressed in 
metric units such as 1/72 of an inch (see AdobeRef, page 151) or 
millimeter. In order to convert the output file 6 to the raster data file 
9, a current transform matrix (CTM) may be used (see AdobeRef, 4.3.2 
Transformations, pages 152-154). This matrix converts the data in the 
output file 6 into data in raster data file 9 taking into account any 
difference in resolution between the co-ordinate systems of the user space 
(the device independent language) and the device space (raster imaging 
device 10). Thus, a distance of X units in the user space defined in 
output file 6 is converted by the CTM into the appropriate number of 
pixels Y in the device space which result in the same distance in device 
space as is represented by X units in the user space. Hence, by use of CTM 
the distance produced by the imaging device 10 is independent of the 
resolution of the imaging device 10. 
On the other hand, data in output file 6 which is defined in device space 
is left untouched by the CTM. Thus X distance units of device space 
defined in output file 6 result in X distance units in device space. The 
actual size, in metric units, of elements defined in the device space is 
device dependent--the size depends on the number of dpi (microdots per 
inch) of the imaging device 10. For instance, the device space distance X 
printed by a 300 dpi printer would 10 times greater than by a 3000 dpi 
printer. In general, specifying data in device space is discouraged as the 
appearance of the data is device dependent and may seem to be deformed 
relative to user space. 
Further explanations of the terms such as user space, device space, 
pattern, pattern cell, tiling, fill, CTM, scan conversion may be found in 
AdobeRef which has already been incorporated herein by reference. 
The microdot spacing and the element spacing of a control field 30-39 is 
preferentially defined in device space, whereas the dimensions of a 
control field in accordance with the present invention may be defined in 
user space. Control fields 30-39 are preferably scalable and their size 
may be defined by the user. Depending on its size, a control field 30-39 
is filled up with as many elements as required, the elements being clipped 
at the boundaries of the control field. As the pattern elements are 
defined in device space, their actual size on the imaged substrate is 
dependent on the resolution of the imaging device. On the other hand the 
size of the field itself is set by the user. 
A control field 30-39 according to the present invention may be generated 
in PostScript.TM. in the following manner with reference to FIGS. 10A and 
B: 
______________________________________ 
&lt;&lt; 
/PaintType 2 
/PatternType 1 
/TilingType 2 
/Bbox[0 0 8 1] 
/XStep X.sub.-- StepL 
/Ystep 1 
/PaintProc { 
8 1 true [1 0 0 1 0 0] 
{&lt;80&gt;} 
imagemask 
} 
&gt;&gt; 
matrix 
makepattern 
/OnePixelLinesVer exch def 
______________________________________ 
This program listing fragment defines a pattern "OnePixelLinesVer" which is 
8.times.1 pixels in a matrix of 8.times.8 pixels in device space. The 
pattern consists of a vertical line 47 (FIG. 10A or B) having a thickness 
of one microdot or pixel in the device space. The pattern is defined in an 
8.times.8 matrix so that the line width may be amended to be up to 8 
pixels in thickness in other fields. The repetition distance between two 
matrices, X.sub.-- StepL, is defined in user space (not listed above) so 
this repetition distance is output device independent. The complete field 
is defined in user space: 
EQU 0 0 X.sub.-- fieldY.sub.--field 1.0/PixelLinesVer setpattern rectfill 
which generates a field of the required size which is filled by the pattern 
OnePixelLinesVer and is clipped at the boundaries of the field. Note also 
that the field is scalable in user space without altering or deforming the 
pattern of the element OnePixelLinesVer which is defined in device space. 
As shown in FIGS. 10A and B, the pixel line 47 may be a black line 
surrounded by white or vice-versa. 
If pixel line fields are used, their performance may depend upon whether 
the lines lie parallel or perpendicular to the fast scan direction in the 
imager 10. To provide an indication of differences between the fast scan 
and cross-scan directions it is advantageous to use pixel lines which are 
orthogonal to each other, i.e. to have part of the control field with 
vertical lines and part with horizontal lines. 
To create a checkerboard pattern, the above script may be amended to create 
alternating, X.times.X black and white squares in the 8.times.8 matrix 46 
(FIG. 9). In this case the distance X.sub.-- StepL is not defined. 
Instead, the 8.times.8 matrix is specified as the pattern cell and tiled 
within the field 30-39 by means of the rectfill command. This generates a 
pattern of black and white, for example 4.times.4 device pixel squares 
whose size is device dependent and non-scalable (see FIG. 9). The field 
(30-39) dimensions are specified in user space using scalable dimensions. 
FIG. 9 shows a checkerboard field in accordance with the present invention. 
The checkerboard may be made up of a repetitive element or pattern cell 46 
of X.times.X black device pixels combined with Y.times.Y white or 
transparent pixels. If X=Y the result is a theoretical dot percentage of 
50% and should produce a grey tone value of 50%. 
FIGS. 11A and B show schematic representations of a raster field. Each 
halftone cell contains one halftone dot 48. This results in a regular 
array of dots 48 in lines at the screening angle. The grey tone value is 
determined by the size of the halftone dot 48. As shown in FIG. 11B, a 
raster field may be formed by white dots on a black background. 
The ISSDV sensitive fields 39 may be generated using uniform grey tone 
fields having stochastic or FM screening methods such as the Agfa 
CristalRaster.TM. technology provided by Agfa-Gevaert, N.V., Mortsel, 
Belgium or other types of ISSDV sensitive screening methods such as the 
Bayer halftone screening method mentioned above. Alternatively, 
checkerboard, pixel line or raster fields may be used which are 
characterised by a relatively large perimeter "P" per unit surface "A" 
(see FIG. 2D). 
To assist in adjusting the imager 10 a correspondence table may be produced 
as shown in Table 1 below. The values have been calculated for an output 
device 10 with a resolution of 2,400 dpi and a sensitive control field 39 
having a checkerboard pattern of 4.times.4. The dot gain "x" in Table 1 
refers to the increase (+x) or decrease (-x) in the radius of the spot 
over the specified value for the field "0" of the control strip 20 of FIG. 
5A, i.e for a theoretical 50% dot percentage. 
A 50% dot percentage, according to table 1 below, is realised by defining a 
square supercell having two square halftone dots. The length and width or 
size of the supercell is 8 microdots. The size of each halftone dot is 4 
microdots. The first microdot is located in the top left corner of the 
supercell. The second microdot is located in the bottom right corner of 
the supercell. As such, both microdots touch each other at a corner point 
situated right in the middle of the supercell. 
We suppose that in the ideal situation each spot fills exactly one 
microdot. At a resolution of 2,400 microdots per inch (2,400 dpi), the 
size s of the ideally square spot is the same as the size of a microdot, 
i.e. 25.4 mm/2,400=10.58 .mu.m. The size of an ideally square halftone dot 
in a checkerboard pattern formed by 4.times.4 ideal spots or halftone dots 
is 4*10.58 .mu.m=42.33 .mu.m. This size corresponds with a halftone dot 
having no dot gain nor dot loss, or a dot gain of 0 .mu.m, as shown in the 
middle line of Table 1 below. The area of one such halftone dot is (42.33 
.mu.m).sup.2 =1792.11 .mu.m.sup.2. Two such square halftone dots are 
placed in a supercell, composed of 8.times.8 microdots. The size of the 
supercell is 8*10.58 .mu.m=84.66 .mu.m. The area A of such a supercell is 
A=(84.66 .mu.m).sup.2 =7168.44 .mu.m. The dot percentage of two such 
halftone dots placed in one such supercell is: 
100%*2*1792.11/A=50.00%. This percentage is also found in Table 1 below for 
a dot gain of 0 .mu.m. 
If the dot gain is -1 .mu.m, which means in fact a dot loss of 1 .mu.m, 
this means that each side of the ideal square halftone dot shifts by 1 
.mu.m to the centre of the halftone dot. This means that the size of such 
a non-ideal halftone dot having a dot loss of 1 .mu.m is effectively 42.33 
.mu.m-2 .mu.m=40.33 .mu.m. The area of both square halftone dots in the 
supercell is then (40.33 .mu.m).sup.2 =1626.77 m.sup.2. The dot percentage 
is then reduced to 100%*2*1626.77/A=45.39%. This percentage is shown also 
in Table 1 below for a dot gain of -1 .mu.m. In the same way, the 
theoretical dot percentages associated with other dot loss values may be 
computed, according to the equation: 
EQU y=100%*2*(d.sub.0 +2x).sup.2 /A 
In the above equation, 
y is the theoretical grey tone value in percentage; 
d.sub.0 is the size of the ideal halftone dot, expressed in .mu.m; 
x is the dot gain of each individual spot and thus the shift of each side 
of the ideal halftone dot towards the centre of the halftone dot for 
negative values of x; and, 
A is the area of the halftone cell or supercell comprising the two halftone 
dots. 
The above equation is not valid for positive values of x or dot growth. It 
is however clear to the man skilled in the art that a dot growth of a 
black halftone dot may be assessed by the thus caused dot loss of the 
neighbouring white halftone dot, for which the equation holds. This 
explains why the dot percentage found for a dot gain of +1 .mu.m, i.e. 
54.61% is complementary to the dot percentage found for a dot loss of 1 
.mu.m, i.e. 45.39%. 
TABLE 1 
______________________________________ 
halftone dot size for 
theoretical dot 
dot gain x [.mu.m] 
checkerboard [.mu.m] 
percentage y [%] 
______________________________________ 
-5 29.17 32.33 
-4 32.89 
-3 36.83 
-2 41.00 
-1 45.39 
0 50.00 
+1 54.61 
+2 59.00 
+3 63.17 48.33 
+4 67.11 50.33 
+5 52.33 
70.83 
______________________________________ 
Table 1 may be used when the ISSDV sensitive field has a 4.times.4 
checkerboard pattern. The dot percentages of the ISSDV insensitive fields 
are preferably set to the grey tone values corresponding to the 
theoretical values of the "y" column caused by the dot gain of -5 to +5 
micron of the basic spot. If the visual control strip shows correspondence 
at "-2" (corresponding to -2 .mu.m dot gain of the basic spot) between the 
ISSDV insensitive and sensitive fields, then the image produced by the 
imager 10 at 50% dot percentage has an effective grey tone value of about 
41% instead of 50%. From this converted value of achieved grey tone value, 
the necessary adjustments to the imager 10 can be made. Preferably the 
screen ruling of the sensitive fields is the same as the screen ruling of 
the useful image data. The screen ruling of the insensitive fields is 
usually smaller. 
Alternatively, table 1 may be used to control the degree of over- or 
under-exposure. Thus, if under-exposure should be used deliberately, the 
degree of under exposure may be selected form the table and the imager 10 
set up to accordingly. 
The visual control strips 20-24 (FIGS. 5, 6, 7, 8) in accordance with the 
present invention may be used in the following way. Where the visual 
control strip is a digital control strip, the digital representation of 
the strip 20-24 is incorporated into a digital representation of a normal 
page in the computer 2 as, for instance, an EPS file. This file may be 
imaged directly onto a printing plate. The control strip is preferably 
located in an image-wise functionally irrelevant part of the page layout. 
As an example, when used to check the quality of the imager used to image 
an offset printing plate without mounting the plate on the press and 
producing a proof print, the control strip may be located in an area of 
the plate which is outside the zone to be inked. An imaged offset printing 
plate 90 is shown schematically in FIG. 12 and comprises a substrate 91, 
e.g. aluminium or polyester on which an image has been formed after 
imaging in a raster imaging device and possibly a subsequent developing 
step. Printing press location or registration holes 92 may be provided. 
Within the confines of the location holes 92, an inkable area 93 is 
defined. It is within this area 93 that the normal pages or graphic images 
have been imaged onto the plate 90. There are many arrangements for 
securing the printing plate 90 to the plate cylinder, as e.g. described in 
U.S. Pat. No. 4,643,063 as but one example (plate securing on web offset 
presses). When mounted on the offset printing press, the inkable area 93 
will be subjected to the ink rollers. Outside the inkable area 93, there 
is a perimeter area 94 which is not inked and serves no image-wise 
purpose. This area has the mechanical function of locating and securing 
the plate to the press but has no function with respect to the 
reproduction of the image itself, i.e. no image-wise functionality. It is 
in this area 94 that one or more of the control strips 20-24 in accordance 
with the present invention may preferably be placed. It is particularly 
preferred if the control strip 20-24 of the present invention, is placed 
in the plate portion of zone 94 which is received in the plate locking or 
clamping device of the printing plate cylinder. 
As the control strip 20-24 in accordance with the present invention is 
preferably scalable, it can be fitted to the available space in ink-free 
zone 94. Because the field elements are preferably not scalable, they 
remain suitable for quality control purposes independently of the size of 
the fields, provided these are each greater than a minimum size of 
preferably 2 mm. 
Further, in a preferred embodiment, the type of field, e.g. checkerboard, 
pixel line, raster field, the type of screening method and the screen 
angle are set for both the ISSDV sensitive and insensitive fields as well 
as the target value(s) of the sensitive field(s) in accordance with 
default values. The operator can alternatively select any of these 
variables from a menu to tailor the visual control strip 20-24 to the 
needs of a particular job. 
Where the visual control strip 20-24 in accordance with the present 
invention is an analogue strip for use in photomechanical screening or 
contact illumination, the strip may be used in the following way. The 
visual control strip 20-24 comprises a piece of film which can be included 
in the page layout film. Again this piece of film including the visual 
control strip 20-24 is preferably located on a part of the layout film 
that lies outside the useful printable and inkable area of the printing 
plate produced from the layout film. 
Further, the visual control strip 20-25 in accordance with the present 
invention has been described with reference to a plurality of ISSDV 
relatively insensitive fields and a single ISSDV relatively sensitive 
field but the invention is not limited thereto. The visual control strip 
in accordance with the present invention also includes a single ISSDV 
relatively insensitive field 39 and a plurality of ISSDV relatively 
sensitive fields 30 to 38 (the visual control strip would still appear as 
shown in FIGS. 5 to 8 but the sensitive fields would be insensitive and 
vice-versa). The relative sensitive fields 30 to 38 could each have 
differing grey tone value target values, e.g. 30%, 35%, 40%, 45%, 50%, 
55%, 60%, 65%, 70% ; and the insensitive field could have a grey tone 
value of 25% or 50% or 75%. 
The simplest form of visual control strip 20-25 in accordance with the 
present invention is a single ISSDV relative insensitive field located 
adjacent to, or surrounded by a single ISSDV relative sensitive field or 
vice-versa. Such a visual control strip can be used to identify when the 
pre-determined grey tone value of the ISSDV relative insensitive field is 
the same as the target grey tone value of the ISSDV relative sensitive 
field. 
FIG. 13 shows a control strip in which a single ISSDV relative sensitive 
field 51 is located adjacent to a plurality of ISSDV relative insensitive 
fields 52. The sensitive field 51 represents a 50% grey tint and is 
screened according to the Agfa CristalRaster technology, a type of 
frequency modulated halftoning. In reality, the size of a frequency 
modulated halftone dot, according to the test as described herein below in 
conjunction with FIG. 13, is 21 .mu.m. The insensitive fields 52 have a 
dot percentage of 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70% and 
75%. These are screened according to the Agfa Balanced Screening (ABS) 
technology, as disclosed in U.S. Pat. No. 5,155,599. The line ruling of 
the ABS screen is 120 lpi (lines per inch). Due to the shorter outline 
with respect to the area of each halftone dot in the ABS screening at 120 
lpi, the fields 52 are less sensitive to overexposure and underexposure. 
FIG. 14 shows another test strip and shows a sensitive field 53, having the 
same structure as the sensitive field 51; and insensitive fields 54, 
having the same structure and dot percentages as the insensitive fields 
52. However, according to FIG. 14, the insensitive fields 54 are 
completely and separately embedded in the sensitive field 53, whereas in 
FIG. 13 the insensitive fields 52 are arranged adjacent to each other and 
are also placed adjacent, i.e. underneath, to the sensitive field 51. A 
series of exposures were accomplished with both control strips according 
to FIG. 13 and FIG. 14. Log H of subsequent exposures increased by 0.05 
from exposure to exposure. Two types of offset printing plates were 
exposed: 
1. Lithostar LAP-0 0.30 mm, digitally exposed by a Stinger system at 2400 
dpi (dots per inch) 
2. Setprint SET-HN-J 0.20 mm, digitally exposed by an Agfa SelectSet 
Avantra 25 system at 2400 dpi. 
The exposed offset plates were offered to five test persons for 
identification of the control strips having a correct exposure. All five 
test persons preferred this evaluation on the control strip according to 
FIG. 14, where the insensitive fields 54 are completely surrounded by the 
sensitive field 53. Evaluation of the correct exposure on the strip 
according to FIG. 14 is more convenient, fast and accurate than evaluation 
on the strip according to FIG. 13. The circular regions 54 shown in FIG. 
14 tend to disappear in the background 53 when there is a correspondence 
of the circular region 54 with the background 53. In FIG. 13, a border 
line between fields 51 and 52 is perceived, even where the insensitive 
field 52 corresponds to the sensitive field 51, due to optical illusion. 
Accordingly, evaluation of the correct exposure of a control strip 
according to FIG. 14 is more consistent and less subjective than 
evaluation of the exposure of a control strip according to FIG. 13.