Docstoc

NAA V8 Update

Document Sample
NAA V8 Update Powered By Docstoc
					                         NAA Technical White Paper
                               Presented at Annual Conference Miami, 2000



                               Is a densitometer useful
                                in QC of newspapers?
                                                  Greg Imhoff ∗ and Martyn K Elmy ∗∗

                                                                   Keywords

                                   CtP, Lithographic, Dot Area, Densitometer, Newspaper Plates

                                                                    Abstract

The advent of CtP in newspapers and the printing industry in general has highlighted a problem with
quality control, namely that conventional densitometry is not reliable enough for consistent readings of half-tone
values, making calibration difficult to nearly impossible.

Fortunately, there is a solution, which has been used in laboratories for several years based on video
frame grabbing techniques, more recently using a CCD camera. This technology has now been
implemented in a hand held unit known as a digital DotMeter, a description of some of the problems
encountered and the solutions will be explained, together with a comparison with analogue
densitometers.

                                                  Newspaper pre-press equipment

Newspapers were some of the very first pioneers of CtP equipment decades ago, however with some
very prominent exceptions this important section of the industry has not moved as quickly as others to embrace
this technology.

Currently many of the CtP machines installed in newspapers are large and relatively expensive which has
slowed the take-up of CtP in the UK and the US, however in Belgium and Norway CtP is the norm.
Perhaps it is a problem with the English language! It is actually, more likely to be because of the age or
the press, the cost of labour, the number of plates required, the cost of the output device, the work-flow
which is needed to drive it and significantly the current cost of the plates.

We are now seeing world famous newspapers starting to install CtP, we are seeing a range of output
devices, e.g. 2 smaller machines for the price of 1 bigger machine. We are seeing the development of thermal for
the newspaper industry that could prove to be an exception to a perceived rule of this particular
technology. However, this year will certainly see the emergence of blue-violet laser diode based recorders
and ‘near conventional’ plates in terms of speed to go with them.

At the 1996, NAA sponsored ‘ICC color management work group’ meeting in Chicago, all aspects of
process control were on the agenda and a large focus was on profiling methods and models, among
various software companies.

The simple fact is color management works, and one of the best examples at this meeting was the Apple
sponsored BMW Knight Ridder ‘Proof of concept ad’ tear-sheets. This focused on profiled newspapers
including the St. Paul Pioneer Press, Duluth News Tribune, San Jose Mercury News and the Miami Herald.




∗
    Greg Imhoff, President, GRIPdigital, Inc., LaGrange, IL, USA. Tel. +1 708 588-1150 sales@gripdigital.com
∗∗
     Martyn K Elmy MIOP, Director, Centurfax Holdings plc, London, UK. Tel. +44 208 441 7788 sales@centurfax.co.uk
There were many good points raised in this group meeting. In particular the potential failure points of the 5 M's:

                             Man – Machine – Methods – Materials - Measurement

Color continues to improve in newspapers, which may also help to improve ad revenues. As these ad revenues
increase, costs will decrease while quality is known to increase in a digital workflow. The place where all
or most of this quality control begins in not in color profiling, but in dot quality which makes up the half-
tones and effects the 5 M's. This ‘half-tone issue’ is what your suppliers and crews should know and
understand, and is what sets your CtP or digital systems to standard.

Profiling is after the fact, if you want to get QC right with its many attending benefits, why not begin
correctly, in control of the halftone process, especially when considering a film-less workflow.


                                               Quality - who cares?

Some of the producers of high quality ‘glossy’ publications who are not directly involved in this strategic
market have been heard to say, ‘well, it’s only for a newspaper, they will be lighting fires with it tomorrow’.
This is a very narrow view of the quality requirements for the newspaper industry. The old adage ‘quality
in - quality out’ is as valid for a newspaper as it is for the front cover of a prestigious financial report and
accounts document.

In fact the need for the best quality originals, best quality reproduction, best quality plates, blankets etc.
may be even more important in a section of the industry which prints at very high speed on low quality
stock. A dot that will ‘hold’ on a sheet-fed litho press will often give the press operator ‘no chance’ in a
newspaper plant.

A major concern with mass circulation publications which may be printed at several remote sites is data
integrity, to a great extend this has been overcome with the emergence of digital work-flows including
PDF and TIFF-IT, however it is also essential to know that the print quality is consistent at these remote
plants. A traditional reflection densitometer or even a spectrophotometer will greatly assist in a post mortem
of a print run, however by monitoring and if necessary adjusting the dot output many print related problems can
be avoided.

The ever growing demands of the advertisers for their product to look good in a mass circulation
newspaper, an up-market magazine, a direct mail flyer, on a bill-board poster and on their web site
certainly puts down great challenges for the trade crafts-person.

Therefore, what tools are there around to help the QC process? Well to day you can purchase also sorts
of meters and gadgets, for the purpose of this paper, we intend to focus on the current biggest seller - the
traditional densitometer.


                                        Why densitometers do not work

Densitometers have been with us in the Graphic Arts industry for many years now. It has been the one
tool available to us that has the ability, though in practice seldom used, of putting some science into what
is still considered by many to be an art form, as someone said ‘Color Reproduction is a Black Art!’

Its use in transmission mode for the stable production of film output, and calibration thereof cannot be ignored.
Neither can its use in reflection mode in the pressroom for the control of ink weights and associated
factors be understated. However when it comes to the area of checking quality of printing plates from a
CtP system, this is one area that densitometry cannot be easily applied.
                                                    Who says?
                                                                       1

• 3M-Imation in a presentation at the TAGA proceedings in 1997.

• Another important body of opinion to share this view is that of other plate manufacturers, all of who rely on
planimeter and / or microscopy techniques in their labs to obtain accurate results.
• A growing number of traditional densitometer manufacturers who are now producing or marketing CCD
technology devices.

• Also a large number of densitometer users - in many respects the most relevant witnesses in the case -
have found that these devices are less than helpful in a CtP environment.

• In addition, last and probably least, we are saying so right now!


                                            Why don't they work?

Before the advent of CtP, the production of printing plates was a reasonably predictable process with
various control mechanisms in place to ensure faithful transfer of known film to a known plate. The film is
of course 'known' thanks to the good ol' densitometer. These transfer techniques of film to plate usually include
the reproduction of a grid / wedge to give the correct exposure level. Such systems
                                            2

have been emulated quite successfully on CtP systems. However, exposure level alone is not the only
requirement when it comes to CtP. Linearisation of the percentage dot on the plate is also a
                                                3

function of the recorder aperture and size and not just exposure level.

The grain profile of the plate will also have an effect on the accuracy of the densitometer readings, as will
variations in plate material and emulsion. On plate, the actual density of the emulsion is generally non-
critical and certainly on thermal plate can only be of interest to the offset lithographic plate manufacturer.
In general, the density (solid tone) on plate bears little relationship as to its ability to hold or reject ink and
transfer to paper. What are critical are the percentage dot areas on the plate that will transfer an image to
paper. That is what this business is all about!

Another important aspect is the human factor. Operators may have specified an incorrect tint value. With
good luck, this would be noticed on film, by either eye or densitometer, before the press was run. This is far more
difficult with CtP where a densitometer cannot be relied upon. For example, how can you check a highlight
dot in the middle of a plate, a distance away from normal calibration zone?




Reflectance itself brings further problems. If the surface being measured was a perfect mirror, then in
theory the reflected density would be infinite, because all of the light would get reflected straight back to
the source with none ever reaching the sensor (light detector). If the surface were totally randomly rough
then a random proportion of light would get into the sensor.
This phenomenon is, of course, only due to the surface reflectance and has nothing to do with measuring
the reflected density, other than make it more difficult. To over-come this problem a wellknown
modification to the famous Murray-Davies equation - the Yule-Nielsen equation - was developed, originally
for the specific application of ink on paper. When applied to plates it can only be effective provided the user
knows the mystical 'n' factor for the plate for each type of plate.

Finally, there is also a potential problem with electronic stability, which will become increasingly critical, as
the density range to be worked with gets smaller.

Hence, the opening statement, densitometers 'do not work' on plates and therefore cannot be relied upon.
To justify this we need to look at the workings of a reflection densitometer.


                                       How densitometers try to work

The above graphic shows a typical model, which complies with the ISO specification. 4 for such devices. A
collimated light source is used to illuminate the sample and detected by a sensor at 45 degrees. This is
then converted to density via a log amp and displayed on a meter, perhaps with other compensations applied.

When attempting to use a densitometer to measure percentage dot, it is necessary to know the white and
black level of the plate. In practice, here is the first problem - namely that any variation in the white or
black level across the plate will have a direct effect on any reading taken. In practice, variations of +/-
13% have been seen using a conventional densitometer across the plate background (white area). Again,
the only way that a densitometer can be used as a DotMeter for plates is by very careful calibration of the
black and white levels within very close proximity of the target area. This also assumes that you know the 'n'
factor for the plate.

The accuracy of the result obtained is the next issue to address. If the 'black' of the plate is say 1.0 D, this
means that in reflectance terms 10% of light is reflected, and the 'white' by definition is 100% reflected. A
50% tint area is made up of equal areas of 'black' and 'white' so that in reflectance terms a total of 55%
(i.e. half of the white plus half of the light from the black area) is reflected.

As density = -log (Reflectance) this equates to a density of 0.2596 as compared to the 0.3 one would
expect in a perfect world. A chart showing the effect of D.max is shown below.


                              Corresponding density for various tint values
 D.max R. % 1%                 10%       49%       50%            90% 99%

 3.00    00.10      0.0044      0.0457      0.3094      0.3006      0.9996      1.9590
 2.00    01.00      0.0043      0.0453      0.3053      0.2967      0.9626      1.7011
 1.50    03.16      0.0042      0.0442      0.2957      0.2875      0.8912      1.3840
 1.00    10.00      0.0039      0.0410      0.2668      0.2596      0.7212      0.9626
 0.90    12.59      0.0038      0.0397      0.2563      0.2495      0.6710      0.8708
 0.80    15.85      0.0037      0.0382      0.2435      0.2371      0.6150      0.7775
 0.70    19.95      0.0035      0.0362      0.2279      0.2220      0.5535      0.6829
 0.60    25.12      0.0033      0.0338      0.2089      0.2037      0.4867      0.5872
 0.50    31.62      0.0030      0.0308      0.1862      0.1817      0.4150      0.4907
 0.40    39.81      0.0026      0.0270      0.1592      0.1555      0.3389      0.3935


Although this is, of course, taken care of by the densitometer manufacturers as far as the user is
concerned, the fact is that the smaller the density range to work with the more susceptible the unit is to
errors due to electronic drift from noise and temperature.
                                         Where densitometers go wrong

A comparison between the arrangement of dots on a piece of film and the equivalent arrangement on an
average printing plate makes it easy to understand why a densitometer is able to give a reading from the
former, whereas it encounters enormous problems in interpreting the latter (as the following graphic
shows).




The distribution of dots on the smooth surface of the film is easily calculable and hence represents
something very much like an 'Ideal World'.

In the 'Real World' of the printing plate, however, there is no such comfort. The 'noise' from the plate material
makes it effectively impossible to obtain trust-worthy results with traditional methods and devices
designed to operate on film

                                                 Which one is 50% ?
                     (actually, they both are, but you can see why a DotMeter is able to get good results)


 Densitometer - image view                                                              DotMeter - image view
                                           What are the options?

So how can we accurately measure percentage dot area on both CtP and conventional plate systems?

There is the age old 'eyeball' method which, by definition, implies a significant element of what we must
call 'guesswork'. Following this route, you could expect a level of accuracy that has a potential error
margin of +/- 20%. At some time the skilled 'dot etcher' will leave the company, then the delta may become
as much as +/- 30%. A good quality loupe and operator experience may bring it down to +/- 10%. You could use a
densitometer and hope for the best and you may get to within +/- 5% of the correct reading.

For many years plate manufacturers and other research laboratories around the world have used video
techniques involving either planimeter or computed results. The planimeter in this application should
perhaps be called a 'cyborg' as it is part computer and part human. A photomicrograph is taken of the dot
and is then put on a digitising tablet and traced around by hand and a judgment made of the dot edge. The
computer then dot calculates the area of the dot. The application of these techniques leads to a dependable
accuracy to within +/- 1%.

The important part of this process is that the human operator decides where to 'threshold' the image, that
is to define the border between the black and white levels of the plate. Clearly, however, such an
instrument is not particularly portable or quick to make readings. For many years now there have also
                                                                                                      5

been all-electronic planimeters using a microscope, video camera and computer system . Using off the
shelf components has again meant a lack of portability.

Within the last few years advances in electronics have enabled the realization of a portable system, and
Centurfax a UK pre-press innovative organization was the first company to realize this concept in the
commercial world when a working device was openly demonstrated at Imprinta 97.

Many of the practical problems of using such a system have previously been highlighted and include:
                                            1. Focus of image 2.
                                           Exposure control 3.
                                           Thresholding 4.
                                           Aperture errors.


These problems have been addressed by the development of a commercially viable DotMeter. What is a


                                           DotMeter?
A DotMeter works via the principle of combining a CCD camera with a microscope. The camera takes a 'snap-
shot' of the area being measured and literally counts the black and white pixels in the image. Rather than
taking an average of dot density (as with a densitometer), the DotMeter is actually measuring image area
and providing an absolute value of dot coverage. However, it is obviously not as simple as just counting
the pixels. A checkerboard made up of nine squares is instantly recognized as a 50% tint, but as you can see
there is a ratio of black 5 : 4 white squares.
Other key features of the very best DotMeters are that it can offer automatic calibration in a single shot
(rather than having to be re-calibrated against a known value area before each new reading); it provides
additional data on screen ruling and screen angle; and it furnishes users with the ability to read film, plate
and paper with one instrument.

As the device is based on CCD camera technology, it is obviously possible to capture the image of the
dot area. These magnified pictures can then be saved emailed for viewing on a remote monitor, which
makes the task of QC at remote print sites easier. The operator only has to ‘snap’ a picture of area of the
control strip and forward them to the central control point for approval in a matter of seconds.

It should be stated at this point (Jan 2000) that - to the best of our knowledge - all the abovementioned
facilities are currently only offered by the CCDot from Centurfax.


                                            How a DotMeter works

The design of a high quality DotMeter includes a glass disc of approximately 25mm diameter, which is placed on
the sample plate. This design makes is easy to keep the medium flat, thus maintaining good contact with
the sample. It is after all a camera and focus is critical. Depth of focus is typically less than 0.2mm for any
such system.

A key design criterion of a high quality DotMeter was to ensure that errors in focus would not be
introduced into the unit because of movement, as in the case of units based on a traditional densitometer
concept. Instead, a high quality DotMeter uses a partially silvered mirror to give a genuine WYSIWYG
viewing system with no moving parts, which makes it virtually wear, and maintenance free for the user.

Exposure control is another very crucial area in setting up any image analysis system. Most video
systems have automatic gain control built in. This will tend to reduce the contrast when placed on, say, a
10% tint, as it tends to take the average and not the peak density. The other possibility is to set exposure level
dependant on the white and black levels in much the same way as a conventional densitometer. This
approach used by some suppliers does make the system susceptible to variations in density, just like a
conventional densitometer.




A high quality DotMeter6 on the other hand always finds the best contrast between printing and non-
printing areas irrespective of the tint value being examined. A complementary colored light source
guarantees optimum contrast and ensures that process colors are measured accurately. It can take a little
while to determine the optimum value as the nearer the value is to a limit, the more difficult it gets to distinguish
signal from noise.

Providing that the exposure level has been set correctly, and that the image is in focus, then the image
may be thresholded at the 50% value with minor errors. The biggest single problem is the level of optical
noise from the medium, however using DSP (digital signal processing) techniques this noise can be overcome.
Assuming that a high quality DotMeter is used within its working range (typically 85 lpi - 215 lpi) then errors
due to the aperture can be kept to within 0.5%.
                                            Where do you need a DotMeter?

The answers to this question are simple and relatively obvious. A DotMeter is vital for linearising any CtP
RIP and is the only viable tool for professional Quality Control to within any acceptable level of accuracy.

This is all very well if the object of the exercise is only the pursuit of technical excellence. However, is this
merely the beauty of the abstract? Is there any commercial benefit in being sure that you have it 'nearly
right' or is this just a lot of fuss attempting to seek unnecessary levels of near perfection? Surely there is
more to it than this.

A recent submission by a major US commercial print corporation offered the results of research and
analysis into the actual cost of errors in the printing industry. This high-profile group produced an estimate
that the average direct cost incurred is approximately US$650 per error, which indicates that a device to
avoid these problems is excellent value for money.

This in itself is significant enough. Nevertheless, to the best of our knowledge, this figure does not take
into account the enormous hidden cost of repeat business lost because of the errors in question.

The development of high quality visible light and thermal plates alongside very accurate recording
engines may eventually reduce the need for manual quality control devices. Until then, please CHECK
your output and adjust if necessary. When this ‘holy grail’ does finally arrive, please continue to CHECK
your output, it will give you a nice feeling to know nothing has gone wrong!


                                                           Appendix
1
 Characterization of Plate Images Part 2, Integrative Sphere Densitometry. S A
Bartels, R S Fisch, D A Nelson, Taga Proceedings 1997
2
    A method for Determining Halftone Dot Area using a Calibrated Visual Reference. David J Romano, Taga Proceedings 1998
3
       Recorder Spot Size and Its Effect on Image Quality and Halftone Reproduction. David J Romano, Taga Proceedings 1999
4
    Photography-Density Measurements Part 4, Geometric conditions for reflection density ISO reference number, ISO 54:1995(E)
5
    The Image Analyzer - A True Dot Area Meter? David J Romano, Taga Proceedings 1996
6   Why densitometers don’t work. S N Colthorpe, G Imhoff, Taga Proceedings 1999
2003 Technology
    Update


Comparison of

CCD PK5.1                                                                PK5.1: 1% FM plate view



and

DMS PK9.1




                                                                         PK9.1: 1% FM plate view



•   Each TIFF image is measured from the same plate target. More image data is clearly seen from the PK9.1.
•   The PK5.1 image reveals roughness and artifacts which are not present in the plate, nor visible in the image of
    PK9.1. The PK9.1 image is superior due to a high-resolution digital camera with premium microscope optics and
    sophisticated image analysis. The handheld DMS PK9.1 operates similar to plate manufacturers’ planimeter based
    plate benchmark testing systems.
•   Faster plate calibration and accurate measurements are achieved with the PK9.1 and due to single button operation
    is simpler to use. Better sampling provides advanced data that can improve results in any CTP calibration and/or
    linearization.
•   Targeting any halftone with the PK9.1 is easier to see and thus faster to measure. Our software includes built in
    fingerprinting for any tonal curve(s) in color.
•   For workflow calibration the PK9.1 is proven as the new standard for both AM (i.e.: 75LPI – 300LPI) and in FM, to
    a 10-micron spot size.
•   The PK9.1 provides unsurpassed repeatability of ± ½ of 1 percent when measuring a 150 line screen. This is twice
    the accuracy, consistency, and repeatability of any other known plate measurement device on the market today.

                                 130 N. LaGrange Rd., Suite 107
                                 LaGrange, IL 60525-2005
                                 Phone: 708-588-1150 Fax: 708-588-1148
                                 e-mail: sales@gripdigital.com

				
DOCUMENT INFO
Shared By:
Categories:
Stats:
views:91
posted:2/28/2008
language:English
pages:9