Re: [OT] Dot shape prevalence
Re: [OT] Dot shape prevalence
- Subject: Re: [OT] Dot shape prevalence
- From: Jim Rich <email@hidden>
- Date: Mon, 10 Oct 2005 14:43:05 -0400
I have been following this thread for a while so here are a few comments
that might bring more clarity to these screening issues.
In the early 1980s I was involved with research about printing four colors
at the same angle. This was at a time when about 50% of the color
separations were done on the camera with contact screens and the other 50%
of color seps were with a drum scanners that used either contact screen or
electronic dot generation (EDG). In one part of that research I created
two forms for a web press. One form incorporated images screened at the
traditional 4 angles. The other form used images screened at the same angle.
The images in both forms used a range of screen rulings from 85 lpi to 175
lpi.
After the forms were printed, the data I collected indicated that
dot-on-dot printing would work without producing a traditional morie
pattern.
However, what was produced was something called an indefinite morie
pattern. This was just more verification of what John Yule (the grand-daddy
of color reproduction) found in the 1940 and 1950s.
The indefinite morie pattern is not like a traditional morie where you get
objectionable patterns. What you do get is color shift due to the change in
the position of each dot on the sheet as it moves through a press.
In the analysis phase, part of the test methodology involved using a panel
of judges to determine if they could see color variations.
>From those tests using the traditional four angle form we found that
one sheet in 20 was considered off-color and not acceptable. That is 95%
of the time color consistency was acceptable.
>From those tests using the the same angle (dot-on-dot) for all four
colors we found that every other sheet was considered off color and was
not acceptable for the work we were doing.
One conclusion was that dot-on-dot printing does work, but...not just well
enough to produce consistent results. So for high quality printing using
dot-on-dot at the same angle was definitely not acceptable.
However at the time, some newspapers were experimenting with dot-on-dot
printing in production runs. The benefit to them was that the rosette
patterns were non existent because there was only one angle, so the images
looked sharper.
That was in the 1982 time frame before PostScript. In the late 1980s some
California companies started writing code using the the dot-on-dot
technology as a basis for PostScript screening.
However, instead of starting the screening at a dot-on-dot position, they
used the same angle for each color but staggered each dot so when 50% dots
were printed the dots would have less chance of being printing with one
color dot exactly on top of another dot. This method is called Staggered
Position One Angle Screening. From what I can tell I don't think that
invention went too far in the graphic arts industry.
Practically speaking, I believe that by using the same angle at a different
screen ruling will allow you to use 5+ colors that are screened. Mileage
may vary.
Lastly, in the mid 1990s I was working with the laser printer group at HP
and I found that they were using dot-on-dot printing. So there are some
applications in the last ten years for dot-on-dot printing.
I am also surprised that dot-on-dot printing is not used more in bill boards
that are printed with grand format printers.
Jim Rich
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