RE: Dot Gain
RE: Dot Gain
- Subject: RE: Dot Gain
- From: Ray Maxwell <email@hidden>
- Date: Tue, 29 Apr 2003 11:45:52 -0700
Dot Gain:
There are two kinds of dot gain. Mechanical dot gain and optical dot gain.
Mechanical dot gain is caused by the spreading of the ink on the paper.
Matt papers have higher dot gain that coated papers. Optical dot gain is
caused by light being trapped under the edge of each screen dot.
For accurate dot area measurement it is best to use a spectrodensitometer.
You will get more accurate and repeatable results than a densitomer.
The Status of the filter used for the density measurements used to derive
the dot area measurements have an effect on the measurements. In other
words dot area measured with a Status T filter will be different than using
a DIN NB (DIN Narrow Band) filter set. Most North American measurements are
Status T.
Effective dot area is measured by measuring the density of the solid, paper,
and the tint and then plugging these measurements into the Murry-Davies
equation. Most instruments will do this for you when you select dot area
measurement.
Dot gain or Tonal Value Increase (TVI) (Newest name for dot gain) is
calculated by subtracting the tint percentage specified in the file from the
measured dot area on the printed sheet.
When people use a single dot gain number they are usually talking about the
maximum dot gain of a system. This occurs around the 50% tint in the file.
On some systems this may occur at a point as low as 40%. The dot gain falls
off as you go above or bellow this point. A 0% tint and a 100% tint have
zero dot gain. To properly represent the full dot gain characteristics of a
printing system you must show the full dot gain curves for each color of ink
used.
With a film work flow you will see typical dot gains as shown below:
From file to film form an imagesetter = 0% dot gain ( The imagesetter is
usually calibrated to be linear) (File Tint = measured tint on film)
When you make a plate from film in a vacuum frame you usually get 3% to 5%
dot gain at the maximum point. (Usually around the 50% tint)
When you print on a sheet fed press with coated paper you usually get an
additional 15% to 20% dot gain at the maximum point.
This gives you a total dot gain of from 18% to 25% on a film workflow.
There are systems beyound the range of numbers I have mentioned.
With CTP (Computer to Plate) you eliminate the film to plate dot gain (3% to
5%). You can compensate for this in one of two ways. You can adjust the
calibration in the RIP that makes the CTP plates to add 3% to 5% dot gain
(at the maximum point) so that you can use files that were separated for a
film workflow or you can re-seperate your original images so that they are
targeted to the 15% to 20% dot gain you get with CTP.
For proofs or press sheets I recommend a spectrodensitometer. For plates I
recommend a imaging dot meter that can actually measure the mechical area of
the dot.
If you want to work to standards I recommend that you order from the NPES
web site the ANSI CGATS.4.1993 document titled "Graphic technology--Graphic
arts reflection densitometry measurements--Terminology,equations, image
elements and procedures".
Finally...Dot gain is not good or bad. It is an effect that happens on
press that must be allowed for when you make your separations. A proper ICC
profile does this compensation automatically.
Ray Maxwell
Creo
Ray Maxwell | Senior Color Systems Engineer, Inkjet Printing
4225 Kincaid Street | Phone (604) 451-2700 ext. 2004
Burnaby, B.C.
Canada V5G 4P5
IMAGINE CREATE BELIEVE
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From: MSD
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Sent: Tuesday, April 29, 2003 7:32 AM
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To: email@hidden
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Subject: Dot Gain
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Good morning,
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Some time ago was a discussion on dot gain.
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Since then, I have been reading - testing - and paying attention to my
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workflow.
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The results seem to be -
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A 30% dot gain does not "grow" a dot by 30%.
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-
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My 3% dot does not end up at 33%.
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Anything over 70% does not end up solid.
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Can anyone explain what everyone thinks they are talking about
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when they discuss dot gain.
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Thank you very much,
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MSD
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