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"Ask Joe"   Archive

JoeJoe answers questions about electro-mechanical engraving, using tranScribe or Collage, and more. Questions with general interest are posted here.

When creating a gamma curve for CMYK, how many points on the curve would you recommend? For litho, it is usually 7 to 10 points. We are experimenting with using the G7 method for achieving a neutral gray print response.

Customer Support answers:

When creating a gamma curve my procedure is to enter points using the 256 point scale. This provides absolute certainty that each output is exactly what is desired for each input. A reduction in points means that you are allowing the computer to select the points in between and this may or may not be correct. For sure this is very time-consuming and may be more labor intensive than you want.

As a general rule it is much more important to control the highlight portions of your gamma curve than the shadows. In other words, controlling your points from 0% to 60% is much more important than controlling the points from 60% to 100%, since a small change in tone at 30% will produce a drastically different tone, while a small change at 90% will be imperceptible.

One common mistake people without much gamma experience make is to try to reduce the shadows to printable dots. As you know, gravure begins to print a solid at around 70% or 80%. If you try to reduce your shadows to these screened percentages your final result will be extremely "muddy" and unacceptable.

What I generally advise is to accurately control your dot percentages up to around 70% or 80% (depending on where you begin to print solid), and then draw a straight line from there to 100%. This gives maximum tone separation, with the darker shadows being represented by additional solid ink, thus loading up the shadows with ink and making them appear as dark as they need to be. Your shadow detail is made up of ink loading.

Also, you must do your best to avoid sharp transitions in your gamma curve. The curve should not have any sharp angles or lines (banding) will appear in vignettes.

If you wish to control your curve with fewer than 256 points, you should have finer point resolution in the highlights and coarser resolution in the shadows.

Many people use a grayscale that has 2, 4, 6, 8,10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 90, and 100% control points. This is 16 points, would be very manageable but would still provide the accuracy necessary to develop a good gravure print. Once again, it is unnecessary to control at the 80 and 90% tones — they should be included just to see what is happening there. It may not be necessary to include the 70% tone in these calculations, depending on whether a screen is printed or not at that density. Some fudging may need to be done in the shadows but if you look at some of our stock gammas you will get an idea of how they work.

A further point I would offer is that densitometers are fairly unreliable for reading dot percentages of gravure dots. They are fairly accurate in the highlights but remarkably inaccurate in the shadows. It is better to use a density reading.

Measure your proof density in each step of the grayscale and then compare that density reading to the density reading on the print grayscale. Ink differences will cause inaccuracies, so the closer your proof ink matches your print ink the better off you will be. The Collage Gamma Editor Fingerprint table will accept density units when calculating and creating the new corrected gamma curve, so this should not be a problem. The Collage Help has a very good explanation of the Gamma Editor and fingerprinting, which you may find useful.

If you should require additional assistance please let me know. Keep in mind we are always available to visit your plant to provide "in person" assistance onsite.