losing contrast with FM screening

[mazengh]

we just moved to FM screening lately(screen spekta 2) and while we calibrated to G7 using curve 2 and our dot gains are acceptable, we are experiencing a loss in contrast in our prints.... any idea why this would happen?

 

[pkleinheider]

mazengh,

how much percent did you have to compensate the ctp lineraisation/calibration curve until you hit the G7 target?

Cheers
Peter

 

[nsk]

Gamma curve should be created for every lpi screen ruling. The highee the lpi the lower will be the contrast.

 

[mazengh]

gamma curve

gamma curve

how would you create a gamma curve? and what's the explanation for this?

 

[Lukas Engqvist]

By gamma curve you mean TVI curve? This is a curve to make sure the different ink coverages give the same tone on paper. Generally a 50% tone (lett's just assume black to start off) should have the the same lightness irrespective of the technique used. The designer wants a predictable colour result irrespective if you use FM, AM or hybrid technology. On coated paper this is equivalet of about 67% ink coverage.
(A gamma curve behaves in a similar way as a TVI curve but has to do with light and not ink)

A TVI curve is made by printing known values, measuring the printed result, and then applying a compensation so that sending a known value will produce an expected (standard) result.

 

[mazengh]

I already have a TVI in place on my rip.... but i don't know what a gamma curve is. In my monitor calibration i set the gamma to 2.2. I am not sure what NSK means by gamma curve.

 

[Lukas Engqvist]

@mazengh
So you have a good TVI where you are matching step by step on the tone curve, but still lack contrast? Contrast is normally lost when tones that should be different are not. Do you mean lack of detail when you say lack of contrast? Could you describe (give an example of), or post an image of what you mean by loosing contrast?

 

[mazengh]

we feel that all our prints are dull.... i will get the dot gains on printed sheets, and post soon....

 

[gordo]

we feel that all our prints are dull.... i will get the dot gains on printed sheets, and post soon....

If your previous presswork was good then that presswork becomes the tone reproduction target for your FM screening. The dot gain is NOT the target. It doesn't matter what the dot gain is. What matters is that you get the same tone reproduction on press with FM that you did with your AM.

The process to build those curves is straightforward and described here: Quality In Print: The principle of dot gain compensation plate curves

That process will make curves for the plate that will make your FM presswork appear tonally the same as your target AM.

If the plate curves are good then the cause of low contrast is the same as with FM - too low solid ink density, poor ink/water balance, poor ink transfer, etc.

best, gordon p