Hello Erik,
Can you please amplify a bit on this. What do you mean by "prepress is also a bit of a mess", and "the design of the press affects the accuracy of the prepress process"?
Thanks,
Al
Hi Al,
I don't want to get involved with a big debate but I will comment on some issues with prepress.
There is still a lot of talk about dot gain or TVI. This is good for process control but has little to do with colour accuracy. If one talks about printing black dots then the tone that you see is related to the measurements of dot gain. But if one talks about CMY then things break down.
When you measure apparent dot size to determine dot gain (CMY) with the densitometer, you actually can not see what was measured. As an example, when you measure Magenta dot gain, you are using the green filter in the densitometer. I can't see green in magenta. Maybe some of the experts can but I can't.
Even if you have the same paper and same ink but print AM and compare it with FM screens at the same dot gain, you may find out that they do not look the same colour. Density and dot gain are not directly related to colour and therefore any system that uses it has a systemic problem.
So all the talk about compensation curves are not going to result in predictable results. Maybe good enough but not necessarily consistently predictable. My point is that mathematically they do not have the capability to do the job. This is because the printing of CMYK in most offset presses is non linear and non independent.
G7 only gets the L line or gray line to match between different devices. It has no inherent ability to match colours.
The talk about all the standards is foolish. There should be a direct standard that says something like reproduce the image, point for point or small area for area, within some colour tolerance. That would be a reproduction standard. The way you get there is your problem. That's how it is done in normal manufacturing.
This approach can be done and if it was designed correctly it would be direct and simple and predictable but the industry has no interest in going in that direction for what ever reasons they have. The increase in computing power and the lower cost of measuring technology now helps to make this potentially a practical option.
Now we can talk about the press again, which is the area I think needs to be corrected to be able to ultimately obtain predictability.
It has long been understood that the ink film applied to the plate in line with the solid patch is not necessarily consistent. The solid patch doesn't represent how the press is printing in line with it. This is due to the printing of the image itself. The image on the plate affects the ink film on the form rollers which then applies ink differently to the plate in a different area.
So if one prints a test form for dot gain or a test form for colour profiling the characteristics of the press with a specific ink, paper, screen combination, the data from those test forms do not fully represent how the press will print another image. The test form is an image just like any other image and it affects how the ink film is developed on the form rollers. Send faulty info into prepress and you will get faulty results out. Probably not terribly bad but not accurate.
There is no surprise to me that Heidelberg has an expensive Image Control technology for their conventional presses but they probably would never suggest it for the Anicolor press, since the Anicolor press will tend to print consistently in line with the solid patch.
These messy issues go on and on but hopefully I have described some major ones. I am sure it is frustrating for some to deal with them on a daily basis. If people would start to understand what is giving them so much grief, they might get angry at the suppliers and industry experts and get things changed. ....Nice thought but not likely.
