Detailed analysis
How to choose the right system for automatic Color Control?
A good system for ink system control must be examined by its ability to perform of the following:
-
Densitometry capabilities
The only during-print-run controllable parameters are ink keys set-up and ink roller speed. Both determine the amount of ink that is put onto the paper: the greater the amount of ink - the higher the density value.
When the type of color is known in advance (CMYK or specific spot colors) - density and its derivatives (such as Dot-gain, discussed below in section e) is the only relevant parameter. Spectrophotometry does not contribute any further additional relevant information. Moreover, current Spectrophotometric devices must use large area color-bar patches, leaving no space to include enough representations of halftones in the color-bar. This limits the color control to full tones only. See detailed discussion below ("Is it really necessary to use spectrophotometry to control the ink keys?").
An online density measuring system - is the right device for color control
-
Ignore irrelevant changes in density that are not caused by the ink system and thus cannot be corrected by changing the ink key setup.
Example: a color-bar patch that has an unprinted spot due to greasy paper or a defect in the plate.
A density measuring device that observes the whole patch in one field of view, will decide that the density has been decreased. As a result it will send a correction signal that will cause an increase the ink coverage. This will erratically darken the image.
A vision based device that observes every color-bar patch with high resolution (many pixels in each color patch) will identify and ignore the white artifact, will analyze only the ink coverage outside this artifact, and will correctly control the ink keys.
A multi-pixel vision device is better than one-field-of-view per color-bar patch
-
identify trends in density changes.
During set-up time density changes may develop from sheet to sheet. Thus as the operator checks a randomly picked sheet, his decisions may be incorrect. This significantly lengthens the set-up time.
A system that observes many sheets can analyze the integrated information and identify the real development of density changes, while ignoring random or specific sheet changes. Naturally this can be done only if the device can observe enough sheets, at full width, in reasonable time.
For example, if you need to monitor 16 sheets to get full width data, and you want to analyze it over 25 sheets - 400 sheets will pass before you come to your conclusion. But if your system is designed to collect full width data from each sheet - you need only 25 sheets.
The above discussion is also very relevant for short runs. As the press starts printing at full speed, color stabilization takes time. For short runs this period may well consume the whole job time. In this case you may not have the required 400 sheets for analysis, but you still do have the 25 sheets as required by a system that analyses this smaller number of sheets.
A full width, every sheet observation device is better than a sampling device that needs many sheets per one full width data collection
-
Multi-functional system (Additional features in one (integrated) device)
Having additional tasks, other than Color-Control, performed by same on-the-press system - is a very significant advantage.
Such tasks may include:
color registration
sheet viewing without stopping press run
Whole sheet quality inspection and bad printouts detection.
If each task needs a specific device it may result in a very significant additional cost as well as in space problems in the press.
The space issue is even more severe because all these devices must be located in the very tight volume of the same last printing unit.
A multi-purpose device is better and cheaper than a battery of separate devices
-
Dot gain analysis in half tones
This is a must for any true color-control system. Color appearance is highly influenced by dot gain. Due to dot gain the same amount of ink may cause printouts to appear different from each other.
If dot gain is changed (due to dust, blanket pressure or other causes) the image color may change significantly.
The dot gain changes appear in full tone and in half tones as well. As the printed image is a combination of many color tones, the measurement and control of dot gain in the whole tone range are very important.
A device that looks on a full tone color-bar patch only, will not identify the changes in the other half tone ranges.
The more patches available in the color-bar, the better the representation of the color tone change achieved, thus the better is the control over the printed image colors. Controlling the full tones only, may guard the colors of the full tones (line works) parts of the image, but will not affect the other (sometimes even more important) half tones parts, such as graphics and shades.
Once such a change is detected, it can be corrected by changing ink keys set-up.
Using a vision system that can monitor small color patches on the color-bar enables the use of mid-tones targets. In contrast, if the vision system uses large area patches - there is room for full-tones only.
Mid-tones patches in the color-bar - is an essential feature.
For this purpose a small patch vision device is much better than a big color-bar patch vision device
-
Use of slim color-bars
The narrower the color-bar, the larger the space left for the image. This influences both the cost of print and the customer's design flexibility.
This is even more important when printing on both sides of the paper, where the two color-bars are located on opposite ends of paper, thus doubling the waste.
A 2mm color-bar device is advantageous
-
Speed
An in line device must be designed to serve maximum speed press.
A system that operates at a speed of up to 18,000 sheets per hour is preferable
