Grafica News - January 2012
How to Use a Standard Screen Print 4-Colour Process Colour Bar
By Mike Ruff, Chief Technology Officer, Nazdar Consulting Services
One of the most important principles in becoming a high quality and profitable 4-colour process screen printers is to learn understand and use a standard 4-colour process colour bar that is sometimes referred to as a "control wedge". (Figure 1, IDEAlliance 12647-5 Screen Print Control Wedge)
It is vitally important to understand the patch set of the colour bar because attempting to print a process colour image with no colour bar is like a pilot flying in the clouds with no instruments to guide the plane. You have no consistent point of measurement that ensures the print will accurately simulate the file or an accurate proof. There are 6 critical elements required on a good quality screen print colour bar.
A. A 100x3 patch.
B. Solid Colours of the cyan, magenta, yellow and black.
C. Tonal values at strategic intervals throughout the tonal range of each color.
D. Grey Balance patches for highlight, quarter-tones, midtones, threequarter-tones and shadows.
E. A substrate patch.
F. RGB patches.
In this article, I will explain the patch sets and the purpose of the patch sets of a standard screen print 4-colour process colour bar and how to use it in calibration and in press control.
A. 100x3 Patch
This patch is Cyan, Magenta and Yellow overprinting at 100% of each colour. The result will be a 3-colour black. A balanced ink set measured with a densitometer will be very close the same density on all three colours. (Figure 2, Balanced Process Colour Inks)
For example when you read the 100x3 patch you will see Cyan, Magenta and Yellow reading between 1.35 and 1.45 absolute density. Try to get the numbers within 2 or 3 points of each other. Balanced values are most important. If you are using a spectrophotometer get your a* and b* values close to zero. The L* will be about L*25. This means that as the colours overprint throughout the tonal ranges, they will not be throwing a cast into your image. The sequence you print in needs to be tested before you begin calibration to determine the correct sequence. Then adjust the densities to achieve neutral. Once this is done, record the sequence and record the densities. After this is completed you are ready to adjust your tonal percentages. NOTE: Normally we find that screen print inks require magenta to be printed first, then cyan, then yellow. Test your inks to see what produces a balanced C,M,Y, 100x3 before you begin other calibration or you will need to go back and do this. If you go back and adjust your ink, then you have to re-calibrate tonal values, solids and gray balance. So start with balanced inks printed in the correct sequence.
B. Solid Colour Patches:
Solid Colour Patches are the most command point of measurement in process colour production. Any printer with a densitometer uses these to measure and control the density of the ink. But a common mistake is using density to control colour. Solid density should be used to control "repeatability". Once density values are determined through calibration of the 100x3. Density should be constant and something that does not move. The reason this principle is not normally followed in screen printing is because the calibration of the solid densities were ever done correctly to start with.
Also, the solid patches provide a point of measurement you can use to get your densities close to ISO Standards before you make final adjustments in the 100x3 procedure. A spectrophotometer like an i1 rather than a densitometer is used to read the patches and make the initial adjustments. Density is a tool that should be understood as a "repeatability" control. L*a*b* will compare your actual ink colour to the target and allow you to calibrate so you will not need to change densities in most cases once 100x3 and solids are set.
C. Tonal Range Patches
This is the most important point of calibration for 4-Colour Process printing. Not that the solid colours are unimportant but solids can be spot on and the print will be a poor reproduction if you don't get these values correct. The correct values are the standard tonal percentages of the target you are attempting to match. (Figure 3 Typical Standard TVI or Dot Gain)
In figure three you see some nominal values on the red line that will produce a nice looking print. This is a starting point and it is where a lot of printers stop. Printers will then begin to adjust densities for hours and finally accept a low quality compromise to what they really want. So, get these tonal values correct, but you are not finished at this point. You still need to gray balance throughout the tonal range.
D. Grey Balance Patches
One the screen print colour bar your will see patches marked H, HC, HR, SC, S. These are named differently from the common 10%, 25%, 50%, 75%, and 90% to eliminate the potential of a printer thinking that this is the values they are supposed to be on the film. They are not. They just represent the nominal gray values for each primary tonal area. In each patch, the colors are mixed based on ISO nominal values very similar to what you see in PhotoShop or other graphic art creation software. (Figure 4 ISO Nominal Grey Balance CMY Mixes)
This is just a starting point of gray balance assuming you are printing on standard white substrate, with standard TVI and with standard ink colors, and with standard transparency. (In screen printing we rarely print on white substrate.) But don't worry about this. This is just a suggested starting point of reference.The way these patches are used is once the solids are set and balanced, we print the colour bar and measure the grey patches. The percentage of the colors are then adjusted up or down in the tonal values to neutralize the substrate and create an image that has no color cast from the inks or the substrate. This is the most accurate output you can produce and your image will look very similar to a gray balanced colour target or an image on a correctly calibrated monitor. You can learn this methodology through being trained as a G7 Professional or following purchasing and following the ISO TS/10128 Graphic technology — Methods of adjustment of the colour reproduction of a printing system to match a set of characterization data. I highly recommend hiring a G7 consultant to train you in the process. It will be a very good investment and will ensure you are doing the calibration correctly.
E. Substrate patch.
The reason you should use a blank substrate patch on a professional 4-colour process colour bar is that if you use a handheld scanner to scan the colour bar like I do, then the software automatically identifies what colour the substrate is without the potential of making a mistake. The software also will calculate how much colour the substrate has in it and suggest a tonal range correction to neutralize the colour cast. (Figure 5, Colour Cast of the Substrate) If you do not adjust the tonal values based on the colour of your substrate you will rarely produce a similar appearance to what you see on an accurate monitor.
F. RGB patches.
The Red, Green and Blue patches represent the two-colour overprint results. Cyan over magenta is blue, yellow over magenta is red and yellow over cyan is green. The colours we are theoretically supposed to achieve based on all graphics creation software will not be achieved using curves only. Normally the result is acceptable but really not accurate because the process colour simulation of RGB in PhotoShop and other graphic design software by default is simulating offset inks that are not as pure and transparent as our inks in screen printing. In most cases it is not critical to get the solid two-colour spot colours spot on because most of the time in process colour images you are not going to be printing a lot of solid 2-colour areas. If you are, then colour management on top of the grey balanced curves will bring the RGB in nicely. (But that's a different article.)
Be aware that the most important thing in using a professional international standard colour bar in process image printing is to first balance your inks then set your sequence and densities based on grey balance. After that, correct your tonal values to neutral and then diligently scan and record the results. By understanding the standard screen print colour bar and measuring your print and your target you then will be able to make small tonal corrections until you have your process accurate and repeatable.