Grafica News - January 2011
Press accuracy with no proof
By Mike Ruff, Chief Technology Officer, Nazdar Consulting Services
A file may not be accurate to the photography and the customer may not even like it, but the printer has only one responsibility in reproducing a digital file if the client does not provide and accurate proof. “Print the file accurately on press”. But how do you do that?
Question: If I were to give you a digital file and you printed the result of image 1a (See Figure 1a) and your competitor printed image 1b, (See Figure 1b) who has printed the image accurate to my digital file? Your answer is probably, “We have no way of knowing which one is accurate without an accurate proofing target.” But I submit to you that there is a way for you to know you are accurateto the file on press.
The way you would know your print is accurate is to pint the digital file without adding a color cast to the image. A color cast occurs when we add more red, green, yellow or blue through our printing process than what was intended in the original file. Therefore, a color cast is added when we do not print in balance. (See Figure 2, Color Cast Example) In this short article I will address a simple way to ensure and prove that you are printing in balance and accurately.
Starting with a calibrated monitor
A calibrated, neutral gray, balanced monitor will display an image very accurately. If the monitor is neutral, it will not be adding a yellow, red, blue or green cast to the image. This is the primary goal of a calibrated monitor. Neutral gray calibration is what drives accuracy to the file. If we trust our monitor and we know it is not adding a color cast to the image on the screen, we will have confidence that what we see on the screen is very similar to what the client submitted. The reason for our confidence is we know we are viewing the file in a neutral condition and our monitor is displaying an accurate image. A good first monitor test is to just look at a good neutral gray image on the monitor. (See Figure 3) RGB neutral is equal Red, Green and Blue. An example is midtone gray is 128 R, 128 G, 128 B. If I make myself a calibrated print of this midtone gray value and the absolute densities of the gray sample print are balanced, I can compare the neutral gray of my proof to seeif I have a color cast on the monitor. If you see a color cast, recalibrate. Gray calibration will be the primary part of all calibration software. Once your monitor is neutral, what you see is very similar to what you should produce on press except for the deep saturated colors of RGB that are out of your color print gamut. Now that our monitor is neutral, if we print to neutral on press the image will have a similar appearance to the neutral monitor.
Moving Though the Workflow
As a digital file moves from the computer to the film or direct to screen, again the important objective is to maintain neutrality. We do not want to add a cast to the file by imaging screens with the tonal values out of balance.An example of what might go wrong in screen making is, if I were to use the wrong mesh on one screen or over or underexpose a screen, then I would not have a balanced print. No matter how good of a printer I might be, I could never overcome an out of balance set of screens. One color will dominate the others and I would get a color cast. The objective is to have screens that are in balance and a repeatable process so I can print in balance.
Printing in Balance
Printing in balance is a very easy thing to do if you have carefully managed the workflow. But a printer that does not understand printing to neutral can still mess it up. Printing to neutral is just printing C, M and Y tonal values that result in an equal response. (See Figure 4, Balanced 3-Color Gray Densities). This can be measured using a simple densitometer set to absolute density. You DO want the value of the substrate in the density reading because we want to neutralize the substrate by managing the tonal values. Do not use the minus paper setting.
If I have an in-line press, it's much easier because I can print the C,M and Y in a quick succession and I then just take a measurement of the gray value. The numbers should be equal. For example the midtone gray should be close to.59 Cyan, .59 Magenta and .59 Yellow. (Neutral) The gray would look natural and neutral. The process image I am printing will look very much like what I saw in the monitor. (See Figure 5, Balanced Midtone Densities)
If I use a one color press, I simply print the cyan first. If my final target is .59 density of cyan in the gray patch then I need to print it a little lower knowing I am going to pick up some cyan from the contamination of the other colors, magenta and Yellow. So I print the Cyan to about .50 density. (See Figure 6, Midtone Cyan Density) This is not “dot area”, “dot gain” or “TVI”. This is absolute density. This includes the color of the substrate.
Next I print the Magenta. When the magenta is printing on top of the cyan in the gray bar I want a balanced Cyan and Magenta value. For example my absolute density may now be Cyan .50density and Magenta .48 when I read the two color gray. (See Figure 7, Cyan Magenta Balanced) When I add the yellow the goal isto have an exact balanced CMY response when I read the CMY tonal percentages. Normally, the three color gray won't be exact but if I'm within a three (3) point spread between density values, it looks great and I will be producing an accurate image. (See Figure 5 Balanced Midtone Densities). If my goal is to print C=.59, M=.59 and Y=.59 but I get C=.57, M=.60 and Y=.58,it is going to look good. If I get C=.55, M=.57 and Y=.54 it will look good. But ifthe density spread gets more than 4 points off I begin to see a slight cast and I then know the image is not balanced. I can normally just adjust the press slightly to move the CMY values closer,If I keep the gray patches balanced and I print at solid densities close to standard, I know my result will be accurate to the file.
I hope this example of printing accurately is a help to you in understanding what printing accurately to the file means. Remember, this does not mean the client will like it. It just means you printed an accurate print of the digital file.
Nazdar Consulting Services
We are grateful to Mike Ruff, the Chief Technology Officer of Nazdar Consulting Services, who is kind enough to support us with his technical articles in Grafica News which will greatly enlighten our readers.
Mike Ruff is chief technology officer for Nazdar Consulting Services, Shawnee, Kansas City, KS. His experience in the graphics-printing industry spans more than 40 years. Ruff is a certified G7 Color Calibration Expert and a regular instructor at the SPTF Graphics Four-Color Workshop in Fairfax, VA, and he has authored numerous articles published in trade publications domestically and internationally.