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Obtaining great four-color proces separations from Photoshop is completely possible!

Separations for Four-Color Process Printing

Obtaining great four-color process separations from Photoshop is completely possible, as long as the correct settings are in place. Those that don’t take the time to make these changes to the default values will quickly experience frustrating production runs, chasing color and tone, and eating up hours on press without good results. To increase your chances of success, we’ll discuss all of the starting values necessary to get close enough for most jobs.

Important note

Before we get into the settings, however, there are some very important ground rules. To begin, the settings mentioned herein only work when converting between > Image > Mode > RGB, and > Image > Mode > CMYK. If a customer provides a CMYK file to begin with, all of the default settings will be embedded in that file.

In Photoshop, choose >Edit>Color Settings>CMYK to access the Custom Settings dialog window:

   

The key to great four-color process is to make sure the Photoshop Color Settings are customized to our printing process. All of the highlighted areas will be adjusted for optimum results: 

 

In order to make the file work for screen printing, it is necessary to convert the original CMYK back to RGB and then reconvert to the custom CMYK values mentioned here. In Photoshop, these changes are made under Image > Mode > RGB and then Image > Mode > CMYK where CMYK are our custom settings.

Custom CMYK Separation Settings

All of the custom settings are found under the Color Settings dialog in Photoshop. The actual location of Color Settings varies depending on which version of Photoshop you have and whether it is in use on a Mac or PC. It is found under the File or Edit main menu settings. Around version CS 3, Adobe standardized the location and are now found at the bottom of the Edit menu on both platforms.

Upon opening the Color Settings selection, you’ll see the default CMYK is set to US Web Coated SWOP V2. We’ll begin modifying the settings by clicking on CMYK. As soon as you click on this setting, you’ll be presented with a dialog that allows you to either Load custom CMYK settings, create custom CMYK settings, or save modified CMYK settings.

We’ll begin with the really easy choice. If your ink manufacturer has already made Custom CMYK settings for their ink brand, all you have to do is get the settings from the manufacturer and choose “Load CMYK.” Navigate to the settings file on your machine and choose them. All of the necessary settings will be loaded. Choose “Save CMYK” and you’re done.

If you don’t have the settings file, we can make the changes by choosing Custom CMYK at the top of the CMYK dialog. Once we check this, you’ll be presented with another window where all the custom areas are available. That dialog window will be labeled Custom CMYK.
The first thing you want to do is name the custom file something appropriate. I typically name it based on the brand of ink I’m using and the dot gain profile for that ink. My experience has been to start with 38 percent as the beginning number.

With the file named, it is now time to set the ink colors. This is where we enter the specific ink values for the brand of ink in use. Most of the major ink companies have these values already done and saved but if you want to do your own, you will need to create a test image where solid CMYK are printed as well as the R (y+m)G (c+y) and B (m+c) overprints. If there isn’t a way of actually measuring the color values, approximate them visually.

The default four-color process colors are SWOP Coated and are not accurate for textile use. Most major ink manufacturers have the necessary CMYK values and the overprint values (RGB +White_Black). These are signifi cantly different than SWOP and every ink set is unique. This means you cannot interchange ink colors between manufacturers without adjusting the settings. 

It’s possible to visually adjust the values in Photoshop to match printed ink colors. Here, you can see the L*a*b* values that were entered into the magenta area in the Ink Colors table. Manually adjust the colors in the Color Picker dialog window until the colors in the top box match what is printed. Then take the L*a*b* value and enter it into the Ink Color Table.

Dot gain curves and separation values

Our next set of curves are accessed under > Edit > Color Settings > CMYK SetUp > Dot Gain > Curves. This allows users to set custom dot gain values that correspond to how the printing looks. There are two different options. The first, starting point is to check the box that applies the same dot gain values to all four process colors.

Dot gain is not always the same for each color but, in the beginning, keep the values the same for all. I use 38 percent for the beginning value. Because both the magenta and black tend to have more dot gain than yellow or cyan, I add an additional 2 percent for these colors.

Dot gain is always measured at the 50 percent dot value and the number is an absolute number. For instance, if the 50 percent value of the first color measured is 88 percent, you have 38 percent dot gain (50 + 38 = percent). The magenta and black would be 40 percent, so you would enter 90 percent next to each of those colors. To enter individual numbers, click the radio button for that color and enter the appropriate value. Do this for each color and click OK.

The next set of values are the Separation Options. Begin by choosing Separation Type GCR. This provides more control over how black is created in the separations. It is much more forgiving than choosing UCR.

Custom Dot Gain is essential for textile printers. A good starting point is 38 percent dot gain. To set this value, add 38 percent in the 50 percent box and make sure to check “All Same.” After you print an image, evaluate all the colors. It is typical to have to increase both the magenta and black to 40 percent if they are printing too dark in your image.

 

 

 

 

 

Black Generation Graph
DIAGRAM 1: Here are the settings for the custom Black Generation curve inside the CMYK Set Up dialog. These settings will minimize the amount of black in the final printed separation. Put points on the curve and adjust as indicated.

 

Black generation curve

In the same Color Settings > Custom CMYK > Separation Options dialog, the defaults are: Light, Medium, Heavy and Maximum. We will be choosing Custom. When the dialog window presents, add points to the curve at the values indicated in Diagram 1.

This curve was produced based on many years and thousands of separations for T-shirt printers. It is an average of typical results and represents compensation for too much black in a typical screen printed image.

Shaping the Black Generation curve determines exactly how the black ink will be incorporated into your separation when you go from RGB to CMYK. It will not work on an existing CMYK file. It works once, and only once, at the time of conversion from RGB to CMYK.

This is important for textile printers because the most common complaint about Photoshop separations is they print with too much black.
Dot gain in the black is especially destructive to the deep colors of an image. The addition of black darkens and overpowers the color, forcing deep colors to appear as color-casted blacks. For instance, a very dark blue will appear as a blue-black. Consequently, it’s desirable to reduce the amount of Black Generation to help minimize color destruction. This is effectively a modification of the UCR/GCR function in Photoshop.

The more experience one gains at process color printing, the less the Black Generation will need to be corrected. It is a matter of taste, but Diagram 1 is a good starting point that balances control with color accuracy.

Save the curve and load different values. To find the value that works best, separate the same file with several different Black Generation settings. Print them all at the same time, on the same shirt. Evaluate the image and pick the one that looks the most desirable.

Finally, the last three values to set are Black Ink Limit (100 percent), Total Ink Limit (240 percent), and UCA amount (0 percent). These are relatively simple and the only really critical one is Total Ink Limit.

If it looks like too much ink is being printed, decrease from the above value. The minimum is 210 percent. If there is excessive dot gain, decrease in 10 percent increments until you find the value that works best for you. For most general separations, 240 percent works well.

Halftone settings

Once the separations settings are complete, it’s time to move onto the halftone settings. This can be done inside Photoshop, Illustrator, CorelDraw, or at the RIP you are using to output to your film or computer-to-screen device. The three settings you will be concerned with are frequency (lpi), dot shape and screen angle.

Frequency

The frequency of the halftone determines the number of halftone dots per inch. For manual halftone printing, choose 45 lpi or less. For automatic printing, 55 lpi is a very good starting point. The idea is to choose the lowest number possible in order to control dot gain and reduce moiré. A difference of 10 lpi may not seem like a lot, but we need to think of it as how many additional dots per square inch we need to manage.

At 45 lpi, there are 2,025 halftone dots per square inch. At 55 lpi, the number increases to 3,025, and at 65 lpi, 4,225 dots. The more dots per square inch, the closer together the dots. This makes very small changes in dot gain appear much more severe.

Higher line counts will carry much finer detail, but at the expense of moiré and tone transition. Lower line counts are easier to carry tone and minimize moiré, but not so good at carrying fine detail. Like so many parts of process color, we must take a balancing position between the two.

Dot shape

The subject of dot shape comes up with every discussion of halftone printing, usually evolving into two main subjects—the effect of halftone selection on dot gain and moiré generation. There are many misunderstood and downright incorrect assumptions made regarding common halftone practices.

The choice of dot is determined by how the midtones (where the dot joins adjacent dots at the corners) behave. This is a huge problem for our printing process. When the corners of the joining dots connect, they immediately bleed together. We know this as dot gain. The associated, simultaneous corner growth is referred to as “midtone jump.” The geometry of the joining corners, combined with the high viscosity of screen printing inks contribute to how much midtone jump you experience.

The dot percentage for the dot being formed is determined by the tone value of the image in the area. There are countless combinations of how dot shape is determined; the most common are round, square, diamond and elliptical. The best choices for screen printing are diamond and elliptical, as they are developed to overcome the excessive dot gain of round and square dots.

Not everything is good with these dot shapes. Both are particularly susceptible to moiré.

The flatter dot also creates a more visible halftone pattern. This can be particularly distressing in areas such as a sky gradient or a flesh tone. Careful selection of the halftone angle set can help to minimize these visual disturbances.

Resolution

The formation of the halftone dot shape is controlled by the resolution of the output device—the minimum output resolution must be 16 times the halftone lpi in order to take advantage of all 255 tonal values possible. (I.e., in the case of a 50 line halftone, the minimum output resolution would be 800; 16 * 50 = 800.)

Since most inkjet output is at 720 dpi or higher, this is not too much of an issue. What little tone range is lost at the expense of output resolution is pretty much hidden by other printing factors. Very low output resolution (400 dpi) reduces the number of tone steps you can reproduce.

The real challenge comes when low-resolution devices attempt to render complex angles and transition shapes. In many cases, it’s not possible to form the desired halftone dot. The dot that is formed is misshaped and irregular.

Under magnification, the dots appear to have “nibs.” These nibs are easily blocked by mesh treads, resulting in highly irregular and unpredictable moiré patterns. About the only solution is to use the finest thread diameter you can obtain in an effort to minimize the interference.

Screen angles

There are two schools of thought when selecting screen angles (those that coincide with the mesh threads and the mesh knuckles). The first is to select a single angle for all colors. This is done to minimize moiré and other variables.

The two best angles (mathematically) for moiré reduction are 22.5 and 67.5 degrees. The worst angles for moiré are 0, 45 and 90 degrees as they present the highest likelihood of threads or knuckles blocking the halftone dot opening. The two best angles are exactly halfway between these conflict angles.

The second choice is to select an angle set that will form a rosette pattern when all the colors are printed. The rosette pattern increases the range of color you can reproduce. If you are a real pro, and have critical colors and transitions, this is the choice I would recommend.
The angle choices for optimum rosette formation and minimum moiré are as follows:

Yellow 5 degrees
Magenta 20 degrees
Cyan 80 degrees
Black 50 degrees

If you experience moiré in the black screen, it’s acceptable to swap angles with the cyan; resulting in black at 80 degrees and cyan at 50 degrees.