Set up and tools for digital color separation
Dot Gain and Color Profiles
Making the effects of printing visible on a computer screen is the key to creating complex color separations. For CMYK separations, information about the printing conditions is handled by the Color Profile. For Spot color channels, print information is held in each channels Color, Opacity and Dot Gain Settings.
Dot gain is a measurement of the increase in size when a dot is printed. The gain is caused by the spread of the ink beyond the edges of the printing stencil. The amount of gain depends on the type of printing and the materials used. A single Dot Gain number refers to the gain amount in the 50% value range. For instance; 40% Dot Gain means a 50% dot is printing at 90%. (50% dot + 40% gain = 90% total). The best approach to finding your specific dot gain is to measure the actual dot gain from your print set up.
The Working Color Profile
The default color spaces used in Adobe Photoshop are determined by the working space color profile. RGB and CMYK both have working space profiles, and combined with dot gain settings and color management policies, can be set up to manage both the input and output color spaces.
The Color Settings dialog
The settings for handling Color Profiles and Dot Gain are found in the Color Settings menu. The Color Settings determine the programs default profiles and behaviors for the various color modes.
To change the color settings select:
Edit / Color Settings.
In the color Settings menu :
• More Options: Select to display advanced settings.
Working Spaces: are the default profiles and dot gain settings for the color modes.
• RGB: Color Profile Setting for the RGB mode. Use the drop down to choose Adobe RGB
• CMYK: Color Profile Setting for the CMYK mode. Choose Web Coated SWOP, or load a custom screen printing ink profile.
• Gray: Select the same dot gain value as selected in the Spot color settings.
• Spot: The spot setting is a dot gain percentage applied to spot color channels. Use the drop down menu to choose a custom setting.
The Spot setting
The spot setting controls the Dot Gain used for all Spot Color channels. Adjusting this setting changes how spot color channels appear on screen, it does not change the actual pixel information. For screen printing, the Custom option is used to enter a Dot Gain setting. A custom Dot Gain Curve can be entered representing actual dot gain from a printed sample.
• Be sure the More Options button is clicked (in older versions look for the Advanced Mode check box).
• Select the Spot: drop down menu and choose the Custom option.
Entering 85% in the 50% box will display a 35% dot gain on screen. Changing this setting will not change a separation, but can help show how it may look when printed on a manual or automatic press.
In addition to the press type used, other factors that affect dot gain in screen printing include stencil thickness, tension, mesh count and off contact, squeegee pressure, angle, durometer, and edge shape, ink viscosity and sheering qualities.
Color management Policies and Conversion options, choose:
“Preserve embedded profiles” for all three modes.
• Profile Mismatches and Missing Profiles: Check “ask when opening” for all.
• Conversion Options: set Adobe (ACE) for engine and Perceptual for the intent. Leave Black Point and Dither checked.
Setting up the Channels
Four main factors affect the way screen printed inks appear on press. Three of these, Hue, Saturation and Value are the qualities of color discussed in the last chapter. The forth factor, Opacity, is a quality of the inks used in printing.
An inks opacity affects the ability of colors to blend with each other, as well as how much substrate shows through the print. The more opaque an ink is the better it will cover a dark garment, but the less it is able to blend to make secondary colors.
Primary colors like CMYK use transparent inks. This allows for a large range of color blends, but only over a solid white base.
The more a separation relies on primary colors, the more transparent the colors need to be. If we increase an inks opacity to print over darker substrates our ability to blend colors on press is diminished.
Translucent color channels
CMYK channels are always completely transparent and are assigned values based on their color profile. Spot Color Channels, however, allow us to control their color and opacity directly. With spot color channels an inks opacity can be changed to allow some or all of the inks below to show through. This allows for a semi-opaque, or translucent color channel. Semi-opaque inks provide better coverage over dark garments while still allowing for limited color blending.
Spot Color Options
By double clicking on a channel you can open the channels options dialog. The channels options dialog displays inputs for the channels Name, Color and Solidity. For Alpha channels there are also the options of converting to a spot color channel and changing the selection mode.
Opacity / Solidity
The option for adjusting a spot color channels opacity is called “solidity”. This setting is entered as a percentage, and affects how much the colors printed beneath this color will show through.
Opacifiers such as white ink mixed into a color can affect the solidity setting, as well as transparent components such as extenders or reducers. Solidity settings can range from 0% for process colors, 5% to 30% for spot colors with extenders, and 80% to 100% for under base whites and opaque ink colors. Printing your inks and comparing them to the on-screen image can help to determine a setting that reflects your print results.
Setting the Colors
Clicking on the Color box in the channel options dialog opens the color picker. The color picker provides a way to choose colors using one of four color modes, a hexadecimal number, or with a color library for Pantone® spot colors. With the color picker we can find the Hue, Saturation and Brightness for channels in a color separation and select the best spot colors to match.
On the lower right side of the color picker are settings for the color models HSB, RGB, Lab and CMYK. On the left side is a color selector consisting of a slider bar and a field. For direct control of the Hue, Saturation and Brightness, for instance, select one of the HSB radio buttons.
• The button checked determines the color mode used and the specific component controlled by the vertical slider bar.
• The remaining components of the color mode are displayed in the color field.
Looking at the HSB buttons, select the H button. This changes the color bar to adjust the Hue of the color. When blending colors, the specific hues used can have an effect on the colors available by mixing on press.
Shifting a hue towards the color it is blending with will improve the transition between the two colors.
A greenish blue, for instance, will blend nicely into a green or yellow ink. This same blue however, may not work so well when blending into a red color. If a color needs to blend with opposing primaries, it is best to find the purest hue for that color. If a blue, for instance, needs to mix with both red and green, the blue’s hue should be placed right in the middle.
Looking at the HSB buttons again, choose S. This will set the bar to adjust saturation. As the slider is moved down the saturation is decreased. As our colors become less saturated our ability to mix pure colors becomes limited.
Reducing an inks color saturation reduces its ability to blend secondary colors, but can help by increasing the overall ink deposit.
The trade off here is in print density, a fully saturated color will need to be printed as a halftone for most images. Reducing the saturation to match a dominant color allows the ink to be printed with 100% coverage in the brightest areas.
The closer to 100% coverage an ink is, in the the most color saturated areas, the cleaner the resulting print will appear.
Looking at the HSB buttons, choose b. This will change the vertical bar in the middle to represent the brightness of the color chosen. As the slider on this bar is moved down the color becomes darker. As our colors become darker our ability to blend bright colors becomes limited.
The brighter and more saturated a color is the more black may be needed, depending on the image.
Printing too much black tends to muddy and an image, so it is best to print as little as possible. If we darken our colors to match the predominate values we can reduce the amount of black needed.
The Color Libraries
To convert colors to printable ink formulas, click on the Libraries button to display predefined color sets including the ®Pantone color books. Most major ink manufacturers provide ink formulas and mixing components for reproducing the ®Pantone solid coated color book. By using a color Matching system for ink mixing, specific colors can be selected and mixed as needed.
Tools for Color Separation
Adobe Photoshop’s tools for color separation are used for measuring, creating and adjusting channels. The basic tools are:
• The Info Palette -Used to Measure
• Color Sampler -Used to Measure
• Levels – Used to Adjust/modify
• Curves – Used to Adjust/modify
• Calculations – Used to Create/modify
• Color Range tool – Used to Create/modify