Six-color Overprint Solution for CD Copy (4)

Gray balance

Gray balance is the core and skeleton of separation. When using a two-tone process, care must be taken to maintain a high-level color balance. This can be a trick, but it's not too hard. By limiting the amount of black ink in the separation, neutral ash can be obtained by color overprinting. However, when these color inks are overprinted, the amount of yellow, magenta, and cyan inks must be adjusted. Gray is a pure ash with no obvious color cast. Again, these adjustments need to be done in Photoshop.

In order to achieve CMY balance, it is best to make a neutral gray scale and add it to the test image is very helpful. It usually starts with an RGB image, because Photoshop only applies separation settings when converting RGB images to CMYK images.

To make a neutral gray ruler, use the Rectangle Select tool to make a narrow rectangle. Set the background color to (0,0,0) and the foreground color to (255,255,255). This is the RGB value entered by double-clicking the Color Picker on the toolbar. Next, use the Blend tool to apply the foreground color to the background color fill pattern to blend fills in narrow rectangles. Photoshop produces a transition from white to black in a narrow rectangular strip. Now go to the Image>Adujst>Posterize dialog and set the number of steps to 21 so that you have a level 21 gray scale.

Note that the neutral gray is defined by RGB values, so to test the neutral gray, you must also use the “pipette” tool to extract different points in the image, and see its value in the information column. The definition of neutral gray is R=G= B. For example, (128,128,128) presents a neutral grey midtone color.

Next, the RGB gray scale is attached to the paper area of ​​the color separation image. The bottom of the Kodak Q60 image has a similar grey scale. Interestingly, it is not that the entire scale is neutral gray. This allows the additional scale to be used as a reference.

When converting the color mode from RGB to CMYK, Photoshop calculates and creates a color separation table using the ink values ​​entered in the color settings. This calculation accurately determines the amount of cyan, magenta, yellow, and black that make up the ash balance. Photoshop also decided at which point to darken and start adding black ink. All of these are important because of the need to manually adjust the gray balance in high profile and below 25%. In this particular exercise, the amount of SWOP cyan and magenta was reduced and light green (Lc) and light magenta (Lm) were used instead.

To make this change, first copy the SWOP cyan and magenta channels and name them "light blue" and "light magenta," and specify each channel as a spot channel in the channel options window. You also need to set the color value for the new channel and enter “0” for the field.

To establish the Lc and Lm color values, first create a printed sample of Lc and Lm inks. It is recommended to print using Lc and Lm inks with a density of 50% of the standard cyan and magenta. Using the densitometer, you obtain the L*a*b* value for each color, and enter the value as the channel color (double-click the channel name). Without a densitometer, the value may be adjusted by the human eye until the color obtained in the color picker visually matches the printing effect.

Once these new channels are named, the following curves need to be readjusted for the original Cyan channel and the magenta channel. The goal is to adjust the shaded areas where the standard cyan and magenta fall in the original hue area, while Lc and Lm mostly fall in the high profile and under 25% area. For this sample image, first open the Curve dialog (Image>Curves), move the starting points of cyan and magenta to 25%, so that the standard cyan and magenta below 25% are completely removed. There is no separate adjustment to the intermediate tone, but the curve is automatically adjusted.

For the Lc and Lm curves, turn on Preview and point a node at the 50% dot. Then drag the curve up slowly at 25% mesh until the gray balance of the image looks better. This requires some experience but it is not difficult. After adjusting the Lc and Lm curves, restore the composite image (including all channels) and use Image>Adjust>Selective Color to make the necessary edits to the high-profile and 25% regions. This process is explained in detail in the Photoshop user manual.

After adjusting for overtones and 25% tint, the effect of these two channels on midtones and shadows is reduced by adjusting the Lc and Lm curves and applying a selection of color adjustments. Once all these adjustments are complete, leave the six-color separation setting.

The dark blue and dark magenta versions used in the six-color printing are lighter than the traditional four-color cyan and magenta versions. Note that when these yellows and blacks are color-separated, the meat shades appear to have the opposite appearance, and the high-graded gray scale shifts toward yellow.

(to be continued)