An ICC characterization profile determines how the print system separates the color values of the source color space onto its channels. Black Start is one of the central parameters of black generation. This parameter defines the tone value at which the separation first uses the black channel. Below the defined Black Start, neutral tone values are exclusively built from the process colors cyan, magenta, and yellow. Only above this tone value is black added to the separation – this increasingly replaces the neutral components of the process colors.

Black Start determines the threshold at which the black channel enters into the separation. The other black-generation parameters, especially Gray Component Replacement (GCR), determine how black increases beyond that threshold. In classic CMYK separations, Black Start affects only the use of the black channel. Separate separation strategies control any additional process colors in an extended color configuration, such as orange, green, or violet.

Figure 1: Principle of a Black Start along the gray axis (schematic)

Black Start ranges from 0% to 100% and defines the tone value at which the black channel first enters the separation:

• Black Start 0% – the black channel is already inserted in the lightest tone values. The black curve begins at zero. That means that even the highlights can contain small amounts of black.
• Black Start 30% – tone values up to 30% consist exclusively of Cyan, Magenta, and Yellow. From a tone value of 30%, the black channel is added to the separation and rises continuously toward the shadows, following the defined black curve.
• Black Start 100% – the black channel is virtually unused. The tonal range is built nearly entirely from Cyan, Magenta, and Yellow.

Figure 2: Black curves for different Black Start values (schematic)

You will find the Black Start [1] parameter in a Profiling Project, in the Separation Parameter tab of the Create Characterization Profile step.

Figure 4: The options in the Separation Parameters tab of the Create Characterization Profile step

The parameter determines the tone value at which the black channel enters the separation. Values range from 0% to 100%. A higher value means the separation builds highlights and light midtones from Cyan, Magenta, and Yellow over a wider tone value range. As a result, the black channel enters only in darker tone value ranges.

Shifting the entry threshold controls the tone value at which black halftone dots appear in the printed image. At the same time, black start affects the range in which GCR can take effect, because black becomes available to replace the neutral components of the process colors only from the defined entry point.

The following recommendations apply when using this parameter:

• Black Start 30% (default value) – a black start of 30% balances image quality in the highlights, process stability, and ink consumption. The highlights consist exclusively of Cyan, Magenta, and Yellow, giving them a smooth, homogeneous appearance. From the midtones onward, black enters the separation. This reduces the gray balance's dependence on process-color variation and allows GCR to take effect.
• Black Start 0% (early black entry) – at a value of 0%, black already enters in the lightest tone values. This can reduce process-color consumption, because black replaces the neutral components of the process colors early. This approach is common in ink-saving profile strategies. Note that black halftone dots in light image areas create high contrast against the substrate; depending on screening technology and print resolution, this can make the highlights appear grainier.

• Black Start 60% or higher – at a high black start, the separation builds highlights and light midtones from cyan, magenta, and yellow over a wider tone value range. The finer process-color halftone dots can create an especially smooth, homogeneous image impression, for example in light gradients, skin tones, or large neutral areas. Process-color consumption increases however because black is unavailable to replace neutral components in these areas. Note that the process colors alone then create the gray balance in the highlights and midtones. This leads to the fact the the gray balance is more sensitive to variation in the print process. Additionally, the tone value range in which GCR can take effect is limited

Note that you can also set the Black Start in other areas of the Create Characterization Profile step.

• In the Gamut Adjustment tab, the Use Black Start even for Pure Black option also applies the entry threshold to the generation of pure black.
• Black Start in the Spot Colors tab sets the black channel's entry threshold for separating spot colors. The calculation here follows different logic, as the average of all CMY percentage values is used to determine Black Start.