What Is A Gray Bar?

What Screen Areas Should my Newspaper Use?

Important Considerations When Using A Gray Bar

Use Of The Gray Bar In The Pressroom

Good color reproduction in the newspaper industry requires consistency in the pressroom. This is the area that has the largest affect on tonal reproduction (dot gain) because of all the components necessary to transfer the image from the plate to the substrate (newsprint). Variables such as newsprint, press settings, blanket height and maintenance, ink formulation, and ink film thickness have a major impact on dot gain.

This paper focuses on ink film thickness and how to control it using a gray bar. In the newspaper industry all colored pictures are formed with screens of the three process colors and black. Once the screens have been set for the picture, the amounts of ink used must be controlled to get the proper reproduction off the press. If the ink film thickness is not controlled the color cast, color saturation, and detail in the picture can be lost. The ink film thickness is monitored with a reflection densitometer. In order to use the densitometer effectively a control device must be present in all columns where color is printed and this can be done by color bars.

Color bars containing solid ink density patches that are typically used in commercial printing can not be used in newspapers due to the trim area. Commercial presses place the color control bars in the trim area. Thus, after printing, these color bars can be cut off and not seen by the consumer. In newspaper printing there is no trim area. Color bars would, therefore, be left on the printed copy and detract from the print quality.

In order to control color reproduction, newspapers must utilize other methods to control the print density. The development and use of a gray bar for density control for newspaper will be discussed below.

What Is A Gray Bar?

When the three process colors are printed in the proper ratio a shade of gray is produced. (see What is Gray Balance Volume VI for more detail). A gray bar is the proper screening of the three process colors to obtain the desired level of grayness. Cyan is always the highest screen in a gray bar because of the inherent impurities in the pigments that are used in the process inks. Some examples of typical gray screening combinations are indicated in the table below.

SHADE OF GREY	PERCENT SCREEN AREA
	CYAN	MAGENTA	YELLOW
Midtone	40	30	30
Quartertone	25	18	18

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What Screen Areas Should my Newspaper Use?

The use of a gray bar in the newspaper must be transparent to the reader and advertiser. It should not detract from the reproduction on the page where it is used. A test form (such as SNAP, GATF, or NAA) should be run on the press that includes two gray bars so that the editorial department will have samples to look at and can make a decision on which gray bar screening is acceptable to them. Many newspapers have gone to the quartertone gray bars. It is felt that the quartertone gray bar is more sensitive to variation by the eye, yet still yields high enough density readings to be considered accurate (by the densitometer.) During this test run the solid ink densities are set to industry specifications such as SNAP, GATF, NAA.

Dot gain is determined from this run so that proper compensation can be made in the prepress area. The dot gain is very critical to the proper use of the gray bar. If dot gain increases, color saturation decreases at the same density specifications. What this means if the dot gain increases, a higher density would be apparent in the gray bar if the solid ink density was set correctly. If the density of the gray bar was reduced to standard levels, in actuality the solid ink density levels would be reduced.

Once the gray bar is selected from the test form, the reflection densitometer density specifications for the selected gray bar can be determined by reading the gray bar in the same column where the solid ink densities are in specification. Some examples of density specifications for various gray bar screenings are listed in the table below:

Gray Bar Screens				Density Specifications
C	M	Y	K
25%	18%	18%	0%	0.52 +/- 0.05
40%	30%	30%	0%	0.65 +/- 0.05

It should be noted that values above should be used only as a guide because they are dependent on the dot gains for each color on the press. Modifications based on the press conditions may be necessary.

Gray bars and color bar targets should be large enough to permit measurement. A target height/width or diameter of 3/8² is recommended so that proper measurements can be made. Gray bar targets do not need to be continuous and can be creatively designed across the width of the page.

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Important Considerations When Using A Gray Bar

Listed below are some critical points that must be adhered to when using the gray bar.

• The imagesetter/film processors must be checked regularly to insure that they are outputting the proper film screening for the gray bar. This can be checked with a transmission densitometer.
• The width of the gray bar should be as wide as the target window for both the transmission and reflection densitometers to insure proper readings.
• Densitometers should be checked for proper calibration daily.
• The print density specification range for the gray bar has to be within the density variation capability of your press.
• Always make gray bar density corrections on press from dark color to light. This should be done because the cyan and magenta pigments contain components that effect the yellow component of the gray bars. If the yellow is adjusted first, it would have to be reset after the other colors are brought into adjustment.
• Always take readings on the same position of the gray bar and same plate position (high side or low side) to minimize density variations due to impression on the press.

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Use Of The Gray Bar In The Pressroom

When starting up the press the following steps should be taken when using the gray bar:

• Get page in register
• Get gray bar balance across the page by eye
• Check the gray bar with the reflection densitometer
• Adjust the density if necessary, darkest color first
• Recheck density after a few minutes in the same position and adjust if necessary

The gray bar can be a powerful tool that allows you to have good color reproduction and consistency if it is used properly.

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