How to Calculate the First Dot Gain Curve
Dot gain is a phenomenon that occurs in halftone printing. Because papers have different levels of absorbency, on some papers the darker dots in a halftone image have a tendency to spread. This has the effect of reducing the sharpness of the image, as well as making the darker areas even darker, often to the point the image does not look the way it was intended to look. However, this can be compensated for with a dot gain curve, which adjusts the tones of the image in order to adjust for the expected dot gain.
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Instructions
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Edit your image to the way you want it to appear on the print. Your monitor should be calibrated so what you see on the screen is what you are supposed to get on the printed page.
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Convert the image from a continuous tone image to a halftone image using one of the many stand-alone programs or plug-ins for popular image editing programs like Adobe Photoshop and Illustrator or Apple Aperture.
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Print the image on the paper you will use for the final production. Make sure to add a calibration bar to the image.
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Measure the amount of dot gain in the image. This is a simple process, provided you have the right tools. Using a reflective densiometer, measure the tones on the calibration bar and compare them to what they should be. Typically, the intended values will be printed right on the page, but if they are not, you can see what values you are comparing to on the screen where you selected to print a calibration bar.
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Calculate the dot gain in the image. Do this by determining the number of percentage points the printed tones vary from the intended tones. For example, at 50 percent gray, if your printer produced 35 percent gray, you would have a dot gain of 15 percent. This means you need to brighten your image by 15 percent in order to make it look correct.
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Create a curve to compensate for the dot gain. To do this, open the "Curves" panel in Photoshop. Then, take your measurement at the 50 percent mark and adjust the curve so it matches your difference. This means if you have a 15 percent gain, you need to have the curve at the 50 percent mark be at 65 percent. The rest of the curve will fall into place, so you only need to use the one value.
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Print the image with the calibration bar again. Examine it against the screen and test the calibration swatches again. They should match at this point. If they do not, go back to the beginning and remeasure and recalculate your dot gain.
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References
Resources
- Photo Credit halftone pattern, dots image by Suto Norbert from Fotolia.com
