Screen Frequency / Resolution

Resolution can be confusing, even for people who have been working in the trade for a long time. This is largely due to the fact that, in digital technology, resolution has to be considered in both the input stage (scan) and the output stage (Imagesetter or laser printer). Understanding these two resolutions is vital to the process of getting high-quality images.

Halftone Spots
Halftone spots are the result of turning off and/or on the dots in a halftone cell. This is how an imagesetter creates variant dot sizes.

Screen Frequency (Line Screen)
The number of rows or Lines Per Inch lines of spots in a halftone image within a given distance (lpi). Screen frequency effects the size and sharpness of the halftone spots the imagesetter will render. Choosing the correct screen frequency will depend on such things as the type of paper it will be printing on and the amount of dot gain expected on the press.

Resolution Specifications

LPI	Imagesetter Resolution
133 LPI	2400 DPI
150 LPI	2400 DPI/3000 DPI
175 LPI	3000 DPI

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Advance Scanning

What makes a good scan? In order to ensure a high quality reproduction, the initial scan must capture as much detail as possible. Check your scan against your original image to ensure that you've maintained as much detail as possible. This can take several rescans. You should also expect to do some tonal manipulation in Photoshop or other image editing program. Tonal manipulation is the key to setting the right contrast, overall tonal range and getting the most detail possible within the limitations presented by the paper and printing choices. See below for the tonal range specifications for specific paper stock types.

Screen Angles
When working with multiple colors that overlap, such as duotones or CMYK, it is necessary to angle the halftone screens. To avoid moire patterns, angle the screen 30 degrees apart.

Tonal Range Specifications

Stock	Shadow Dot	Highlight Dot
Uncoated Stock	80-87%	3-5%
Matte Stock	87-92%	5-7%
Gloss/Dull Stock	92-97%	7-10%

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