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Prepress/Scanning & Repro
 

There are a number of things to be considered when preparing originals for scanning, we have produced a few guidelines to assist you with this task.

Calculating the scanning resolutions of photographs

Minimum scanning resolution is calculated as 1.5 x screen ruling at output e.g. if screen ruling on output is 120 lpi (lines per inch) the minimum scanning resolution at 'same size' would be 180 dpi. (Dots per inch). The screen ruling is determined by the paper you will be printing on, in general cheaper uncoated papers will be printed with a coarser screen ruling to retain detail. Below is a guide to screen rulings for our stock papers.

Bookwove/Opaque 120 lpi
Fineblade 133 lpi
Coated stock 175 lpi

Scanning at higher resolutions will not in any way improve the amount of detail captured by the scanner but will only increase file sizes.

Line work should be scanned as 1 bit TIFF’s with a resolution of 1200 dpi recommended.

Compensating for dot gain in greyscale images

Halftone images are created by halftone dots of varying sizes creating different shades of grey giving the illusion of a continuous tone image. on press these dots of ink are absorbed by the paper spreading into each other, causing the image to darken. this effect is known as dot gain. Porous, uncoated stocks tend to exhibit more dot gain than coated stocks.

All greyscale images should be adjusted for dot gain before using them in your layout. Using the 'eyedropper tool and 'Info dialog box' in Photoshop with 'curves' and 'levels', you can adjust your images 'by the numbers' rather than relying on your screen preview.

When establishing minimum and maximum highlight and shadow dot we require a minimum 3% highlight and maximum shadow dot of 95%. Adjust the picture levels first to get the contrast right then apply the curve. As a starting point for your curve set the 20% to output at 17%, 50% to output at 35% and 80% to output at 70%.

Remember that these are rough guidelines and you may need to make further adjustments depending on the tonal range of the scan.

Avoiding moire patterns

Moiré Patterns are one of the more annoying things that can occur when working with scanned bitmap images. They are most commonly associated with scanned printed originals.

The physical nature of moiré lies in the interference pattern created when you superimpose two dot grids with a regular structure i.e. the pixels created by the scanner and the halftone dot pattern in the original.

Many modern scanners even quite low end ones have some kind of descreening facility which should remove this effect however if you do not have this facility then you can download a pdf document from here detailing a method which can be employed using Photoshop to remove moiré.


 
   
 
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