Halftone Scanning

Halftone scanning evolved because someone discovered that they could scan photographs from magazines. While I wont get into the legal issues here, suffice it to say that there will be a legal problem with using any pre-published images. Assuming that you have the permission to use a pre-printed piece, dont -- ask for the original photograph instead.

Halftone scanning is essentially treated like a greyscale scan by most software and the conventional wisdom says to scan the image at a resolution lower than the original in order to reduce the threat of moires. Well, about all you can do is reduce the number of moire patterns that you will have, not reduce the threat. If you scan a halftone, you will have moires. Without getting too technical, the reason for this is that all halftones generated by a press have irrational screens and inconsistent angles applied. Irrational screens and inconsistent angles are used to increase the quality of the printed piece for the type of press being used, they are called irrational and inconsistent because in the case of the press, they are adjusted mechanically by the press operator (If they were generated by a high-end four-color system--Scitex, Chromacom, etc.--the irrational screens were computed elaborately). To properly eliminate potential moires your scanner would have to adjust its resolution, tracking (pickup alignment), and linearity (the range of grey scales) dynamically across the entire image. Elaborate filters such as PhotoShops Despeckle filter is designed to take out these moires, but they do so by slightly defocusing the image and blending the pixels which effectively lowers the quality of the image.

Granted, this may not be a big deal if all you intend is to scan the cover of SIs swim-suit issue to embarrass your colleague (of course, I would never advocate this type of blatantly criminal activity), but assuming your goal is publication quality work, Halftone scanning should always be used as a last resort if at all.

Halftone Screening

Halftone scanning should not be confused with halftone screening. Sound confusing? Halftone scanning is scanning an original from an already screened source such as a magazine, coffee table book, newspaper, etc. However, many scanners now come with plug-ins, or custom software that has an option labeled Halftone. Check your manual about this, but most of these options are for creating a custom halftone screen of the image being scanned not for scanning a pre-screened halftone.

As an example, Microteks Adobe PhotoShop plug-in has a halftone option that allows you to screen an image to a number of preset patterns as you scan the original. This is important because Microtek, and most other scanner makers, have done their homework on their scanners and know how to generate a halftone screen that will best take advantage of their machines capabili- ties with your chosen output device. Another advantage is that once the scan is completed, you dont have a large grey scale file, but rather a simple 1-bit black and white file that approximates the grey range of your selected output device and takes up only a fraction of the space that a grey scale would.

The disadvantage to this, is that your screened scan is now appropriate for really only one output device. For instance, If you perform a halftone scan with the Microtek plug-in set for a 5x5 60 dot #1 pattern (the recommended pattern for the Apple Laserwriter IIf photograde) it will look great when printed to the Laserwriter, but appear like a strange rippled newspaper photo when printed at 1200dpi on a Linotronic.

Halftone screening a scan is not entirely without merit, however. Many manufacturers will give you some screen patterns (such as horizontal or vertical lines) that can create some very artistic effects regardless of output device.

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Compiled and maintained by Jeff Bone, © 1993-1997, All Rights Reserved.