Regarding problems relating to moire patterns, lets consider the challenges related to scanning previously printed items which consists of halftone dots. As is seen (above) in the foreground road and lawn areas there is a problem with this scanned image of a color printed postcard. Below is another example from a different scan of the same item.
When scanning printed items there are two different patterns that mostly likely will conflict with each other. One pattern is the rows of halftone dots on the item being scanned. The extreme closeup (below, left) of the postcard reveals the halftone dots that are being used in the printing process. The second pattern is with the rows of pixels that the scanner creates (below, right) as it scans the item and assigns colors to each pixel. Chances are great that the two patterns will conflict with each other creating an unwanted moire pattern. (For further description, see earlier post on Moire Patterns.)
When scanning, these small halftone dots are actually the subject matter to be most concerned about (not the building, the sky, the lawn, etc.). The goal is to try and scan the dots accurately. To do this, higher settings are needed for resolution due to the smallness of the dots. The example shown (above, left) was scanned at 600 pixels per inch.
Aside from scanning at higher resolutions, another trick is to try scanning the item by laying it on the scanner at different angles. Since the unwanted moire is coming from a conflicts with two different patterns of rows (of dots, and of pixels), sometimes the difference between the angles will make matters worse, other times it will make things better. This generally will work (trying different angles) more successfully with items that were printed originally in only one color. Multi colored items will consist of different halftone angles for each of the different colors being printed making it a bit more challenging to find an angle for scanning that does not conflict with any of the different colors.
For judging quality of the scans, here is another example (above). The scan (left) captured the halftone dots reasonably well. It was scanned at a higher resolution and at an angle. The scan (on the right) captured none of the halftone dots, and consists of a pronounced and unwanted moire.
A note of caution: When judging scanned images for quality, this really needs to be done only by zooming in close enough for seeing the actual halftone dots. This is to avoid yet another instance where moires can occur. The computer being used for scanning and viewing your images, has a pattern all of its own built into the display. Even with a good scan of an item, with good capturing of the printed dots, when viewed at different zoom sizes, you will see different odd looking moire patterns appear on your screen. The odd (and cool looking) pattern seen in the above example is only due to the size I was viewing it on screen. It was due to a conflict of patterns with my monitor. If I were to zoom in close though, the quality of the halftone dots would be evident.
Now that quality dots have been scanned, the next step will relate to what we now do with them to avoid a problem that can occur during print production. Subsequent posts will related to how to refine these types of images for production.
In spite of the problems and challenges that come from these patterns, I do find them to be fascinating and fun when found in the "real world." This comes to mind almost every time I travel the PA Turnpike and drive under one of the many bridges with large screened fences on both sides of the road above. Between the motion of my travel, and the conflicting patterns of the two screens above, I get to watch a constantly changing and moving moire.
Photo information: The color postcard is of Lamberton High School, Carlisle PA. The second photo is the interior of the "A Street Church" (home of Carlisle Brethren in Christ Church from 1920-1952) scanned from a dedication booklet published by the Planning and Development Committee in 1972.~~~