Screening/Halftoning - II

There are two ways that this halftone screening equivalency is usually measured.

One is equivalency of detail rendering - the ability of the screening to render image detail. The other is lithographic equivalency - how they perform on press lithographically. Note that in both cases, because the respective screening technology is so different, equivalency can only be an approximation.

Equivalency of detail rendering

Since halftone dots form the printed image - more dots per linear inch translates into more detail that can be rendered.

With an AM screen the detail rendering ability is specified in lpi (or lpc) - i.e. halftone dots per inch (e.g. 175 lpi or 60 lpc).

Since an FM screen has no "lines per inch" determining the equivalent detail rendering equivalency is usually done by drawing a line through the FM screen and counting how many dots are intersected (crossed) in a distance of one inch.

For example the distance measured is 1/16th of an inch. In that 1/16th of an inch approximately 36 dots are intersected. So, in one inch about 576 dots would be intersected (16 x 36). Put another way, there are 576 dots per linear inch - 576 lpi - to render detail, i.e. this FM screen is equivalent to a 576 lpi AM/XM screen.

Lithographic equivalency

Lithographic equivalency is a bit more complicated to figure out. It is usually measured by counting the number of edges (transitions) in a square inch.

Comments

Post a Comment

Popular posts from this blog

Packing and Cylinder Pressure

PRESSMAN TIPS  Packing and cylinder pressure seem to be the number one culprit in printing related problems. It is absolutely necessary for the press operator to check and maintain proper printing pressure between plate, blanket, and impression cylinders. Press manufacturers should be able to supply this valuable infor mation. Most of the time the packing information is located on a plate on the press or in the operating manual. How much pressure is too much? Most operators feel that is .004" squeeze is good, then .008" must be better, and if that doesn't work then an extra .002" - .004" will certainly help. This will without a doubt effect quality and production. Most presses should operate with .004" - .006" squeeze between cylinders. Any more than this can cause problems like dot gain, emulsification, bearing shock marks (streaks), poor register and fit to name a few. It would take several paragraphs to explain how each one of these problems ...
Read more

Pre-Press-Color halftone & Screen angles

In reproducing colour pictures that were originally continuous tone, printing requires us to overlay the CMYK images as halftones. If the halftones are AM (amplitude modulated), they will be formed from regular patterns of fixed frequency. The need to rotate the angles of the halftone patterns comes from the fact that printing cannot precisely plac e down one halftone dot on top of a previously printed other-coloured dot. Commercial-quality colour printing typically has 60 dots per cm in both the horizontal and vertical direction. Slight inaccuracy in placing four identical patterns on top of each other will result in an unpleasant moiré or screen clash. By rotating the screen patterns at 30° from each other it is possible to reduce the frequency of the moiré to a level which is below the visual threshold – it becomes too small to be obtrusive. That reduced moiré is the pattern which is commonly called the printing rosette. As the halftone patterns are crossed lines at 90°, we only hav...
Read more

Ink Mileage

Ink mileage is usually defined as the number of copies produced with a kilogram of ink, or more commonly quoted in the newspaper industry as the kgs of ink required to print a thousand impressions on paper. . Estimating ink mileage has always been a difficult task. This is because there are several interest ing factors that control the amount of ink required to achieve the final ink density. The factors that most influence the ink requirements are as follows: 1) Strength of the ink 2) Substrate 3) Print Density Control 4) Ink and Water Balance 5) Print coverage Strength of the ink The key factor here is the level of pigment in the final ink formulation which determines how much ink is required to achieve density. The pigment is the ingredient in the ink that either absorbs or reflects light. When ink is printed on paper, what we are trying to do is to change the light reflection off of the paper so that the reader perceives color. The pigment type has certain characteristics to ...
Read more