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Following the introduction to halftones, we can now proceed to learn the complex association between the desired screen ruling and required output resolution. If you have not read the Halftone Screens pages, then please do so first.
|CELLS & Dots
Digital halftone dots are constructed within a Cell or defined grid area, and each Halftone Cell is the same size as the others. Output devices that use a laser for the creation of our output images also work with a defined grid where the entire page is divided into areas like pixels in a bitmap (sample on the left), except these tiny virtual areas in an output device are called Dots.
Therefore we can have Halftone Dots and Output Device Laser Dots. Remember these very different but correct references.
|Various techniques are used
to direct the laser light onto the film emulsion, but
shown on the left is one where the laser beam is directed
by a rotating mirror.
The film is advanced by the equivalent of the device resolution each time the mirror completes a scan across the width of the film. As the mirror scans across, the laser beam is either switched on or off in sync according to laser resolution, so producing (or stepping over) the tiny image dots of the mapped array (raster).
Your simple text and line images (solid colour) will be constructed in a fashion similar to that shown by the top image.
The ideal Halftone cell has a grid matrix of 16 X 16 = 256, allowing up to 256 different halftone dot sizes. The small animation above shows this, only on a 5 X 5 cell array.
A defined 'Grey Level' is one of the different values of halftone dots, 100% to 0% and any value in between, that can be output on a Laser Output Device.
To ensure that the maximum number of grey levels can be obtained at output for a single channel of a Black to White or CMYK image, we must be able to use the 16 X 16 Cell, at the required Halftone Screen Ruling, at an Output Device Resolution that will allow this.
Dot size calculation
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