In order to reproduce different shades of gray, the printing technology uses the screening technology. A dot matrix is the most economical way to produce shades of grey.


Screening-matrix
of an image point simulating grey through single image setter dots

Fig. 2	An image dot from the scanner is transformed into a screening matrix (in general a 16x16-Matrix). If a screening dot is black, up to 256 image-setter pixels can be set in a screening cell. In a screen of 152 lpi there are 152 screening dots in a line. The unit of measure lpi (lines per inch) is often confused with printer resolution. The unit of measure for printer resolution is generally dpi (In some countries, lpcm (lines per centimetre) is the unit of measure for the image setter's resolution as well as for the screening frequency). Once again the units of measure: Printer resolution: dpi /dpcm (lpcm) Screening resolution: lpi /lpcm The images on the left show the effects of different scanner and screening resolution. Image 1 shows a normal scan with 220 dpi printed on a laserprinter with 120 lpi. Image 2 shows an enlargement of a selection. Image 3 shows a scan with very low resolution (under 72 dpi) printed with 120 lpi. Image 4 shows a scan from image 2 printed with a line screen of only 20 lpi.
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