In order to reproduce different shades of grey, 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 measurement lpi (lines per inch) is often confused with printer resolution, which is generally dpi. In some countries, lpcm (lines per centimeter) is the unit of measurement for the image setter's resolution as well as for the screening frequency. Once more the units of measurement: Printer resolution: dpi /dpcm (lpcm) Screening resolution: lpi /lpcm The images on the left show the effects of different scanner and screening resolution. Fig. 1 shows a normal scan with 220 dpi printed on a laser printer with 120 lpi. Fig. 2 shows an enlargement of a selection. Fig. 3 shows a scan with very low resolution (under 72 dpi) printed with 120 lpi. Fig. 4 shows a scan from Fig. 2 printed with a line screen of only 20 lpi.
Fig. 3
Fig. 4