To many people a printed image is indistinguishable from its photographic counterpart. That so many can be fooled is a testimony to the sophistication of modern printing technology. While a full range of tonal values can be used to generate a photographic print, printers don't have that luxury. There is no gray ink. Ink dots must be arranged and printed so that the illusion of a continuous tone is presented to the viewer.
The process of converting a continuous tone image into an image that can be printed with one color ink (grayscale) or four color inks (color) is referred to as halftoning. Halftoning relys on the inability of the human eye to distinguish spots that are closely spaced.
The image below illustrates this concept. If you stand far enough away and squint you will not be able to distinguish the continuous gray levels from their conventional halftone equivalents.
Conventional Halftone
The image below was scanned from a hardcover book printed with only black ink. At a normal viewing distance it is indistinguishable from a scanned photograph.
A halftone image
The enlargements of the woman's face show how grayscale values are represented in a conventional halftone. Lighter shades of gray are printed with small spots. Darker shades of gray are printed with large spots.
Halftone spot detail
A wide range of spot sizes is visible in this enlargement. The smallest spots, seen in the forehead and the shoulders, cover approximately 10% of the spot cell. Spots along the hairline cover about 50% of the cell area, and spots in the hair cover nearly 90% of the cell area.
Halftone cell grid
The halftone cell grid is visible in this enlargement with rows at a 45 degree angle to the horizontal axis. The image has 11 rows of halftone spots per 1/16" giving a screen, or grid, frequency of 175 halftone spots per inch.
Digital Halftoning
In digital halftoning the dot size remains constant while the frequency of occurrence of the dots is varied within many small halftone cells. The figure below displays a simple halftoning scheme with halftone cells consisting of 3 x 3 dots, thereby providing 10 levels of density per cell. In this case, each pixel is represented by one halftone cell.
Halftone spot composition
Each halftone cell consists of a number of printed dots. For example, printing 75 lpi halftone grid on a 600 dpi laser printer produces a halftone cell that is 600/75 = 8 pixels wide, for a total cell of 8x8 or 64 laser printer dots. This cell can represent 64 shades of gray, a function of the total number of printer spots. The human eye can discern up to about 200 different levels, or shades, of gray.
Scanning Resolution
The key to getting the right resolution for halftones is to scan in relation to the desired line screen frequency.
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For halftones with a line screen 133 lpi or higher:
scan at the line screen x 2 x scaling of original -
For halftones with a line screen less than 133 lpi:
scan at the line screen x 1.5 x scaling of original
For example, a 3"x5" photo that will be reproduced as a 6"x10" (200% of original) using an 100 lpi line screen, would be scanned at 100 x 1.5 x 2.0 = 300 spi.
Related Links
- Resolution Inch by Inch
- This reference has more thorough introductory information on image and printing resolutions.