The recommended resolution for a bitmap graphic depends on the line screen frequency that your printer will use to digitally create the halftones.
Halftones are used to print continuous tone images, such as photographs and gradients, on paper. Halftones are made up of small dots that vary in size according to whether the area of the image is darker or lighter. Darker areas use larger dots, lighter areas use smaller dots.
The line screen frequency is the number of halftone dots in a linear inch or centimeter. This is expressed as lpi (lines per inch) or lpc (lines per centimeter). The higher the line screen frequency, the more detail the printed picture will have. Typically, the line screens used for printing will fall into the following categories:
- Newspapers and newsletters at 90 to 100 lpi
- Brochures and magazines at 133 to 175 lpi
- Fine art books and magazines at 150-200 lpi
When your printer creates a digital halftone from a digital image, the resolution of the image must be high enough to make the halftone at the specified line screen frequency. The most widely-accepted rule of thumb is that the resolution of the bitmap must be twice the line screen frequency. This means that if your printer will use a line screen frequency of 110 lpi, the resolution of the bitmap must be at least 220 ppi.
If the resolution of the bitmap is lower than recommended——especially if it is significantly lower——the image may look pixelated when printed.