Pixels, PPI, DPI, and LPI for Scanning, Printing, and

Web Publishing

CSC361/661Digital MediaSpring 2002Burg/Wong

A Pixel A pixel on your computer monitor is

a point of light What is the maximum resolution of

your monitor? Probably 1024 X 768

How many pixels per inch is this? ~ 91 ppi

What is standard for computer monitors?

A Pixel A pixel in an image file is a piece of data. It could be represented in 1 bit, 1 byte, or

3 bytes How many colors can you represent if a

pixel is 1 bit 21 = 2 1 byte 28 = 256 3 bytes 224 = ~16 million

Resolution Resolution of your computer screen

is measured in pixels per inch (ppi). Resolution of an image file is

measured in pixels per inch. What’s the relationship? There’s a one-

to-one relationship between pixels in the image file and pixels displayed on the computer screen. However, this does NOT mean that the picture, when it is printed out, will be the same size as the picture displayed on the computer screen.

A Dot Resolution of a scanner is measured in

dots per inch (dpi). This is essentially the same as pixels per inch (ppi) in the sense that it is pieces of information per inch.

If you scan at a resolution of 72 dpi (or ppi) you’re getting 72 pixels of information about the picture in each direction for each inch of the picture. (5184 pixels per inch2)

A Dot The resolution of a printer is measured in

dots per inch (dpi). Each dot is a spot of color output by the

printer. The dot is the smallest dot of color the

printer creates. Your printer is probably 300 dpi, 600 dpi,

or 1200 dpi. Note that this is really dots per inch in both the x and y directions.

Inkjet Printers The dot of ink sprayed out by an inkjet

printer is wet. One dot blends into another.

In general, when you’re scanning a picture to be printed out on an inkjet printer, a resolution of no more than 1/3 the resolution of the printer is sufficient.

Thus, for a 600 dpi inkjet printer, 200 dpi resolution should be enough.

Laser Printers, Halftone Dots, and LPI A dot is the smallest printable unit

for a laser printer. A line is created from a group of

dots. The line is a series of half-tone dots. (p. 59, Avoiding the Scanning Blues, p. 87, The Non-Designer’s Scan and Print Book.)

Thus, laser printers are characterized by the lpi at which they print as well as the dpi.

From p. 59, Avoiding the Scanning Blues by Taz Tally, Prentice-Hall Publishers, 2001.

Half-Tone Dots

Each line printed out by a laser printer is a series of half-tone dots. For a grayscale picture, a half-tone dot looks more or less gray depending on how many primitive dots it is composed of.

Laser vs. Inkjet Printers Laser printers use AM screening

(amplitude modulation). That is, the size of the halftone dot changes to change the color.

Inkjet printers use FM screening (frequency modulation). That is, all dots are the same size, but the number of dots in the picture changes.

(p. 44, Avoiding the Scanning Blues)

Common DPI and LPI for Laser Printers 300 dpi 60 lpi (5 pixels2 per

halftone dot)

600 dpi 85 lpi (~7 pixels2 per halftone dot)

1200 dpi 110 lpi (~11 pixels2

per halftone dot)

Resolution When Printing From a Laser Printer

Experience shows that a good rule of thumb is to scan at a resolution that is 1.5 to 2 times the lpi of the laser printer you’ll be printing on.

e.g., for a 600 dpi laser printer, scan your picture at about 130-170 ppi