Dots per inch (dpi) represent the quantity of dots per inch a printer is
capable of putting onto paper. Dots per inch is also known as the
resolution of the printer.
Lines per inch (lpi) represent the number of halftone dots (per inch) that
a screen contains Lines per inch is also referred to as the screen
frequency or screen ruling.
The halftone dot is quite different from the dpi dot. The halftone dot
can actually be a variety of shapes: lines, ellipsoids, squares, dots,
random flecks, and a variety of other solid black shapes. Typically dots
and ellipsoids are used. However, for the more artistic representations
other shapes are often used. The number of shapes per inch is measured
by lines per inch, that is, shapes per inch equals lines per inch.
With the exception of daisy wheel type printers, output from computers
has always been based on a quantity of dots placed together to represent
alphanumeric characters and graphics. The first printers of this nature
were the dot matrix printers. Laser printers and ink jet printers use this
same principal. The laser and ink jet dots are smaller than the dot
matrix printers, but they work the same way. The number of dots these
printers produce is the measured in the dots per inch (dpi)
notation.
Prior to the use of the dot based printers the pre-press/printing industry
used various methods of placing solid forms of ink onto paper (gravure,
letterpress, offset, and so on.). These methods do not use dots to form
the alphanumeric characters. Similar to the daisy wheel printers, the
characters are fully formed ink impressions. The problem with using solid
ink comes when attempting to print graphic images which consist of more
than solid lines or solid blocks (line art). Some method to vary the
amount of ink placed on the paper is needed in order to represent various
levels of gray. Otherwise, printing photographs and other continuous tone
artwork would not be possible.
To address this need, a technique was developed to print various sized
black halftone dots on the paper. An area of various sized, solid black
halftone dots will trick the eye into thinking it is seeing various
levels of gray.
In the traditional technique used to create these various sized dots a
screen is placed over the photographic enlargement paper to break the
continuous-tone photograph into halftone dots. A screen can best be
described as a sheet of plastic which contains the inverse (negative)
gray scale pattern of halftone dots. This sheet of semi-transparent
plastic allows varying amounts of light to pass through and contact
the photographic paper placed beneath it. In the computer technique,
there is no physical screen used. The halftone dots are created via
computer image processing.
The number of halftone dots in the screen is represented by the lines
per inch (lpi) measurement. That is, 65 halftone dots per inch equals
65 lines per inch. The number of halftone dots per inch for a screen can
vary. Typical values are 65 lpi for newspaper photograph halftones to
200 lpi (or higher) for art reproduction books.
There is a relationship between dpi and lpi. The relationship exists as a
limit. The lpi of a printer will always be less than the dpi of the
printer, because the halftone dot is composed of a matrix of the dpi
dots. Higher dpi does not directly indicate higher lpi, only the
possibility of higher lpi. Both a 300 dpi printer and 600 dpi printer can
produce halftone dots of (possibly) 53 lpi, 75 lpi, 83 lpi, 106 lpi, and
150 lpi, but the 600 dpi printer would be able to produce additional,
higher lpi.