LPI, DPI and PPI - What is
the difference?
We are constantly being asked "What is the difference?"
We will try to give our opinion, but we are sure there are many
other thoughts on this difficult question.
LPI.
Lines per inch. This term goes back to the graphic
arts industry. In fact, way back. When printers were trying to
find a way to print photographs, they had a major problem. Printing
presses would only print ink or NOT print ink. There was no way
to print a continuous tone image. They came up with a method of
etching vertical lines on a glass plate. They took 2 plates and
put them at right angles, then photographed the photograph through
these plates onto high contrast film. This gave a halftone negative
with varying sizes of dots that could be printed by a printing
press. The number of lines per inch on the glass plates was referred
to as lines per inch. LPI has been used to this day, referring
to the number of halftone dots that occur on a linear inch.
DPI. Dots per inch.
This is the hard one, as there are different ways of thinking
about this.
It's easy if you are thinking of a graphic arts
printing press. There are so many halftone dots per linear inch
of image. As an example, there are 75 half-tone dots per inch
(lpi) in a newspaper, 133 half-tone dots per inch (lpi) in a magazine
or 175 half-tone dots per inch (lpi) in a high quality brochure.
Do you see the problem? Are we talking LPI or DPI? We are talking
about both. True at 75 dots per inch (lpi), there is room for
only 75 equally spaced areas for half-tone dots per inch in a
newspaper --BUT there are many different sizes of these dots which
can fit into each designated area.
In the digital world of 8-bit data there are 256
different sizes of these dots but still only 75 different dots
per inch. The graphic arts industry usually refer to the dots
as 0% to 100% density of printing area.
If we are referring to a continuous
tone photo printer (like the Chromira, Lambda or Lightjet)
things are a little different. The photo printers print at 300
pixels per inch. If we can think of the 300 pixels per inch as
300 different areas into which we can print it may be easier.
Each area is 1/300 by 1/300 of an inch in space. Into this we
are not making different size dots but exposing the whole (1/300x1/300)
area to make a density from 0-255 (0=no color, 127=mid-tone and
255=solid color). Since the eye and the photographic material
can only resolve so much data, these areas blend together to form
a true continuous tone photograph.
The ink jet printers
are again different and yet still use the same terms. For example,
a printer may be referred to as 1440 x720 dpi. Most ink jet printers
can only print one size dot. (Our Roland HiFi 8 color ink jet
and the new Epson 10000 do print 3 different size dots).
If we divide a square inch up into an area of 1440x720
squares, we have the area in which we can place a dot of ink.
So if we are printing a very light area we leave a lot a squares
with no ink around the few dots that make up the image. If we
need a midtone area, we will fill about one half of the areas
with dots of ink, and if we need a solid colored area we will
fill each of the squares with a dot of ink. Because the dot sizes
are all of the same size on most printers, in very light areas
the cyan and magenta would be very noticeable. Some printers use
light magenta and light cyan ink to combat this problem.
Advanced printers have gone to 3 sizes of dots for
a smoother more continuous tone look. This allows dots to be placed
closer together in the light areas, giving a much smoother finished
print. This is the reason the Roland HiFi 8 color ink jet printer
and Epson 10,000 have gone to 3 sizes of dots for a smoother more
continuous tone look.
PPI.
Pixels per inch. "Pixels per inch" refers to the number
of pixels that are in a digital file. If the RGB file is an 8x10
at 200 ppi the files size is 9.16 MB. RGB color consists of 200
ppi of information in each of the 3 colors. Each pixel consists
of 8 bits of information per color. This equals 256 levels of
color in each color--or a total of over 16 million possible colors
(256x256x256).
PPI can also refer to the total number of pixels
a scanner can capture. Our Tango
(PrimeScan) can capture up to 11,000 pixels per inch from the
original while scanning. Other scanners refer to the total number
of pixels that they can capture while scanning the largest image
area. Flatbed scanners usually refer to 600x1200 pixels over the
whole scanning bed. Some of the consumer scanners such as the
Polaroid 4000 refer to the total number of pixels it can capture.
Confusing, isn't it? I have spent over 50 years
dealing with these issues, and now that we are blending photography
with the graphic arts world the issues become even more complex.
It's our expertise that sets us apart
from the rest.
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