Recently, a former student asked Rajendrakumar Anayath some questions about PPI, DPI, and LPI. The good doctor who is currently head of the Heidelberg Print Academy in Chennai shared his answers with us. Readers are welcome to send us any of their technical or other questions which the IppStar technical staff will get answers for with the help of industry experts.

Most of us generally confuse the terms: PIXELS per inch (PPI) for DOTS per inch (DPI) or even LINES per inch (LPI). PIXELS per inch (PPI) exclusively refers the computer’s digital image representation. It could be how an image is comprised (in Photoshop, on the web, or displayed on a monitor), but no matter how it is depicted, it is always digital (that is -- electronic, computer, binary, etc.) and in any form, it is something you can NOT touch! (You can not pick-up a pixel with your fingers or serve a cup of pixels for breakfast.) The resolution of digital images are defined as ‘PPI’ (with a ‘P’). There is no such thing as a digital image having a ‘DPI’ resolution!

DOTS per inch (DPI) are physical (tangible) marks on media (such as paper) and are the direct results of a laser or inkjet printer. You can TOUCH a DOT (image on paper). You can NOT TOUCH a PIXEL (which is buzzing around inside the computer and bouncing off your computer screen)!
LINES per inch (LPI) is the term for resolution inherent in the offset (commercial and high volume) printing industry and is necessary for the reproduction of tonality in images by breaking down the image into larger and smaller dots using a screen of a particular frequency. Halftone screening is a modern equivalent of earlier image repro techniques such as etching, wood cuts, engraving and other line art methods used widely from the 17th through the early 20th century. These images actually comprised a lot of black lines against a light or white background. The lines that broke up the image could vary in size, thickness and direction although from a distance the human eye resolved the image as continuous and consisting of detail and tonality (shades of grey, white and black).

The more lines that could be fitted within an inch, the more fine detail appeared in the image. Most press runs use between 65 lpi and 200 lpi, depending on the coarseness of the paper (i.e. porous newspaper is 65 lpi to 85 lpi, while gloss enamel can be 133 lpi 155 lpi or higher). One of the big differences between a desktop printer’s DPI and an offset press’s LPI is that the dots (from DPI) are ALWAYS the same round size, while an offset press’s LPI relies on varying the size of each impression, as well as using non circular impressions laid down in specific directions (or degrees).

Nowadays, in the commercial printing field, the division between each is becoming more and more blurred (no pun intended), as commercial digital presses use dots so small, that when clumped together, give the same appearance as a non-concentric LPI impression.

Printing from a computer relies on two distinct and mutually exclusive entities: The computer, which uses binary data to represent, among other things, words and images; then the printer which converts electronic data into a physical product which are marks on paper (often in the form of an image). The computer, as it sends digital data out, could care less where the data is going, just as long as it transfers. (Of course we humans DO care where the data is sent: to monitors, modems, printers, etc.).

By the same token, a hardware printer could care less what information it receives, as long as long as it can convert the information into some type of mark, using ink dots on paper to illustrate an image (probably several hundred dots per inch of paper). (The results are dots on paper are tangible -- they can be touched! But you can not touch or pick-up any of the electronic, binary ‘pixels’ as they stream from the computer to the printer.)

The quality of the digital image and the quality of the physical out put are both exclusively independent of each other and mutually interdependent in optimizing the final result.

Since inkjet printers produce a stochastic image (random dot pattern), Epson recommends a file output size of 1/3 the printer’s resolution (i.e. Printer @ 720 dpi = 240 ppi, Printer @ 1440 dpi = 480 ppi). -- Any more will clog the system and degrade the image. -- Any less are insufficient data.

Also bear in mind, because inkjets (and most other colour desktop printers) rely on creating a ‘stochastic’ or random image pattern (representing each pixel with multiple clumps of ink), images produced by these printers are not conducive to being reproduced on a press or photocopy machine. (A laser printer can produce a structured dot pattern resulting in reasonably good press or photocopy quality.) Further, marketing statements as 720x1440 (or any A x B reference) tend to be misleading. The only number of true relevance is the first, lower number. The second is a synthetic interpolation used primarily by the marketing folks to try and impress us mortals and sell their product. Finally, using the appropriate and accurate terms when referring to various resolutions promotes effective communication. (Conversely, improper use of these terms merely advertises our ignorance.)

The relationship between LPI (lines per inch) and PPI (pixel per inch). In offset printing to print an 100 LPI, the input resolution what we need for the image is 200 PPI (always double the amount of LPI). For grayscale images the multiplication factor can be 1.5 times of the LPI– as per GATF documents. But many European systems say it is always better to have double the input resolution.

Relationship between LPI and DPI (dot per inch)
A normal healthy human eye can see only 256 gray levels. Any reproduction done with more than this will not be seen by the human eye, and hence could be described as a waste of time and effort.

The formula below will help us to understand and to decide on the right DPI and / or LPI we should use for our requirement. +1 ≥ 256 (Gray levels)

For example
– To print with 150 LPI we need to have the output as 2400 DPI
– To print with 80 LPI We need to use 1200 DPI. It will save the processing time enormously.
– To know an approximate DPI for a particular LPI see the below table: ( This is from our experience in CtP settings )
– 85 lpi 1024dpi
– 100 to 120 lpi 1693/2032 dpi
– 133 to 150 lpi 2540 dpi
– 175 to 200 lpi 3386 dpi
– 250 lpi and above 5040 dpi

To answer the question in the mail, if I understand it correctly: On an empirical way you can say that ‘multiply the lpi with 16’ to get the minimum resolution of the printer to be used.

For example if you use 55 lpi, the minimum resolution of the printer irrespective of whether it is a laser printer or inkjet printer should be, 55 X 16 = 880 dpi. Similarly, for 75 lpi it must be a minimum of 1200 dpi (75 X 16 = 1200).