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).

Procedure for Complying to the ISO 12647 standards

Procedure for Complying to the ISO 12647 standards
- By Ron Augustin (IPP - September - 2009)

We have already oultined the prerequisites for the application of the ISO 12647 standard in our previous articles in Indian Printer and Publisher and Packaging South Asia but are still briefly summarising these, before explaining the testing procedures used in the standardisation and certification process.

Before a company registers for ISO 12647 certification, it has to check that it has the required measurement devices and that it is capable of producing proofing and printing results within the tolerances of the standard. Usually, the certification agency (in continental Europe Ugra and Fogra certified partners, in Asia-Pacific several other certifying bodies are also coming up) will discuss and draw up a contract, in which the certification steps and examination tasks are to be described in detail. After a preparation stage, an appointment will be made with an auditor of the certifying agency, who will be present to supervise the final printing process and select both OK sheets and a set of sheets from the printrun. After an appropriate drying time, these will be controlled by the auditor and, depending on the agency, by an additional certifier. ISO 12647 certification has a validity of two years.

Audit and certification costs depend on the agency and on the kind of presses (sheet- and/or webfed) to be included in the process, but should not exceed INR 300,000 including both prepress and print production. A prepress-only certification amounts to roughly INR 150,000.

What will be tested:


Prepress
1) The company has to show that it has standard for file acceptance and communication with customer for print quality and colour managed file exchange.
2) In data processing, it has to show its capability of processing, checking and correcting intake data for faultless print preparation.
3) In colour management, the company has to show that it is capable of handling colour profiles and that it has understood the principles of colour management and its implementation including control of output curves on its platemaking device.
3) In proofing, it has to show that it is capable of performing a print simulation consistent with customer specifications and that it is able to demonstrate its colour consistency metrologically.

Print production
1) In platemaking, the company has to show that it is able to create printing plates that are equipped with adequate control elements to be checked with suitable measurement tools. It also has to show that it is capable of maintaining consistent exposure results over a longer period of time and that it is able to compensate batch variations at any time.
2) In makeready, the company has to show that it can produce an OK sheet in a suitable setup process within the acceptable ISO 12647 tolerances.
3) In print run stability, it has to prove its capability of controlling, monitoring and maintaining a stable print run, keeping variations within the acceptable ISO 12647 tolerances.

How to get involved in the development of the ISO standard

How to get involved in the development of the ISO standard - By Ron Augustin (IPP - August - 2009)

The next two meetings of ISO’s Technical Committee on Graphic Technologies, TC 130, will be held in September 2009 in Beijing and in February 2010 in Miami. Although there are other Technical Committees concerned with our industry, such as TC 6 (paper, board and pulp), TC 42 (photography) and TC 171 (document management applications), participants at the Monsoon Summit on ISO 12647 recently held in three cities across India stressed the need of Indian involvement in TC 130 for a more balanced representation in global print colour perceptions.
If the Indian printing industry is serious about standardisation, it has to act fast and, if the deadline for September is too close, make sure it will be in the February meeting.
The procedure to get there is as follows:

ISO is a network of ‘national bodies’ (NBs), i.e., the national standards institutes of some 160 countries, of which the Bureau of Indian Standards (BIS) is one. Although participation in ISO is open to all, there is a protocol that must be observed. For each Technical Committee, the NB can choose to be a P (participating) or O (observer) member. TC 130 has around 15 P members and 20 O members. For practical purposes, the TC is divided into working groups (WGs) where most of the standards development work is done and in which there are technical experts participating as individuals nominated by and responsible to their NB. Once an NB has established a relationship with a TC, it identifies the technical experts who will represent it in the various TC and WG activities. The working documents of the various TC projects are made available to these technical experts through the TC Secretariat, usually via access to a secure Internet connection.

As an example, in the USA, ANSI is the official NB in ISO, but has delegated responsibility to various industry groups to monitor US participation in each TC for which the US is either a P or O member. These groups are called Technical Advisory Groups (TAGs). For ISO, each NB is free to manage this process in its own way. The TAGs have the responsibility to recruit technical experts and endorse their participation to the TC Secretariat. Therefore, anyone who wants to participate in the work of a TC must contact the TAG for that TC. The TAG for TC 130 in the US is the Committee for Graphic Arts Technologies Standards, CGATS, which has its Secretariat at the National Association for Suppliers of Printing, Publishing and Converting Technologies, NPES. For the Indian industry, it must not be hard to imagine and build a similar structure.

Availability of ISO 12647-2 Standards in India

Abstract for ISO 12647-2:2004

ISO 12647-2:2004 specifies a number of process parameters and their values to be applied when preparing colour separations for four-colour offset printing or when producing four-colour prints by one of the following methods: heat-set web, sheet-fed or continuous forms process printing, or proofing for one of these processes; or offset proofing for half-tone gravure.

The parameters and values are chosen in view of the complete process covering the process stages colour separation, film setting, making of the printing forme, proof production, production printing and surface finishing.

We enquired Bureau Of Indian Standards(BIS) Delhi to find out availability and Price for ISO 12647-2 Standards in India.
ISO 12647 -- Process control for the production of half-tone colour separations, proof and production prints -- Part 2: Offset lithographic processes


Cost of the Standards is mentioned below:
Cost for ISO 12647-2 : Rs. 4410
Procurement Charges : Rs. 441
Postage Charges : Rs. 79

Net Total : Rs. 4930

Interested Professionals can send a Demand Draft of Rs. 4930/- (including Packing & Postage charges) in favour of Bureau of Indian Standards, payable at New Delhi, at the following address.

Bureau of Indian Standards
9,B.S.Z Marg
New Delhi - 110002
P.H No.- 011 23230131,2323375

Book Review - Understanding Color Management

Title: Understanding Color Management

Author: Dr. Abhay Sharma

Publisher: Thomson Delmar Learning

As the name says “Understanding Color Management”, a topic that has become the need of the hour in today’s printing world. The book starts from the point of view of a beginner in color and color management.

Abhay explains the basics of colour science and colour management, and provides an in-depth look, starting from basic attributes of colour and how the human eye perceives it. Good examples such as the “Airline analogy” (Pg-10) have been used to make the reader understand how CMS works. The book is filled with pictorial representations for nearly all topics which makes it easier to understand colour. Details about the range of measuring instruments and profiling software are also available to the reader.

International Color Consortium (ICC) profiles are discussed in detail and methods for profiling scanners, digital cameras, monitors, inkjet printers, and printing presses are thoroughly explained. After reading this book you will actually feel that colour management is not rocket science. Making this book an ultimate guide to understand color management. We also used this book as the textbook for our Certificate Course on Colour Management conducted in Delhi and Chennai in 2006 by Paul Lindstrom.

About the Author - Dr. Abhay Sharma is currently the chair, school of Graphic Communication Management, Ryerson University and a member of the International Color Consortium.

Trying to build a technical advisory group for TC 130 (ISO 12647)

Given below is a list of persons and the text of an email that we sent to them. This is essentially the follow up to our earlier post and an effort to convert the usual wishful thinking and talk talk into some kind of community action. To a large extent I see this blog (to which we will invite a team of experts) and the comments as both a byproduct and an inspiration to the overall quality printing and standardisation movement. We have already received two email responses and one phone call over the last three days. This is obviously not good enough or fast enough. Can blogging help? Will you help?

After the recent Monsoon Summit to discuss ISO 12647 standardisation many participants said that it would be good if the Indian experts also get involved in the ISO TC 130 committee. As Alan Dresch said, "Get inside the tent!" instead of complaining about what is good or bad and contribute to the evolution of the discussion and the standard. To this end we have talked about this with some of the senior people who attended and they seem to agree that it would be best to develop a discussion group that would become the technical advisory group for the TC 130. I am listing below some of the people that we thought could take part in these discussion on a regular basis -- by email, or by telephone and also face to face.

The next meeting of the TC 130 in Beijing is 21 to 25 September and our discussion group or the industry should attend and this discussion group should try and make its views and priorities clear so that whoever attends from India is able to listen and speak on our behalf and also to report to us on the issues, discussions and decisions.

I am making an initial list of persons to start this discussion. It is open to amendment and addition. Please give your views and also whether you agree with the idea or not. The idea is that we should have representation. That the persons attending the TC130 meetings from India should represent an Indian technical advisory group. If the persons attending are not self-supporting in their travel or not supported by their companies, we will have to financially support these trips and participants. My view is also that for the Beijing meeting although the issue of grey balance came up and no one seems much against it, we should for the most part listen at the first meeting we attend. That we should bring back the issues for the consideration of the group and reserve the right to give our views at the next meeting.

Here is the initial list of my nominees and this email is being sent to all of the persons on the list over the next day or so. If you are a nominee please agree or disagree about being a part of this group! Also suggest other names if you like. You can also vote against anyone on this list if you like.

1. Ritu Sharma -- Bureau of Indian Standards, Delhi
2. Vilas Gupte -- AGS Mumbai
3. Vishnu Kamat -- AGS Mumbai
4. Professor Madhura Mahajan -- Pune
5. Rasheed Mistry -- Comart Mumbai
6. Amit Navarange -- CondeNast
7. Sobhagayanidhi Sheksaria -- New Jack Mumbai
8. Ramesh Kejriwal -- Parksons Packaging Mumbai
9. Parksons Grahics -- Animesh Kejriwal Mumbai
10. Rajendrakumar Anayath -- Heidelberg PMA Chennai
11. Satish Nayak -- Bodhi Solutions, Bangalore
12. Nikhil Mittal -- Sona Printers Delhi
14. Ravi Shroff -- Nutech Photolithographers Delhi
15. Vivek Sachdev -- NPT Offset Chennai
16. RB Kashyap -- Thomson Press Faridabad
17. Gurjeet Dhingra -- Canon India -- Delhi
18. Parshav Jain -- IppStar - Noida
19. Anil Joshi -- K Joshi and Sons -- Pune
20. Harsha Paruchuri -- Pragati Offset -- Hyderabad
21. Naresh Khanna -- IppStar -- Noida
22. Arindam Sarkar -- TechNova
23. Tarun Chopra -- Color Dots -- Delhi
24. Vasudevan -- Epson -- Bangalore
25. Sambit Mishra -- EFI
26. Vaidyanathan -- Proteck Chennai
27. Debshish Sengupta - DIC

Best Practices, Colour Management and Standardisation

One of the outcomes of the Monsoon Summit was not only a great indication of interest in standardisation practices and also the need for the Indian print community to "get into the tent." That is, to become a part of the ongoing standards discussion in the TC 130 committee of the ISO. IppStar is initiating with industry help and approval, a local discussion group that will send representatives to the TC 130 committee starting with the meeting in Beijing from 21 to 25 September 2009. This discussion group will also interact with the Bureau of Indian Standards and hopefully evolve into a technical advisory group. If you would like to be part of this discussion please send your email to Parshav Jain at standards@ippgroup.in . In addition we are launching an experts blog on Best Practices, Colour Management, and Standardisation on our website.

We are hoping that this blog will become a how to discussion among experts in India and the Asia Pacific Region that will discuss not only standardisation but other technical issues pertaining to the printing industry. We will also be running two other blogs: the first will be a general and all-inclusive type of discussion called the Print Asia Blog and the second, a publishing blog that takes up some of the professional and 'backroom' issues in publishing' which will be called 'Content and Media -- Asia Pacific.' All the blogs will be team blogs with experts from primarily the Asia Pacific region invited to lead the conversations. If you would like to be invited to be one of the regular team bloggers please write to edit5@ippgroup.in .