Is there any relation between DPI and LPI?

[rafaqat]

We use unit Dot per Inch (DPI) for desiging and when we go on Out put a unit uses Line per Inch (LPI). Is there any relation between these units and what kind of difference between these units ?

[Stephen Marsh]

DPI is generally accepted as the term for the resolution of a device, such as 2540 dpi.

PPI is generally accepted as the term for the resolution of an image, such as 300 ppi.

LPI (or per metric CM) is generally accepted as the term for the amplitude modulated halftone screen traditionally used for print, such as 150 lpi.

Pixel input for a halftone screen ruling generally has an accepted "quality factor", which is often x1.5 to x2 times the output lpi of the input ppi of the final sized image to the linescreen LPI, such as 225 ppi to 300 ppi (this figure is often image content dependent). So, yes, there is a relationship between the image ppi and the output screen lpi.

EDIT: A 150 lpi screen often has a 300 ppi image as input, which is the x2 quality factor (150 x 2 = 300).

EDIT #2: http://www.heidelberg.com/www/html/e...ening_tech_pdf


Best,

Stephen Marsh

[maxon]

A bit off-topic, since you asked about the relation between PPI and LPI :
in addition to what Steven said there's no particular relationship between DPI and LPI considering modern rip algorithms i.e. supercell screening. Well, modern since mid 90's. You as a designer won't have to deal with DPI but it's useful to learn more about it. I have seen output devices (platesetters) set at 1800 dpi exposing plates at 175 lines which is very much reasonable. I even saw a Trendsetter at 1200 dpi pushed to expose 175 lpi jobs, sound weird, the dot structure is bad indeed star shaped or anything but round or square but they say offset sheets are sellable as a final product. IMHO it's only true for average quality jobs.
You can find all the details you need regarding DPI, LPI and PPI on Gordo's blog Quality In Print as well as searching this very forum it's a matter long discussed already.

[graficworx]

I'm so glad you brought up this topic. On my site I have an article on just this subject. Graficworx - Honolulu, Hawaii It by no means covers everything, but it should give you a general idea. There is a formula for figuring out the DPI needed for a certain LPI, but I have seen the formula vary wildly from very complicated to DPI/10, so I chose to leave it out and opted for a table of common values. BTW, laser printer manufacturers are the ones that claim DPI/10 = LPI, so 600 dpi ~ 60 LPI.

[J]

@graficworx

I think that you should revise the information on your lpi/dpi webpage as it appears to be very wrong.

For example, first you say: "Dots per inch (DPI) are tangible marks on media, and is a measure of the resolution of a printer." A printer is an output device.

Then you say:

"In offset printing, to print at 100 LPI, the input resolution needed is 1692 dpi." Now you're using DPI as an input. That appears to say that the scanned image I need to print at 100 lpi needs to be 1692 dpi. Which is ridiculous.

Better to use Gordo's site for this kind of information: Quality In Print: lpi/dpi

J

[graficworx]

It's not at all ridiculous. The DPI values listed are what is required by an imagesetter or platesette to produce a halftone image. If you input a 300DPI image the RIP (Raster Image Processor) interpolates the remaining dot pattern and sends the output device the correct image for whatever line resolution it's set at.

300x300 DPI, a low end inkjet for example, equates out to 90,000 individual dots. A 300 DPI PSD file doesn't have anywhere near the needed number of points, but yet it looks fine. The printer driver, which takes the place of a RIP, must do the same thing, and interpolate the data the printer needs.

Maybe my post was on the more technical side of how an output device works, but I assure you it's correct.

[maxon]

If you input a 300DPI image the RIP (Raster Image Processor) interpolates the remaining dot pattern and sends the output device the correct image for whatever line resolution it's set at

You don't input a 300 dpi image to a RIP, you send a 300 ppi contone file. A RIP doesn't simply interpolate a dot patern, it creates color separations made out of dots with a specific shape and size placed at very precise coordinates to conform to angles it has chosen, in that a RIP like harlequin for instance may modify to a certain degree the nominal angles so-called maximum deviation to ensure moire-free separation.

300x300 DPI, a low end inkjet for example, equates out to 90,000 individual dots.

Sorry to say you're mixing up things again, if you refer to final dots produced by the RIP you'll find many more dots due to the dither, error diffusion screening applied.

A 300 DPI PSD file doesn't have anywhere near the needed number of points, but yet it looks fine.

Again, you probably refer to pixel per inch - of course it looks fine on screen being a contone image.

The printer driver, which takes the place of a RIP, must do the same thing, and interpolate the data the printer needs... BTW, laser printer manufacturers are the ones that claim DPI/10 = LPI, so 600 dpi ~ 60 LPI.

To some extent yes, a laser printer driver does process a prn/ps/whatever format job into raster but with a limited set of features - you'll find 45 degrees raster in most cases and the halftone you see on the printed paper is created according to the laser engine requirements, it is set to match the imaging optics characteristics - beam scanning, focus, drum etc. A laser engine is not a shrunk-down version of an imagesetter, a typical laser printer will not acurately place dots on paper to compose a rosette as in offset printing. As said before, you'll find Gordo's website very informative.

[J]

It's not at all ridiculous. The DPI values listed are what is required by an imagesetter or platesette to produce a halftone image. If you input a 300DPI image the RIP (Raster Image Processor) interpolates the remaining dot pattern and sends the output device the correct image for whatever line resolution it's set at.

Maybe my post was on the more technical side of how an output device works, but I assure you it's correct.

RE: "The DPI values listed are what is required by an imagesetter or platesette to produce a halftone image."

That's not what you are saying on that web page since you do not say that you are referring to imagesetter/platesetter DPI. Instead you say "input resolution" - "input" is what gets sent to an imagesetter/platesetter - i.e. the image resolution.

The information on your web page is also incorrect if you think that you need, for example 3386 DPI for the imagesetter/platesetter in order to do 200 lpi. That might be true for old, pre-1990 RIP screening technology but is certainly not true of post 1990 systems that use supercell screening technology which is practically all of today's workflows.

Your post was technical...but it is incorrect.

J

[rafaqat]

Thank you very much all for such informative posts. I shall study more about this topic.

[jrhmobile]

In short, there are a couple of significant relationships between DPI and LPI.

A number of people here have offered opinions and self-made rules, as well as a divergence that includes relevant information about PPI. Let me try to provide some factual answers, because it's worth a detailed explanation of how all three of these numbers interact. I've taught a lot of printers and graphic designers how these three figures interact in a print environment for nearly 20 years now, so I'm willing to take a shot.

Be forewarned that this will include some math. But I'll provide answers as I go through this explanation that should reduce the pain factor dramatically.

First, let's break these abstract numbers into simple mechanics: Pixels, Dots and Spots.

Pixels=PPI. Pixels are magical things. They are what are input into your computer prepress system when you bring in a digital image through a digital camera, a scanner or most types of digital image files. They're also what you see when you look at digital images on the computer screen. The great thing about pixels is that they're square, so they fit together perfectly and there are no gaps in the information. Another great thing about them is that they are the actual color they portray -- green really is Green, not some approximation made up of Cyan and Yellow, with maybe some Magenta and/or key color BlacK thrown in.

The only problem is, you can't print a pixel. You have to translate that information into Output Dots and Printer Spots (halftone cells) to translate those pretty pixels into printable pictures.

Output Dots-DPI. This is the resolution of your output device. if you'll allow me to set aside some imagesetters/platemakers that have switchable resolution settings, these numbers are the constant of the process. A 600 dpi laser printer has output dots that are 1/600th of an inch. They don't get any bigger, and they don't get any smaller. A 1200 dpi imagesetter or platemaker, like you'd find at many newspaper plants or a typical small job printing shop, makes output dots that are exactly 1/1200th of an inch.

Printer's Spots/Halftone Cells=LPI. To give the impression of darker and lighter shades of printing, your output device groups bunches of Output Dots together to create Printer's Spots, also known as Halftone Cells. The more of those Output Dots that are clumped together and the less space there is between those clumps, the darker the shade appears to our eyes. The smaller those clumps are and the more space there is between them, the lighter the shad appears to the human eye.

So that's the first relationship there is between DPI and LPI. DPI defines -- literally -- how finely we can construct those halftone cells.

The second relationship between DPI and LPI is that it defines how many shades I can define between absolutely solid black and absolutely no shade at all.

Let's talk about grayscale halftones, because the math is easier -- though it's not exactly simple. The mathermatical relationship between Output Dots and Printer Spots/Halftone Cells is:

(DPI/LPI)squared, + 1 = number of shades of gray you can define. In absolute terms, more is better.

Let's take that abstract equation and translate it into real terms, using a 600 dpi laser printer and a 1200 dpi imagesetter. Let's say that we want to run a job at a 133 linescreen, as a typical image screen for commercial printing.

With a 600 dpi laser printer, at a 133 linescreen, the math is as follows:

(600/133)squared + 1 = # of shades
600/133 = 4 (actually 4.511278 and a whole lot of other numbers for the fraction, but fractions don't count)
4x4=16 + 1 = 17

That's the maximum number of shades we can reproduce between 0% white and 100% solid black.

So, why do you care?

Well, because we said more shades are better. And 17 shades aren't much. Let's say that you need to print a 20% screen. That's not going to happen with a 600 dpi laser printer and a 133 linescreen, because the gaps between shades are roughly 5.88 percent. You can print roughly a 17 1/2 percent screen or a 23 1/2 percent screen. In simple terms, 20 percent ain't gonna happen. and if you try to print a 133-linescreen image on that 600 dpi laser printer, the wide range between shades will translate into poor-quality, posterized pictures.

Using the same linescreen with a 1200 dpi imagesetter or platesetter works a heck of a lot better:

(1200/133)squared + 1 = # of shades
1200/133=9 (actually, 9.022 and a bunch of other numbers, but remember fractions don't count)
9x9=81 +1 = 82

We still can't print a screeen at precisely 20%, but the gap between 19.5 percent and 20.7 percent is a lot finer. And when you conside the human eye can detect between 80-100 shades, printing a good digital file from a 1200 dpi imagesetter or platemaker will give the impression of a quality photographic image.

This doesn't even take into account highlight or shadow dots -- the lightest and darkest shades you'll actually be able to reproduce on press. Nor does it account for dot gain as ink is absorbed by paper. Sorry about the novella. But this does precisely answer the original question: Is there any relation between DPI and LPI? The short answer is: absolutely.

Randy Hagan
htech
415 Congress, Suite 202
Portland, ME 04101
207-699-1316