LPI vs DPI

Discussion in '4-Color Offset Presses +' started by dotfordot, Apr 15, 2009.

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  1. dotfordot

    dotfordot Member

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    I am not new to offset printing. I have brokered offset printing for 5 years. I am familiar with calculus concepts using algebra. I know what to look for in print providers.

    It seems that whenever I am looking at high lpi rated print the dots are much smaller

    Also it seems that the higher the dpi the sharper image quality.

    Is there any type of relation between lpi and dpi?

    I have done tons of research to find out that people only know how to define and describe each, which I already understand. But I want someone to explain to me the relationship (if any) between the two. If you have to go into math concepts I am sure you won't stump me.

    Sorry for seeming like an idiot if this is a dumb question. But I am looking for an expert or two to set me straight.
     
  2. Printer Mike

    Printer Mike Member

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    More dpi gives more dots to form the lpi dots; Thus, less jagged, and more rounded lpi dots?
     
  3. dotfordot

    dotfordot Member

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    Im not sure if this is what you are saying... so smaller dots give you capability for a higher concentration of lines(lpi)? I have made this deduction already, but i guess im just confused on why I see inconsistency of line screens with certain advertised DPI.

    IE 175 LPI @ 300 dpi or 250 lpi @ 300 dpi,, and etc..
     
  4. Printer Mike

    Printer Mike Member

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    Here's an example; If I image a toner based polyester plate on my 1200 dpi laser machine, the maximum decent looking lpi screen I can use is about 120 lpi. My film image setter has a maximum of 3000 dpi and has imaged at 200 lpi.

    Sorry, that's all I know. Hopefully, someone can answer the question better than me. I probably overkill with file sizes trying to be safe.
     
  5. dotfordot

    dotfordot Member

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    LOL, I do too..
     
  6. madan

    madan New Member

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    LPI - Line per inch which is for positive outputing
    DPI - Dot per inch (also known as resolution) for printer output or computer screen display
     
  7. madan

    madan New Member

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    more DPI - image resolution - in photoshop - give more sharpness for Image (300 DPI, 166 DPI or for line art 1200 DPI etc.)

    &

    more DPI - output resolution - for positive - give more sharpan positive ( 3000 DPI, 2400 DPI, 1800 DPI)
     
  8. klecman

    klecman Senior Member

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    DPI vs. LPI

    Coming from the computer business to the printing business during the first years of desktop publishing, I learned a bit about the two which caused a lot of confusion when lithographic people first started talking with computer people.

    My understanding is that dots per inch refers to the number of increments that the print head can advance in one inch. However, these are not necessarily discrete dots and at a certian point, the dots will overlap and appear as a continuous line.

    Whereas Lines per inch refers to the number of discrete lines that can be discerned through a magnifier. Thus the two have little relation to each other because the dpi standard does not specify the size of the dot.

    Dpi is the standard for printers and LPI is the standard for printing which does by definition imply the size of the dot. Therefore 300 lines per inch is usually a lot higher resolution than 300 DPI depending on the size of the dot.

    The pixel is equivalent of a dot for stating resolution on a video display. Thus on a 40 inch display, the pixels, or dots, will be twice the size as on a 20 inch display and will have only half the number of DPI on the 40 inch compared to the 20 inch display. Pixels refer to the total number of dots on one horizontal scan line. A resolution of say 1280 or 720 refers to the total number of pixels on one horizontal scan line. If you look at a video display through a maginfier you will see the individual pixels but when you look a an image printed by a desktop printer, the dots will merge into continuous lines when the dots begin to ovelap. It would make more sense if printer manufacturers would indicate the size of the dot when stating the DPI number. So in one sense, pixels are more similiar to lines since they are discrete and formed by a fine grid mask imbedded into the display screen.

    I would have to draw diagrams to make it more clear but I hope this helps.
     
    Last edited: Jun 28, 2009
  9. RichardK

    RichardK Senior Member

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    dpi - desktop printers, the higher the dots per inch the sharper the print

    ppi - monitor resolution and image files, including ccd arrays

    lpi - final plate output resolution: newsprint around 60lpi, sheetfed from 133 to 300 depending on platesetter/imagesetter and resolving capabilities of the plate/press/paper stock/ink combination

    All the above are subject to the golden rule: GIGO garbage in, garbage out

    If the original file is low res, say 72ppi, then you can add as many pixels as you like but you won't increase it's 'quality'

    check out www.b4print.com for detailed info on prepress issues such as these.
     
    Last edited: Jun 28, 2009
  10. klecman

    klecman Senior Member

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    So, scanners work in pixels per inch. I wasn't aware of that. But it makes sense because the first electronic imaging used equipment and standards from television and video. I knew that laser and inkjet printers operate in dpi because desktop image printing started with the dot matrix printers.

    When I worked for Kodak in the microfilm division we had a test image that allowed us to measure relolution in lines per inch. Film photography, then required no translation when set to burn a plate image so there was no translation confusion.

    I seem to recall that the first versions of desktop publishig software produced digital images in either dpi or ppi which did not match the normal screen standards used in printing. They generated Hewlet Packard's Post-Script files. If my memory serves me correctly, there was quite a bit of debate at the time in getting the two standards to match up.

    I recall a debate between a representative from Apple Computer and a fellow who had worked as a typesetter. The Apple representative argued that laser printers produced sharper letters than typeset print because the printers produced 300 dpi which she thought meant lpi.

    I learned the distinction by hashing it out with my brother, the printer. I came from a backgroung in TV, computers, and film photography and he explained resolution from the printing end.

    My brother was a consultant for a company who developed air operated oscilating rollers. He worked both in the design and testing phases. He was also hired by Crabtree to testify in several lawsuits involving printing quality from a couple of presses. He was able to convince the jury that the problems were caused by poor quality ink and bad chemistry rather than anything wrong with the presses.

    You certainly have a broad knowledge of all aspects of the printing process. It is difficult to explain to some people why you can't get a sharper image than you started with. But now with digital image processing, that may not always be true. I know from my studies in visual perception that the eye can be fooled into seeing a subjectively sharper image than can be measured by objective image analysis.
     
    Last edited: Jun 28, 2009
  11. RichardK

    RichardK Senior Member

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    Hi klecman, you're making me blush again :)

    If you love and have enthusiasm for the work you do, you make sure you learn as much as you can....

    38 yrs in and I'm still only certain of one thing - there's still so much I don't know. :D
     
  12. Satweavers

    Satweavers New Member

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    What Is The Optimal PPI For A Given LPI?

     
  13. Navigateur

    Navigateur New Member

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    Hi Satweavers! Rank beginner/outsider here, but according to this website http://desktoppub.about.com/cs/intermediate/a/measure_lpi.htm

    2 x desired LPI for uncoated paper stock
    1.5 x desired LPI for glossy magazine stock

    The reason why the paper matters is because glossy stock is able to hold a higher resolution of lpi than uncoated, where the ink would soak and spread out more.

    So there's no mathematical formula per se, just choose a higher lpi for glossy, and then you don't need so much dpi.

    I presume you're trying to save print time, which is why you want the minimum dpi for your target lpi, yes?

    Regards,
    N

    P.S. a real mathematical formula would involve the physics of that particular paper, surely :p
     
  14. carlosls

    carlosls New Member

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    The MATH way: ppi > 2 lpi ... why? ... It is a linear systems formula for sampling rates:
    The sampling rate of any analog function (ppi, pixels per inch is the sampling rate made by a scanner for example) for an effective digital representation must be greater or equal to 2 times the maximun frequency (lpi, lines per inch is the frequency of printing) ... therefore, if you print at 250 lpi (we are talking AM, not FM or stochastic) you better have your images made at 500 ppi, although 300 dpi will continue working it will have better transitions and dynamic range with 500 ppi.

    Have you ever think why your CD player has a sampling rate of 41 KHz and even more? .... it is the same science ... the maximum audible frequency is 20 KHz.

    SR > 2 Fmax
     
  15. Marlon

    Marlon New Member

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    Dot size

    Hi All,

    I have a basic and simple question:

    What is the size (in micrones) of a single and fine dot (not cell) on a tshirt that can come out from 230, 305 and 355 mesh?
    Another way of asking it is what is the thicness of a line from 230, 305 and 355 mesh?

    Thanks
     
  16. Meny

    Meny Senior Member

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    This is simple - here we go

    DPI - the number of dots the printer (plate printer too) can electronicaly put side by side on one intch to form the image.

    LPI - screen rulling for offset - the number of dots / lines that will form the image dots in print.

    if you work at 100 LPI and use a 1200 DPI printer , every printing dot you put to paper will be built by 12X12 printer dots to form one printing dot.
    this makes your printing dot nicer & rounder apposing to a dot made by fewer printer dots.

    this is why you need a high DPI # in order to get nice and sharp dots at higher LPI.

    if you will print a 100 LPI printing dot at 200 DPI , every printing dot will be built by only 2X2 printer dots , i.e. 4 only.

    by having a printing dot that can differ from others in small details , you get more gray shades and a better image.

    hope this made the whole issue clear now - if not , talk to me , will be happy to dive deeper.
     
  17. Marlon

    Marlon New Member

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    Dot size

    Thanks,

    I am looking for the size of a dot from those meshs I've mentioned that come out from a single hole/opening in the mesh.

    All I am after is the dimensions (on the tshirt) of the singe and basic dot that comes out from a screen. I need just 1 example.
     
  18. archipelago

    archipelago Senior Member

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    Marlon, it seems you are asking about screen printing not offset printing, right?
     

  19. Marlon

    Marlon New Member

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    Dot size

    Yes,

    Screen printing on garments.