Commercial LPI vs DPI

Spatial resolution is a measure of how finely the image information is grouped to be reproduced or rendered by the output device. With the digital imagesetters used in commercial printing, this is where the line screen (or screen frequency) comes into play.

Using the typical 150 lines per inch (lpi) as the assumption, the printing dots are arranged in rows that are placed 1/150" apart. The spatial resolution is then 150 lpi. Now output the same image at 85 lpi, and you've lowered the spatial resolution (and reduced the detail of the image). See Figure 2.6 for an exaggerated example.

How does lines-per-inch (lpi) relate to dots-per-inch (dpi)? A 150 lines-per-inch image will probably be output on a commercial imagesetter at 2,400 dots per inch. The addressable resolution of this device is, then, 2400 dpi; the spatial resolution is 150 lpi. The 2400 dots are used to print the 150 lines.

Clear as mud, right?

Clear as mud, right?

Lpi Printing

10 lines per inch

Figure 2.6 Two spatial resolutions for the same image for offset printing.

150 lines per inch as each dot or spot having its own address on the paper, and all this is measured in dots per inch (dpi). (Imaging scientists actually have other ways of talking about resolution, too, but I'll leave the arcane terms and definitions to them.)

Do you know the story of the blind men and the elephant? Six blind men encountered an elephant for the first time. Each touched a separate part of the beast and was then asked to describe the whole animal. They did so but in very different ways. The elephant was either like a snake, a wall, a spear, a fan, a tree, or a rope depending on which blind man spoke.

And so it is with "addressability" and dots per inch. Those numbers you see listed on every print device's spec sheet and in every advertisement only give you part of the picture. And each print-device manufacturer talks about it differently.

Take inkjet printers. The Epson Stylus Pro 4000 printer's maximum resolution is listed as 2880 X 1440 dpi (Note: virtually all digital-printing devices have multiple modes that allow for more than one resolution setting; naturally, only the maximum is advertised. The smaller the resolution numbers, the faster the printing, but the lower the image quality). The maximum resolution on the HP Designjet 130 is 2400 X 1200 dpi. For the Canon i9900, it's 4800 X 2400 dpi.

So what do these numbers mean? The 2880 (or 2400 or 4800) refers to the horizontal axis and is the maximum number of dots the printer can cram into one inch across the paper, or in the direction of the printhead's travel (see Figure 2.7). The other number (720, 1200, or 1440) is the maximum number of dots the printer can place in one inch down the paper (in the direction of the paper feed).Keep in mind that these are not separate little dots standing all alone; they are frequently overlapping or overprinting on top of each other.

Figure 2.7 Inkjet printers have the higher-resolution numbers in the horizontal or printhead-travel direction.

Printer image courtesy of Hewlett-Packard Company

Why are the horizontal numbers usually higher? Because it's a lot easier to position the printhead precisely than it is to position the paper precisely. As software developer Robert Krawitz explains it, "The printhead typically doesn't actually lay down a dot every

1/2880th of an inch in one horizontal pass. What happens is that different nozzles on the printhead pass over the same line or row to fill it in. It might require up to eight passes to print all of the intermediate dot positions and complete the row. This interleaving of dots is sometimes referred to [in the case of Epson] as 'weaving.'" (See Figure 2.8.)

2880 x 1440 dpi y

i

ft

1 1 -

r.i

r\

■ 3

-<-printhead travel

1/2880 in.

Figure 2.8 Multi-pass droplet offsetting or "weaving" is one factor affecting an inkjet printer's addressable resolution. (Note: the dot sizes and positions are representative only; actual printing dots are more variable.)

The idea is the same for the other inkjet brands, although each has its own way to arrive at the maximum resolution numbers. HPs do things like "color layering" to change both horizontal and vertical resolutions. Canons combine "dot layering" with other factors including small ink droplets, small nozzle structure, and a small nozzle pitch (the distance between nozzles on the printhead) to reach high dpi numbers.

What does all this mean? Honestly, not that much. Is 2880 X 1400 really 36 percent higher—if you simply multiply the two numbers together—than 2400 X 1200 dpi resolution? I've seen outputs from many printers with these stated maximum resolutions, and I would be hard-pressed to say one is that much better than the other.

The theory is that higher printer resolutions produce finer details and smoother tonal gradations. This is true up to a point, but you eventually reach a position of diminishing returns. The negatives of high dpi—slower printing speeds and increased ink usage—eventually outweigh the positives, especially if you can't really see the differences. (For more about this, see "Viewing Distance & Visual Acuity" below.)

When it comes right down to it, the dpi resolution numbers on a spec sheet are irrelevant. They only tell a very small part of the story, just like the blind men's elephant. There are many factors that go into what really counts—the image quality a particular printing device is capable of producing. Factors like printer resolution, the number of ink colors, the size of the ink droplets, the precise positioning of the dots, how the inkjet nozzles are arranged and fire, the order of the colors, the direction of printing, and the screening or dithering pattern of the image pixels—they all come into play. My advice: Don't put too much stock in the dpi numbers alone, and don't use them to compare printers of different types or brands. Instead, use dots-per-inch resolution only to weigh different models of the same brand. Then, at least you're talking the same language.

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Responses

  • tecla onio
    What does mean max resolution 2880 x 1440 dpi?
    7 years ago
  • amethyst
    How to setting dpi lpi for news paper printing?
    7 years ago
  • tamara
    What is the difference between 2400 x 2400 dpi vs 1200 x 4800 dpi print?
    7 years ago

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