Resolution (DPI, PPI, SPI & LPI)

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1 Resolution (DPI, PPI, SPI & LPI) DPI, SPI, LPI, and PPI Resolution is one of the trickier things to get a handle on when you're learning graphic design. Even I struggle with it from time to time, but hopefully by the time you're done with this feature you'll understand not only the different types of resolutions, but also what resolution to use. Resolution Definitions There are several different types of resolutions, and they all have their own acronyms. LPI or Lines Per Inch LPI is the resolution used in printing, and it's very important. When documents that include photographs or shades or tints of a color are printed, they're screened. This used to be done by laying a piece of film with dots printed on it over the film before exposing it, but now it's all done through the imagesetter. The principles are the same. The dots were arranged in rows, or lines. LPI referred to the number of lines per inch. The higher the LPI, the smoother the shades look. Some typical LPI: o 50 LPI for laser printers o 85 LPI for newsprint o 133 LPI for uncoated stock o 150 LPI for coated stock o LPI for color magazines o DPI or Dots Per Inch Here's just the beginning of the confusion. Dots per inch also describe printer resolution. Not only do we refer to the number of lines per inch, we also refer to the actual number of dots that can be printed per inch. The higher the DPI, once again, the smoother both photographs and line art will appear. LPI has no effect on line art, since line art isn't screened. Line art is art made up of shapes like rectangles and ellipses, and lines. Line art can only be one color, so it doesn't have any shading. When I started out in graphic design, 600 DPI printers were uncommon, but if you looked at type from a 300 DPI printer versus a 600 DPI printer, there was a noticeable difference. And there definitely was a difference in the photographs! Today, 600 DPI is fairly standard for laser printers. SPI or Samples Per Inch Although most scanner manufacturers refer to DPI, they really should refer to spi. Scanners use CCDs (charged couple devices) to capture information, and they do this by taking a fixed number of samples per inch. The thing to watch out for in scanners is optical resolution vs interpolated resolution. If a flatbed scanner is making claims of higher than 600 DPI, you can be pretty sure that they're referring to interpolation. Interpolation increases the resolution by software. The highest resolution your scanner can capture without interpolation is the optical resolution. PPI or Pixels Per Inch

2 Finally, your monitor sees everything in pixels per inch. This is really confusing! Most monitors display graphics at 72 PPI. So if your graphic is 72 pixels x 72 pixels, it's actually one inch by one inch. What if you changed the resolution of the graphic to 36 DPI? All of the sudden your graphic would be two inches by two inches (36 x 2=72). And if you changed the resolution of your graphic to 144 DPI, your graphic would shrink down to half an inch by half an inch. A brief recap. When it comes to printers, if you increase the DPI of a graphic, it will remain the same size, but it will look smoother, because more dots are being used to create the graphic. On the other hand, if you increase the PPI that a graphic will be displayed on a monitor, you'll actually decrease the size of the graphic. Confused? Let's take a slightly closer look at PPI. The Problem with Pixels For printers, a 4 x 5 graphic always remains 4 x 5. Increase the graphic's resolution, and you'll be able to display more shades of gray, but the physical size of the graphic remains the same. For display, the size of the graphic is dependent on the resolution of your display device. The higher the resolution of your display device, the smaller your graphic appears. The resolution of the graphic itself doesn't matter to your monitor. The three graphics above all have the same dimensions: 72 x 72 pixels. The one on the left is 36 DPI, the one in the middle is 72 DPI, and the one on the right is 144 DPI. Why are they all the same size? Because they all have the same physical dimensions, 72 x 72 pixels. You can see from this example that more resolution (more spi when scanning, for instance) doesn't make any difference when you're displaying the graphic on a monitor, like you do on the Web. In fact, the higher resolution is wasted on the monitor but it will result in a larger file size, which means a longer download. And you don't want that. However, if you were to print out these graphics, you'd very definitely notice a difference. Now let's take a look at how these same graphics appear at the most common monitor resolutions: 640 x 480, 800 x 600, and 1024 x 768. The graphics below are actually the same graphic, just resized slightly to simulate the different monitor resolutions. We run into that pixel problem again when taking screen captures, because the screen capture program only takes the capture at 72 PPI. You can see the same results by changing the resolution of your monitor.

3 640 x x x 768 The above should convince you that it's a good idea to check out your Web graphics in these resolutions, which are the most common ones for monitors. Generally, the larger the monitor the higher the resolution it's capable of displaying. Today more and more computers are shipping with larger monitors. So what screen resolution should you design for? It's a much debated question, and you're welcome to discuss it on the bulletin board. My personal opinion: always check your graphics at all three of these resolutions. Just because most desktop monitors can display at 800 x 600 these days, that doesn't mean they're actually set to that resolution when they're shipped. Often they're left at the default 640 x 480 (even my 21 inch monitor was set at 640 x 480 when it arrived two years ago!). And there are a lot of laptop users out there who can only comfortably display 640 x 480. Better safe than sorry in my opinion. Nothing like scrolling horizontally to turn viewers off. Printing and Resolution It's hard to show the difference resolution makes in printing on the screen, but the graphic below makes an attempt. It's the same picture, the same size, the same resolution. The only difference is the photo on the left was printed at 300 DPI and the photo on the right was printed at 600 DPI. It's easy to see that the photo on the right has more dots and looks smoother than the photo on the left. If you have a non-postscript 600 DPI printer and you try to copy photos you printed on it, what you get is pretty much mud, right? The problem here is that you can't control the LPI. Non-PostScript 600 DPI laser printed photographs are much too fine

4 to be reproduced by a copier. With a PostScript printer, you can set the LPI to 85. This will look better when copied, although still not great. The best solution when you want photographs copied is to spring for Xerox Docutech copies. They're 600 DPI copies produced from your digital file; they look very good, and they don't cost a whole lot more than regular copies. What resolution do you need to scan in at for printing? The general rule of thumb is your scanning resolution = 2 x LPI. So, let's say you're printing stationery on uncoated paper, and it includes a photograph that needs to be scanned. Most likely it will be printed at 133 LPI (but ask your printer!), so you'd scan it in at 266 PPI. You're probably wondering right now what difference it makes. You understand that a lower resolution won't print well, but why not scan in at the highest PPI your scanner allows? Won't you get a better photograph? No. Like the Web, anything over 266 PPI won't be utilized by the printer. And you still technically have to consider download times to the printer, that is. A larger file will take longer to print. While that may not matter too much to you for a simple stationery project, on more complicated projects it could drive up the cost of your output. Some service bureaus charge you extra if your file takes a long time to print. Resizing vs Resampling Resizing bitmap images usually doesn't work very well. Tell the program to make the bitmap 150% larger, and your graphic will be very pixelated. Make it 50% smaller, and chances are it will look blurry. When you resize a graphic, you're telling the program to change the size of the graphic, but do nothing to the resolution. When you resample, you're asking to your software to change the size of the graphic and the resolution, which makes all the difference in the world. It gets very confusing in Photoshop, because you do both resizing and resampling via the Image Image Size dialog box. The key to resampling in Photoshop is to make sure the file size box is checked. This is one way you can get scan in a graphic at a low resolution, and then up the resolution in your software. Resample your graphic 50%, and you actually double the resolution. Here's our mermaid again, scanned in at 200 PPI. Now we're going to reduce the size 50%, by changing the width to.49 inches and checking both the proportions and file size boxes under constrain. Watch what happens in the resolution box.

5 See the resolution? Now it's 400 PPI. This is a sneaky way to get higher resolution graphics. Dots per inch From Wikipedia, the free encyclopedia. (Redirected from DPI) Dots per inch (DPI) is a measure of printing resolution, in particular the number of individual dots of ink a printer or toner can produce within a linear one-inch space. A close-up of the dots produced by an inkjet printer at draft quality. Actual size is approximately 0.25 inches square. Individual colored droplets of ink are visible; this sample is about 150 DPI. Generally, printers with higher DPI produce clearer and more detailed output. The DPI measurement of a printer is dependent upon several factors, including the method by which ink is applied, the quality of the

6 printer components, and the quality of the ink and paper used. A dot matrix printer, for example, applies ink via tiny rods striking an ink ribbon, and has a relatively low resolution, typically in the range of 60 to 90 DPI. An inkjet printer sprays ink through tiny nozzles, and is typically capable of 300 DPI. A laser printer applies toner through a controlled electrostatic charge, and may be in the range of 600 to 1200 DPI. The DPI measurement of a printer often needs to be considerably higher than the pixels per inch (PPI) measurement of a video display in order to produce similar-quality output. This is due to the limited range of colors typically available on a printer: most color printers use only four colors of ink, while a video monitor can often produce several million colors. Each dot on a printer can be one of only four colors, while each pixel on a video monitor can be one of several million colors; printers must produce additional colors through a halftone or dithering process. The printing process could require a region from four to six dots on each side in order to faithfully reproduce the color contained in a single pixel. An image that is 100 pixels wide may need to be 400 to 600 dots in width in the printed output; if a pixel image is to be printed inside a one-inch square, the printer must be capable of 400 to 600 dots per inch in order to accurately reproduce the image. There are some ongoing efforts to abandon the dpi in favor of the dot size given in micrometres (µm). This is however hindered by leading companies located in the USA, one of the few remaining countries to not use the metric system exclusively. A resolution of 72 dpi for example equals a dot size of about 350 µm, 96 dpi 265 µm, 160 dpi 160 µm, 300 dpi 85 µm, 4000 dpi 6.4 µm. Going the other way, 1 µm dpi, 30 µm 850 dpi, 200 µm 127 dpi. Note that = 1 dpi µm, so dividing by a measurement in one of these units gives the measurement in the other unit. Some have also proposed using dpcm. A pixel image on a computer display may require many more than printer dots to accurately reproduce, due to limitations of available ink colors in the printer. [edit] Misuses of DPI measurement Owing in part to its conceptual similarity with other measurements of graphical resolution, the DPI measurement is frequently misused. For instance, it is common for an image scanner's sampling resolution to be specified in terms of DPI, though a more accurate measurement would be samples per inch. The

7 number of pixels per inch in a computer display is sometimes specified in this way as well. Usage of the DPI measurement in these cases is considered by some to be inaccurate and misleading, though the intended meaning is usually clear based on context. Another misuse results from an incomplete understanding of what the DPI measurement means. In order for any "per inch" measurement to be meaningful, some number of inches must be specified. A digital image captured by a scanner or digital camera has no inherent "DPI" resolution until it comes time to print the image (and even then, a more accurate measurement of image resolution is pixels per inch); for example, a pixel image could be printed at 4 4 inches and 250 pixels per inch, or at inches and 100 pixels per inch. Digital images contain some number of pixels; the size at which they are printed is relatively arbitrary. When someone asks for a "300 DPI image", they may be expecting an image with 300 pixels per inch of printed output; unless the size of the printed output is known beforehand, the measurement is meaningless. A more complete specification would include the desired print size in addition to the number of desired pixels per inch. A yet more complete specification would also include the DPI capability of the printer that will be used to print the image; if the printer is only capable of faithfully reproducing 100 pixels per inch, there is no reason to use a higher-resolution image. Pixels per inch From Wikipedia, the free encyclopedia. The square shown above is 200 pixels by 200 pixels. To determine a monitor's PPI, measure the width and height, in inches, of the square as displayed on a given monitor. Dividing 200 by the measured width or height gives the monitor's horizontal or vertical PPI, respectively, at the current screen resolution. Pixels per inch (PPI) or pixel density is a measurement of the resolution of a computer display, related to the size of the display in inches and the total number of pixels in the horizontal and vertical directions. This measurement is often referred to as dots per inch, though that measurement more accurately refers to the resolution of a computer printer. PPI may also be used to describe the resolution of an image scanner or digital camera; in this context, it is synonymous with samples per inch. Typical circa-2000 cathode ray tube computer displays are generally capable of 72 to 130 pixels per inch. For example, a display that is 11 inches wide by 8.5 inches high, capable of a maximum 1024 by 768 pixel resolution, can display about 93 PPI in both the horizontal and vertical directions. This figure is determined by dividing width (or height) of the display area in pixels, by width (or height) of the display area in inches. It is possible for a display's horizontal and vertical PPI measurements to be different. The apparent PPI of a monitor depends upon the screen resolution (that is, number of pixels) in use; a monitor in 800 by 600 mode has a lower PPI than the same monitor at 1024 by 768 mode. The dot pitch of a computer display determines the absolute limit of possible pixel density.

8 The measure of pixel density is useful for calibrating a monitor with a printer; software can use the PPI measurement to display a document at "actual size" on the screen. PPI could also broadly describe the resolution, in pixels, of an image to be printed within a specified space. For instance, a 100x100-pixel image that is printed in a 1-inch square could be said to have 100 pixels per inch, regardless of the printer's DPI capability. Used in this way, the measurement is only meaningful when printing an image. Samples per inch From Wikipedia, the free encyclopedia. Samples per inch (SPI) is a measurement of the resolution of an image scanner, in particular the number of individual samples that are taken in the space of one inch. It is sometimes referred to as dots per inch, though that term more accurately refers to printing resolution. Generally, greater the SPI of a scanner, the more detailed its reproduction of the scanned object. Typical consumer-level flatbed scanners are capable of optical resolution ranging from 100 to 2400 SPI; high-end scanners may have an optical resolution of 4800 SPI or more. Many scanners use interpolation techniques to achieve a higher effective SPI rating, with some manufacturers offering nearly one million SPI, though the quality is primarily limited by the optical resolution; interpolated SPI does not provide additional detail to the scanned image. Horizontal and vertical SPI ratings may differ for a given scanner; typical flatbed scanners use a horizontal array of sensors that are passed across the bed using an electric stepper motor. The density of the array of sensors determines the horizontal scanning resolution, while the minimum step size of the motor determines the vertical resolution. Lines per inch From Wikipedia, the free encyclopedia. Lines per inch (LPI) is a measurement of printing resolution in systems that use a halftone screen. Specifically, it is a measure of how close together the lines in a halftone grid are. Higher LPI indicates greater detail and sharpness. Printed magazines and newspapers often use a halftone system. Typical newsprint paper is not very dense, and has relatively high dot gain or color bleeding, so newsprint is usually around 85 LPI. Higher-quality paper, such as that used in commercial magazines, has less dot gain, and can range up to 300 LPI with quality glossy paper. In order to effectively utilize the entire range of available LPI in a halftone system, an image selected for printing generally must have 1.5 to 2 times as many samples per inch (SPI). For instance, if the target output device is capable of printing at 100 LPI, an optimal range for a source image would be 150 to 200 SPI. Using fewer SPI than this would not make full use of the printer's available LPI; using more SPI than this would exceed the capability of the printer, and would be effectively lost.