Scanning 101: Setting the Right Resolution

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Your first goal as a conscientious print designer is to create the highest quality scanned images for your clients. But too often designers interpret this responsibility as a mandate to scan original artwork at very high resolutions, which produces large files that eat up disk space, are difficult to transfer to a service bureau, and take a long time to process at output time. If you really want to do the best job possible all the way around, then you should strive for efficiency in your scanned images as well as quality. Here we'll help you do just that, by focusing on the optimum resolution you should use when scanning images for printed output. To determine the best scanning resolution, you need to consider two factors -- the resolution of the final output device and the type of artwork you are scanning. For the purposes of this discussion, all artwork falls into two basic categories -- black-and-white line art and continuous-tone images. One Equals One Regardless of the actual color used, an image qualifies as black-and-white artwork if it consists of only one color without any tonal variation. Line-art drawings and technical illustrations are obviously black-and-white images. But a blueprint, a purple-hued company logo, or a red text headline could also be scanned as black-and-white images. Scanning black-and-white originals is actually a fairly straightforward process because there is a one-to-one correspondence between the resolution of a scanned line-art image (measured in pixels per inch, or ppi) and the resolution of the printer (measured in dots per inch, or dpi). For example, if you intend to print a black-and-white line-art image on a 600 dpi laser printer, then you should scan at 600 ppi. In general, you can increase the quality of a black-and-white scan by increasing the resolution. Figure 1 illustrates how your choice of resolution affects the final image. At low-resolutions, such as 75 ppi, black-and-white scans can exhibit serious artifacts, including broken lines and aliasing -- which refers to the jagged staircase pattern on curved or diagonal edges. At 600 ppi, those same lines remain unbroken and reproduce with much smoother edges.

Figure 1: The 600-ppi scan (right) of the image appears much smoother than the 75-ppi scan shown at left. Note: Computer monitors display images at screen resolution -- typically 72 pixels per inch -- so the images you see here on screen merely simulate the resolutions discussed in this article. You can download the actual black-and-white scans by clicking the following links: low-resolution image and high-resolution image. Save the files to your hard disk, and then print them using your favorite graphics program and your desktop printer. Provided your printer is capable of generating at least 300 dpi, you'll clearly see a difference in the final output.

You can keep your life very simple by always scanning line art images at the exact resolution of your output device, regardless of whether it is a desktop printer or an imagesetter at a service bureau. In practice, though, there is a limit beyond which increasing the resolution of a black-and-white image has no benefit, because the human eye cannot perceive the difference. So while image setters may be capable of generating black-and-white images at up to 2,540 dpi, scanning the extra data is a waste of time (especially processing time). Table 1 lists the hardware resolution of various output devices along with the optimum scan resolution for each.

Table1: Optimum Resolutions for Line-Art Scans

Photographic Reality When you're working with realistic photographic images, the formula used to determine optimum scanning resolutions changes dramatically. Photographs are classified as continuous-tone images, because they contain smooth -- or continuous -- transitions from one color or gray tone to the next. These subtle color and tonal gradations allow photographs to simulate the shading of three-dimensional objects, to mimic the way light is reflected from different surface textures, and to pick up small arbitrary details (such as a freckle or a stray wisp of hair) that occur in nature. If you were to inspect a photograph very closely -- with the aid of a magnifier, for instance -- you would see that what appears to be a solid color actually consists of thousands of variations of that color. Indeed, a color photograph can contain millions of colors. In the same way, a "black-and-white" photograph actually contains hundreds of shades of gray, with black and white being merely the beginning and ending points of the gray spectrum. When a scanner captures a photograph it must convert the continuous tone information into a discrete number of hues and gray shades. The trick is to capture as much tonal information as possible. If you are scanning a black-and-white photograph, for example, you should make sure you are working in grayscale mode and are capturing a full 8-bits of tonal information per dot, which will yield up to 256 shades of gray. If you are scanning color photographs you should capture 24-bit bits of data per pixel to reproduce the full spectrum of 16.7 million colors of which an RGB monitor is capable. (Scanning at higher than 24 bits may make sense for you. If you're feeling daring, check out Bruce Fraser's recent article on using high-bit color.) One word of warning: Your particular scanning utility may use arbitrary naming conventions to designate different scanning modes. For example, many scanning utilities slip back into common parlance and refer to black-and-white photographs instead of grayscale pictures. It's a phrase that's easily confused with black-and-white line art -- a totally different scanning mode discussed earlier in this piece. And there are some older scanning utilities that use the term "Color Photograph" to identify an indexed color scanning mode, which uses a fixed palette containing only 256 colors. To be absolutely sure that you have chosen the correct scanning mode, double-check the documentation for your particular scanner. Going Half Way Provided that you choose the right scanning mode, and capture 8-bit grayscale images or 24-bit color images, you'll find that your computer monitor can handle the subtle variations in tone and color, because each red, green, or blue pixel on a monitor can display one of 256 levels of color intensity. Printing devices do not have this capability. Instead, printers simulate the 256 different grayscale values present in an 8-bit image using only black ink on white paper. Likewise, the multitude of color values in a 24-bit scanned picture must be reproduced using only four primary inks -- cyan, magenta, yellow, and black. Printing devices create the illusion of gray shades or color variation through the use of halftones (see sidebar, "Halftoning and Halftone Cells"). Things can get pretty confusing when you are trying to find a relationship between the resolution of a scanned image and the resolution of a printer. The most basic scanning rule states that the resolution of a scanned image should always be based on the capabilities of the output device. But when you are measuring a printer's ability to simulate gray shades or color variation, the output resolution is measured in the number of halftone lines per inch (lpi), not hardware dots per inch (dpi). With the exception of some PostScript devices, printer drivers don't list (or allow you to specify) halftone resolutions. Nevertheless, both desktop printers and service bureau imagesetters employ fairly standard halftone resolutions. When you read Table 2 (which lists the recommended scanning resolutions for different types of printing devices) pay special attention to the halftone resolution (or linescreen frequency) associated with each device.

Table 2: Suggested Scanning Resolutions for Continuous-Tone Images

Table 2 summarizes commonly used halftone resolutions. It's obvious from the table that the halftone resolution increases along with the hardware resolution of the output device. However, you should be aware that the halftone resolution also increases as the quality of the paper increases. Traditionally magazines have been printed on glossy (or coated) papers at a resolution of 133 lpi, but 120 lpi is increasingly used to accommodate thinner (read: less expensive) paper stocks. The highest line screen frequency -- 150 lpi -- is reserved for high-quality print products such as coffee table books and corporate reports. Of course, there is no law compelling you to use these standard line-screen frequencies. You can use a non-standard line-screen setting -- meaning one that is not summarized in Table 2 -- by specifying the halftone resolution from within your graphics or page layout application (such as QuarkXpress, Adobe InDesign, or Adobe Photoshop). You can compute the optimum scanning resolution with one of the following two formulas:

  • minimum scanning resolution = lpi x 1.5
  • maximum scanning resolution = lpi x 2

In most cases you will find that the minimum scanning resolution produces quality images. There are valid reasons for using the maximum scanning resolution. It is useful when a picture contains small details, thin straight lines, or sharp demarcations between solid color areas. But in general, you should scan at the maximum resolution only when you are printing at high line-screen settings (such as 133 or 150 lpi) and on premium coated paper stocks. Oversample Your Data You may be wondering why the formulas given above require you to scan your photographs at resolutions higher than the actual line-screen frequency used to print the picture. The reason is oversampling. You may be familiar with oversampling as a feature of your stereo CD player. The concept is the same whether applied to digital music or digital images. Basically, oversampling gathers excess information that allows the computer to average the value of a sampled sound (in the case of a song) or a sampled dot (in the case of a picture), to arrive at a more-precise value. When you scan a photograph at 1.5 or 2 times the line screen setting, you are oversampling the data to ensure that the tonal and color values of the scanned image are accurately translated into a halftone pattern. Here's the important point: Choosing a resolution that is greater than 2 times the line screen setting doesn't improve the accuracy of the halftoning process. It simply wastes computing resources. Life as a print designer is one big circle. You start with hard-copy images -- in the form of drawings or photographs. You scan them into a computer where you modify them with image-editing software or incorporate them into a DTP layout. And then you print the pictures back out to paper. By understanding the relationship between scanning resolution and output resolution, you can ensure that the circle of quality remains unbroken. Read more by Luisa Simone.

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