Scan 101 ­ Appropriate Scan Resolutions and Color Settings for Your Inkjet Printer - The Online Magazine for the Sign Trade.
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Scan 101 ­ Appropriate Scan Resolutions and Color Settings for Your Inkjet Printer

Resolution can be a confusing concept in the scanning & output world, but here are some facts to help you better understand and not get caught up in the myths.

By Keri Collet

Resolution can be a confusing concept in the scanning world. A common misconception is that higher resolution scans produce higher quality prints. Another mistaken belief is that since the printer uses CMYK inks, the image color space should also be CMYK. It always depends upon the final print device.

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  • Most large-format inkjet printers in use today have a limit as to the amount of scanned information they can accurately reproduce. Plus, you should always take into consideration the viewing distance of the sign or banner. Although inkjet printers use CMYK inks, they can print a much wider variety of colors than the CMYK color space.

    Scanners and digital cameras grab information in the form of pixels (pixels is an acronym for picture elements). Pixels are displayed as squares on your computer monitor and measured in pixels per inch (PPI). Inkjet printers print round dots in a random pattern and are measured in dots per inch (DPI). Pixels and dots are not directly related. You don’t need one pixel for every printed dot. A photo can have thousands of colors that the printer must reproduce with either 4 or 6 ink colors. The printer does a much better job if it has more dots of ink available to reproduce one pixel of color.

    A standard measurement for the closest viewing distance is the diagonal measurement of the sign or banner. For example, an 18” x 24” photo enlargement’s closest viewing distance would be 30” away. For a 3’ x 8’ banner, it would be 8’ to 9’ away. The further away the viewer is from the sign or banner, the lower you can set your resolution. The lower the resolution, the smaller the file size, which allows your computer or printer to process the file much faster.

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    For a 600 ­ 720dpi inkjet printer you only need 150 ppi of digital information at the final output size for a color print to be viewed less than 4 feet away such as a 2’ x 3’ photo enlargement or fine art reproduction. For a 3’ x 8’ banner full-color photographic banner, a resolution of 100 ppi is appropriate. Billboards typically use scans less than 50 ppi and are printed from between 4 to 25 dots per inch. The human eye cannot see the dots from so far away and views it as a continuous tone graphic. Use this chart for determining the correct resolution when scanning. All resolutions assume the image is sized to its final printing dimensions.

    Use your scanner’s software interface to set the resolution and final output size. Have the scanner scale the image while it scans from the original. Avoid scanning an image at its original size and then using an image-editing program such as Adobe Photoshop to resample the image. Resampling is where the image-editing program uses mathematical calculations to add pixels to the image and tends to make the image look fuzzy when enlarged.

    For those customers that bring their own high-resolution scanned image on CD, you can use a program such as Adobe Photoshop to change the resolution to a more appropriate one for the viewing distance. It’s always easier to down-sample than up-sample. Just don’t tell your customer what you’re doing. They’ll never know!

    Also, it can be helpful to hang a display in your shop with three images of the same size, each printed at 50, 100 and 150 ppi. Have the customer stand back at the appropriate viewing distance for the size of their sign or banner and let them see that it’s going to look just fine.

    Digital cameras and scanners by default capture images in RGB format. Keep all of your images that are printed on an inkjet printer in RGB. It has a much wider color range than CMYK and makes photos look more realistic. Inkjet printers can typically match more colors from an RGB image. Let your printer’s RIP convert the image to CMYK, not the scanning or image-editing software. The RIP knows how many colors your printer’s ink system can reproduce and it will always be a wider color range than a CMYK image file.

    If your customer gives you a scan that has already been converted to CMYK, it won’t help to change it back to RGB. The wider color range has already been lost and image-editing software isn’t sophisticated enough to “guess” the rest of the colors that are missing from the photo.

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