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Front Page » August 2, 2005 » Tech Tips » Digital Imaging, Part 3: Digital Scanners
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Digital Imaging, Part 3: Digital Scanners

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Probably the most common facet of digital imaging is scanning technology. Digital photo scanners essentially convert printed media, like photos, illustrations or text into digital images which are stored on a computer for safe keeping. In the early days of digital imaging, scanners were often found only in large, hi-tech companies because of their high cost and complexity. Today, however, they're found nearly everywhere.

Most traditional photo scanners work on the same principle as digital cameras. Light, often powered by an internal lamp, illuminates the material to be scanned. A CCD sensor (much larger than those in digital cameras) then slowly passes over the material, converting the image into digital information (digitizing the image). This information is typically passed onto the PC and saved as an image file, which can be e-mailed, copied, printed, etc.

There are several types of photo scanners:

• Flat-bed. Dedicated flat-bed scanners are one of the most capable types of scanners and may contain better quality hardware than other scanners. Most are measured by the maximum size of paper it can scan. Many flat-bed models can scan up to an A4 (an international paper size). Smaller, more economic models can often scan up to letter size (8�"x11").

• Printer/Scanner combinations. There are many types of printer & scanner combinations, most of which have the printing hardware on the bottom and a flat-bed scanner of sorts on the top. Such scanners provide good quality, but many feel they lack the high-quality components that the dedicated flat-bed scanners often have. Moreover, these scanners often lack the capacity to scan larger page sizes like dedicated flat-bed scanners. The advantage of these combo devices, however, is that images can be scanned and printed all on the same device, without the need for a PC.

• Form-Feed Scanners. These scanners behave much like a fax by feeding the material to be scanned through a scanning device. These scanners are often discouraged because larger materials, like books or photographs can't be fed through the feeder. Moreover, the feeder can often crinkle the material as it is passed through the scanner.

No matter the scanner type, it is important to understand all of the associated terms.

• Resolution. Resolution is often measured in PPI, or pixels per inch. PPI essentially defines how wide & tall (in pixels) the scanned image is. A higher resolution will give the image more detail; however, the higher the resolution, the larger the file size. For home use, 150-300 PPI is usually considered reasonable. In fact, 600 PPI is often considered overkill for most home inkjet printers.

Some scanners have different maximum horizontal and vertical resolutions - an important item to remember when considering a scanner purchase.

Be wary of scanners that claim ridiculously high PPI capabilities. Some scanners will claim they can scan at amazing PPI levels like 9,000 PPI. These claims are a total stretch of the truth.

These scanners essentially scan the image optically at the highest PPI they are capable of (1200, for example), and then enlarge the image using a process called interpolation. This process attempts to intelligently guess what the image will look like at the larger size (9,000 PPI, for example) and then fills in the gaps as it sees fit to make the enlargement successful. As a result, such an astonishing scan isn't genuine.

• Color Depth. Color depth defines how many unique colors that can be used to compose an image. A higher color depth will provide realistic colors, but will make the image's file size larger. Color depth is often listed in bits - how many bits should be allocated to each pixel to describe each pixel's unique color.

Color Depths
Bit Depth # of Colors
8-bit 256
16-bit 65,536
24-bit 16,777,215

Be wary of bit depths beyond 24-bit, which will not provide any extra print quality beyond 24-bit. In fact, once any image correction has been performed on the image (edited using photo-editing software via your PC), the image will automatically be reduced to 24-bit anyway.

Also realize that if your monitor is set to display at a color depth lower than 24-bit, any scanned images will appear to contain less than 24-bit quality. In other words, if your monitor is set to 16-bit, a 24-bit image will display on the screen as 16-bit, but will print as 24-bit.

• De-screen. Magazines, newspapers, posters, laser printed jobs and other commercially printed materials contain strategically arranged dots of ink or toner. Scanning such materials can result in choppy, jagged images. A de-screen attempts to remove the visibility of the dots so the scan is clear, crisp and consistent.

• Optical Carrier Recognition (OCR). Optical Carrier Recognition is a special type of software that intelligently translates the scanned image into text. For example, scanning a page of typed text will result in an image (a picture) of text, not a text document that can be modified. OCR scanning software, on the other hand, will convert the image to a text document that can be edited in a word processing program. Today's OCR technology has become highly accurate. Most scanners come with at-least basic OCR software.

When saving the scanned image, consider saving the image using one of the following image formats:

• TIFF. The TIFF format was designed for TWAIN (an interface standard for scanners), and will preserve the best possible image quality. Also, because TIFF is a major image standard, nearly every computer can view them. Moreover, TIFF compression will reduce the file size of the image without reducing quality (loss-less compression).

• JPEG. JPEG provides good image quality with file sizes that are significantly smaller than TIF, which makes copying or e-mailing photos much faster. JPEG uses loss-ful compression, which means additional compression provides smaller file sizes at the detriment of image quality. JPEG is extremely common and can be viewed on virtually any computer.

Digital cameras, scanners and all the related jargon can be confusing, but a little knowledge can go a long way to help make the process a little simpler.

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