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Scanners | Tech Info Home Page |


General Information

If you are looking to buy a scanner, or need a driver or technical support go to the "Flatbed Scanners-Hardware" page.

Scanners are devices which bounce a light source against an image and convert the reflected light into a computer image. The file is saved in either a photographic format, or a black and white format and the file type is selectable by the user. The common formats include JPEG, BMP, PCX and GIF.

The physical characteristics of scanners fall into 2 categories. The Flatbed operate much like a copying machine, having a movable light source, while the scan image is placed atop on a glass surface. The Sheet Feed type have a mechanism which uses rollers to pull the scanned paper past the scanner sensor while building the image. Some Sheet Feed scanners have detachable heads making the scanner more flexible by allowing the user to physically move the head over a surface. This gives Sheet Feed scanners the ability to scan bound documents.

Typical scanner resolutions are given in Optical and Interpolated values. The interpolated value is usually much greater, but the optical value is the real resolution of the machine. The most common scanning resolution is 300dpi followed by 600.

Three factors have to be considered in scanning documents or images.

First the type of image, which can be either a "black and white" (B/W) or a "photographic" image. A black and white image is typically a text document that does not contain any images. There is a fundamental difference in the real viewable size of two images both having the same dpi setting. As an example, lets say one image was scanned at 100dpi, black and white, and 256 colors and on the screen the size if about 4 inches across. If the same image was scanned in at 100dpi, 256 color photographic format, the photographic image will physically be much smaller. Again, on the screen when viewed side-by-side the same image will take up more screen space in the B/W mode and less in the photographic mode.

Second, the resolution which is measured in dots-per-inch. This can vary from about 72 to the maximum allowed by the scanner. Several factors are at play here. Images will get very large at higher resolutions. If the images are to be printed the resolution can be as low as 72dpi for color printers (100dpi is adequate) or about 100dpi for laserjets (120dpi giving good results). It would not make sense to scan an image at a higher resolution than the printer can use if the document is meant to be printed.

Third, the number of colors used in the image, which can vary from 2 to millions. Here again, your choice can make a very big difference in the output. Images that contain flesh tones will look much better at higher number of colors even if the dpi resolution is low. Images that have very few solid colors will be of much smaller file size if the number of colors is very low, such as 256 or even 16 colors.

There is one more variable to keep in mind, the time it takes to scan the image. As you increase the resolution or number of colors the scanning time increases considerable from a few seconds to many minutes per page.

The file size of a scanned image can get out of hand very quickly when the resolution and color depth are increased. For instance a 8.5x11 scanned photograph will take about 35mb (35 million bytes) per page, in 300dpi at 24-bit color depth (true color).

So the same image at 600dpi and 30-bit color will be so large that it will not be practical to use.

There are also hand held units that the user moves manually over the image. These can be tricky to keep moving straight and usually do not have the same quality optics as the larger more expensive units.

One more item, many scanners come with bundled OCR (Optical Character Recognition) programs. These programs take the scanned image and convert text areas to editable text by making a new file containing words just like if you typed them in yourself. Images are ignored. OCR programs such as OmniPage, are very accurate and can cut down typing time considerably.

Note: images that will be converted to text by using OCR are typically scanned in at 200dpi, in black-and-white format (this is the typical FAX format).

The last item is the type of interface scanners use to connect to the computer. There are two interfaces commonly used, SCSI and Parallel port. The SCSI interface requires a card that must be bought separately if its not included with the scanner and must be installed inside the computer. SCSI driven scanners are much faster than the parallel port type.

On the other hand the parallel port scanners are easy to setup. You only need to plug the scanner data cable to the back of the computer's parallel port plug (just like a printer) and then take the printer cable and plug it into the back of the scanner which has an extra connection for that purpose.

Note: Some scanners might even have a "pass through" connector which allows the printer cable to be plugged in behind the computer by piggy-backing on the scanner plug.

The drawback of the parallel port scanners is that they are much slower than SCSI and typically you wouldn't want to scan an image at very high resolutions and/or color depth.

The transfer rate of the various technologies is as follows:

One final point, scanners use two methods to scan a color image. The one-pass method does just that. The scanner light source moves across the image once and all 3 primary colors are captured and recorded. The "3-pass" method makes 3 passes over the image recording a primary color on every pass, and of course taking three times as long to complete the scanning. This can after a while be very noisy and annoying.

Recommendation for home users:

A one-pass, 600dpi or 600dpi optical resolution, parallel connection Flatbed scanner (parallel is recommended only because of the ease of installation, the SCSI is the better machine if you can handle the installation). If you have a newer computer with s USB port then get a USB scanner. They are the easiest to setup and have the fastest connection possible.

Sensors

The most common type of sensors used by scanners is the CCD (Charge-Coupled Device). 

A competing technology is the CIS ( Contact Image Sensor ) which is used on less expensive scanners.

 

 


 

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