Scanner Faq's

Questions:

1)    What is a scanner and what does it do?
2)    How does a scanner work?
3)    What is TWAIN and what does it do?
4)    About bitmap images
5)    Image Resolution
6)    Pixel Dimensions
7)    Color Depth
8)    File Size
9)    Scanner Resolutions, Monitor Resolution and Printer Resolution
10)  What is the proper scanning resolution for the best result?
11)  Which file format is proper to save the scanned image?
12)  24-bit scanners versus 30-bit scanners and 36-bit scanners
13) Why the scanned image looks much larger on screen than the original?

What is a scanner and what does it do?
A scanner is an raster-input device. It captures either an image of a text document or a picture and transfers it into bits of information, which a computer can understand and manipulate.  Similarly, an image of a scanned document can be converted into editable text with Optical Character Recognition (OCR) software.  Image Edition integrates fully with  Xerox TextBridge Pro (tm) for this powerful feature.

How does a scanner work?
The scanner uses a light source, typically a cold cathode lamp to illuminate the scanned object. The light is then reflected off the object and into Charged Coupled Device (CCD). The Charged Coupled Device collects the information, and through a series of electronic devices converts the analog signal into a series of digital signals which can then be read and processed by the internal electronics of the scanner and subsequently a computer.

What is TWAIN and what does it do?
TWAIN (Tool Without An Interesting Name) was created by a consortium to develop a standard interface between raster input devices and programs. The consortium was intended to be kept small enough to be efficient, while still representing all aspects of the industry. Prior to this standard, users had to scan an image with one application, save it and import the image into another application. The presence of TWAIN greatly simplified matters; now TWAIN-compliant software can call up the scanner user interface without importing or opening other programs.

About bitmap images
Computer graphics fall into two main categories - vector graphics and bitmap images. Vector graphics, as its name stated, are made of lines and curves defined by mathematical objects called vectors. In general, vector graphics are created by drawing programs such as Adobe Illustrator, Corel Draw, etc., therefore will not be discussed in this document. Scanners, other digital image input devices, and paint and image-editing programs generate bitmap images, also called raster images. Understanding the properties of bitmap images will help as you create and edit digital images.

Bitmap images use a grid (the bitmap or raster) of small squares known as pixels (picture elements) to represent images. Each pixel is assigned to a specific location and color value. For example, a basketball in a bitmap image is made up of a mosaic of pixels with orange or background color in that location instead of the shape of a circle. When working with bitmap images, you edit pixels rather than objects or shapes.

Image Resolution
The number of pixels displayed per unit of printed length to represent a bitmap image. Usually resolution is measured as dots per inch (dpi) or pixels per inch (ppi). The image resolution is determined when the bitmap image is created and can be altered with most image editors. If the image is created from a scanner, the scan resolution becomes the image resolution. Of the same printed dimensions, an image with a high resolution contains more and smaller pixels than an image with a low resolution. For image input devices such as a scanner, scanning at a higher resolution usually reproduce bitmap images with more detail and subtler color transitions than at a lower resolution does.

Pixel Dimensions
The number of pixels along the width and height of a bitmap image. When creating a bitmap image from a scanner, the pixel dimensions is the result of scanning resolution multiplies the physical dimensions of scanned area. For example, if we scan a 4" by 6" photo at 100 dpi of resolution, the result will be a bitmap image at 400 by 600 pixels. The file size of an image is proportional to its pixel dimensions; therefore, scanning in higher resolutions will produce larger image files. However, the display size of an image on-screen or printed on paper is determined by the pixel dimensions plus the resolution of output devices - whether it's a monitor or a printer.

Color Depth
The maximum number of different colors each pixel can be displayed in an image, sometimes measured in number of bits. The most commonly adapted color models by bitmap images are Black-and-white, Grayscale, Indexed color, and RGB True color. Their color depths are:

File Size
The raw (uncompressed) file size of a bitmap image is the result of its pixel dimensions multiply its color depth. For example, the size of an 100 pixels by 80 pixels B&W image is 1,000 bytes (100 x 80 x 1 bits). At the same pixel dimensions, grayscale image takes 8,000 bytes (100 x 80 x 8 bits) and RGB true color image takes 24,000 bytes (100 x 80 x 24 bits) to store precise information.

There are several image-compressing formats that can reduce the image file size dramatically. However, please be aware that those formats only reduce the size for storage. It will require at least the space of the raw data file size on the computer to process bitmap images, the raw file size still is the key factor of image processing performance. For example, if you scan a letter-size (8.5" x 11") document at 300 dpi resolution in true color mode, you are generating a bitmap image of 24MB (8.5x300 x 11x300 x 24 bits) in size. The actual buffer space needed on your computer for scanning is even greater than that (sometimes more than 3 times as big as the raw file size). In the same example, if you save the image in JPEG format, it may takes less than 1MB disk space. But when you open the file in an image-editing program, it'll require more than 24MB buffer space again for the program to display and process the image.

The following is the chart for file sizes of 24-bit color images:

Scanner Resolutions, Monitor Resolution and Printer Resolution
As you might have been aware of, there are several types of resolutions when referring to the scanner; namely Optical resolution, Mechanical resolution and Software interpolated resolution. For a typical flatbed scanner, the optical resolution is set by the Charged Coupled Device (CCD) or Contact Image Sensor (CIS) optical module, while the mechanical resolution is set by the stepper motor within the scanner. So if the scanner is said to have 300 by 600 dpi (dots per inch) resolution, it means the optical resolution is 300 dpi - the optical module can sample the image up to 300 segments per inch horizontally. While the mechanical resolution is 600 dpi - the stepper motor can drive the optical module to move as little as 1/600th inch per step vertically, thus the scanner can capture as many as 600 segments per inch of the image vertically. In order to let you scan above the scanner's true hardware (optical and mechanical) resolutions, scanner software often uses an algorithm that estimates the placement and color of the pixels that the hardware missed, thus resulting a much higher scan resolution. This higher-than-hardware resolution is called Software interpolated resolution (sometimes stated as maximum resolution of the scanner). But beware that interpolated scans may look fuzzy in some cases due to the nature of the image and the interpolation algorithm used.

Monitor resolution is the number of pixels or dots displayed per unit of length on the monitor, usually measured in dots per inch (dpi). Monitor resolution depends on the viewable size of the monitor and its pixel setting. For example, the viewable area of a 15" PC monitor is about 11" wide by 8.25" high, if the video card is set to display 800x600 pixels on screen, the monitor resolution is about 72 dpi (800 pixels / 11 inches, or 600 / 8.25). If you set it up to display 1024x768 pixels on the same monitor, the monitor resolution is then about 93 dpi. When preparing an image for online display for uncertain target audiences, for example, a Web page that will be viewed on a variety of monitors, we often assume the target monitor resolutions to be 72 or 96 dpi. Understanding monitor resolutions helps explain why the display size of an image on-screen often differs from its original or printed size.

Printer resolution is the number of ink dots per inch (dpi) produced by an ink-jet or laser printer. A printer uses a small matrix of ink dots to reproduce a colored or gray image pixel, for example, a 600 dpi printer might only be able to render 150 image pixels per inch (ppi). Therefore for best results, scan an image at resolution that is proportional to, but not the same as, the printer's maximum resolution.

What is the proper scanning resolution for the best result?
There has always been a misconception that more is better. But in the case of scanning pictures, that rule does not always hold true. Though you can use image-editing programs to change image properties or to enhance image quality, you would almost always get better results from scanning at lower resolution directly than re-sampling a high-resolution image by software afterward. For example, comparing 2 of 100x100-pixel images scanned from the same 1" x 1" picture; one is scanned at 100 dpi, the other is scanned at 200 dpi (200x200 pixels) then re-sampled down to 100x100 pixels by software. The former would appear much sharper than the latter in most circumstances, plus by scanning at lower (100 dpi) resolution, it would help save disk space to save the image and also cut down the actual time required by the computer to scan and process the image.

Choosing a "good" scan resolution depends on the output devices' resolution. To ensure a high-quality scan, you should always predetermine the scanning resolution to prevent unwanted colors or wasted computer resources from being introduced by your scan. For instance, if the image is scanned for displaying on screen as for Web publishing, slide show, wallpaper, etc., due to the 1-to-1 pixel mapping from an image to monitor display, you should select a scanning resolution accordingly to the monitor resolution. One approach is to scan at the monitor resolution - 72 dpi to 96 dpi, this would maintain approximately the same physical dimensions of the image on screen and its original (a 4" x 6" photo would still be measured 4" x 6" on screen).

The other approach is to determine the outcome pixel dimensions of the image first then calculate the scanning resolution according to the size of the original. For example, if you like to scan a 3" by 2" image and to keep it displayed within one screen on most monitors, the idea pixel dimensions would be 600 x 400 pixels (keep in mind that a VGA monitor is set to display 640 x 480 pixels per screen). Therefore the idea scanning resolution should be 200 dpi (600/3).

To send out an image to a fax machine, scan at 200 dpi to 300 dpi in B&W (Line Art) mode would create the best results. But if you want to print an image, what would be a good scanning resolution? The problem is that the printing technology has not quite caught up with the scanning technology. The stated maximum resolution of a printer is several times larger than the image resolution it can actually print out. A good rule of thumb would be to scan at about 1/4 to 1/3 of your printer's maximum resolution. Most laser printers have output resolutions of 300 dpi to 600 dpi and produce good results with images from 72 ppi to 150 ppi. High-end color printers can print at 1200 dpi or higher and produce good results with images from 200 ppi to 300 ppi.

Which file format is proper to save the scanned image?
Bitmap images can be saved in various file formats that fall into two categories - Uncompressed and Compressed. Uncompressed formats store bitmap images as stream of bits they were created (scanned, captured, drew, etc.), the actual file size therefore is the raw data size (pixel dimensions times color depth) plus rooms for extra pieces of information (file header, remarks, etc.). Common uncompressed image formats are BMP, PCX, PICT, TIFF, PSD, etc. These file formats are usually adapted for advanced publishing purpose because they keep the precise information of the original image, but they require much more disk space then compressed file formats do.

Compressed file formats use various algorithms to reduce the space needed for bitmap images. The compress ratio varies depending on the nature properties of the image and compressing methods used. In general it's a trade off between compress ratio and image quality, the more disk space you save, the less image quality on display. GIF and JPEG are the most popular compressed formats on the Internet and desktop computing today. GIF format is limited to compress 8-bit (or less) color or grayscale images, and is more efficient to store images without rapid color changes. Therefore it is more suitable for storing graphics such as logos, charts, etc. JPEG uses different schemes to compress true color images; it is the most efficient format to store photos, video captures, drawings, etc. Most image processing software not only support these two formats, but also allow you to fine-tune the compression process to save the most storage space without sacrificing too much image quality.

So you should save scanned images depending on the applications of them. In general, for Web posting, Emailing, Faxing, on-screen display or desktop publishing, the compressed formats such as GIF and JPEG are the best choices, because they can save huge storage space (thus reduce transfer time) and still provide sufficient quality to meet these applications' need. If you want to save an image for further editing, graphics design or high quality publishing, then uncompressed formats are better ones.

24-bit scanners versus 30-bit scanners and 36-bit scanners
You might have noticed by now, a 30-bit scanner only outputs 24-bit pictures. A color picture is separated into 3 channels namely red, green and blue. For a 30-bit scanner, the scanner will collect 10 bits of information for each color. As for a 36-bit scanner, the scanner will collect 12 bits of information for each channel. As some users might have noticed, all 24-bit scanners have a tendency to produce slightly darker images. While you can try to compensate by adjusting the gamma or brightness and contrast, you stand a very good chance of losing mid-tone and shadow details if you do so. In the case of a 30-bit or 36-bit scanner, the scanner driver will "look" at the information collected in the scan and discard either 2 bits (for a 30 bit-scanner) or 4 bits (for a 36-bit scanner) of "irrelevant" information from each channel to further enhance the picture. So typically, a 24-bit picture produced by a 30-bit or a 36-bit scanner will look better than one produced by a 24-bit scanner.

Why the scanned image looks much larger on screen than the original?
When displaying image on screen at the scale 1:1, image pixels are translated directly into monitor pixels. This means that when the scan resolution is higher than the monitor resolution, the image appears larger on-screen than the original dimensions. For example, when you scan a 4" by 6" photo at 160 dpi (which means to produce a 640 pixels by 960 pixels image) and then display it on an 80 dpi monitor, it appears to occupy an 8" by 12" area on screen. If the scanned image is prepared for online display (Web page, slide show, screen saver, wallpaper, etc.) only and you'd like to preserve the same physical dimensions of the original, then you could either scan the image at lower resolution close to the ones of monitors (72 ~ 96 dpi), or use an image-editing software to resize the pixel dimensions of the image.