Week 2 - How Scanners Work

Here is an outline of the five areas covered and learned in this session about scanners.

1.) Types & Basic Principles
2.) Anatomy
3.) Process
4.) Resolution & Interpolation
5.) Image Transfer

There are four types of scanners:

• Flatbed scanners - are the most versatile and commonly used.
• Sheet-Fed scanners - much like the flatbed except the document moves while the scan head remains immobile.
• Handheld scanners - use same basic technology as flatbed scanner, but rely on user to move them instead of a motorized belt. Doesn't provide the best image quality.
• Drum scanners - used by publishing industry to capture incredibly detailed images. They use technology called photomultiplier tube (PMT).

and the basic principle of scanners is to analyze an image and process it in some way. Image and text capture (OCR- optical character recognition) allow you to save information to a file on your computer you can then alter or enhance the image.

When it comes to the anatomy, rather than list the actual components, I'd rather discuss the key component the CCD array. The CCD is the most common technology for image capture in scanners. CCD is a collection of tiny light-sensitive diodes, which convert photons (light) into electrons (electrical charge). These diodes are called photosites. Each photosite is sensitive to light - the brighter the light that hits a single photosite, the greater the electrical charge that will accumulate at that site. The image of the document scanned reaches the CCD array through a series of mirrors, filters and lenses. The exact configuration of these componants will depend on the model of scanner, but the basics are pretty much the same.

The scanning process is quite basic, place the document on the glass, hit the start button and let the scanner perform its magic. Rather than go through a very detailed explanation of the inner workings of the scanner, I'd much prefer to move on to more interesting information relating to resolution and interpolation.

Scanners vary in resolution and sharpness. Most flatbeds have a true hardware resolution of at least 300x300 dots per inch (dpi). The dpi is determined by the number of sensors in a single row of the CCD or CIS array by the precision of the stepper motor.

Sharpness depends mainly on the quality of the optics used to make the lens and the brightness of the light source. A bright xenon lamp and high-quality lens will create a much clearer, and therefore sharper, image than a standard fluorescent lamp and basic lens.

Many scanners proclaim resolutions of 4800x4800 or even 9600x9600. To achieve such high resolutions the specifications will usually be labeled software-enhanced interpolated resolution or something similar.

Interpolation is a process that scanning software uses to increase the perceived resolution of an image. Which it does by creating extra pixels in between the one actually scanned by the CCD array.

Another term used when talking about scanners is bit depth, also called color depth. This simply refers to the number of colors that the scanner is capable of reproducing. Each pixel requires 24 bits to create standard true color and virtually all scanners on the market support this. Many offer bit depths of 30 to 36 bits, but still only output in 24 bit color.

Scanning the document is only one part of the process. For the scanned image to be useful, it must be transferred to your computer. There are three common connections used:

• Parallel - connecting through the parallel port is the slowest transfer method.
• Small Computer System Interface (SCSI) - SCSI requires a special SCSI connection. Most SCSI scanners include a dedicated SCSI card to insert into your computer and connect the scanner to, but can use a standard SCSI controller instead.
• Universal Serial Bus (USB) - USB scanners combine good speed, ease of use and affordablity in a single package.
• FireWire- usually found on higher-end scanners, firwire connections are faster than USB and SCSI. Fire wire is ideal for scanning high-resolution images.

On the computer you need software, called a driver, that knows how to communicate with the scanner speak a common language, Twain. The Twain driver acts as interpreter between any application that supports the Twain standard and the scanner.

Resolution refers to the number of pixels in an image. Resolution is sometimes identified by the width and height of the image as well as the total number of pixels in the image. For exampe, an image that is 2048 pixels wide and 1536 pixels high (2048x1536) contains (multiply) 3,145,728 pixels (or 3.1 megapixels). As the megapixels in your camera increase so does the maximum size image you can produce. Hence a 5 megapixel camera is capable or capturing a larger image than a 3 megapixel camera.

A 17" monitor is most likely set at 800x600 pixels, while a 19" monitor is usually set for 1024x768 pixels. What this means is if you open an image that is 640x480, it will only fill up a portion of the screen, whereas if you open an image at 2048x1536 (3.1 megapixels) it will become necessary to move the slider bar around to see the image. Now add the fact the monitor has a finite number of pixels per inch (like 72) so if you are going to display a image on a monitor ony you would want to drop the quality down to 72 to save file space. When you go to put your image on a webpage or in an email you will first want to mke it a useful size (not to big, not to small).

Image size and resolution is important in that the key is selecting a resolution that will scale that input size in inches to that desired output size in pixels. Following is an example.

You have a 4x6 inch photo, and you want the six inch dimension to fill a 800x600 pixel screen vertically. Obviously 600 pixels / 6 inches = 100 dpi scan resolution is required. Scanning 6x4 inches at 100 dpi will produce an image of (6x100) x (4x100) = 600x400 pixels, aligned vertically just fill the 800x600 pixel screen height. Horizontally, this 400 pixel image would fill half the 800 pixel screen width, so we could put two of these side by side. However, if it were a 8x10 inch photo, then 600 / 10 = 60 dpi resolution would produce an 480x600 pixel image which fits vertically, but we would have to crop to fit two of them horizontally.

Here are some pixels settings to remember.

• 640x480 or 800x600 pixels for 14" monitors
• 800x600 or 1024x768 pixels for 15" monitors
• 1024x768 or 1152x864 pixels for 17" monitors
• 1152x864 or 1280x1024 pixels for 19" monitors