Reference Series Table of Contents For This Issue

How Computers Work, Part I
August 2001• Vol.5 Issue 3
Page(s) 84-89 in print issue

A Scanner On Every Desktop
Thanks To Power, Price & Performance
It wasn’t too long ago when scanners were expensive and exotic components that only dedicated professionals could use and afford. They were enormous, complicated, and out of the economic reach of a typical home or business user.

Things have changed dramatically in the past few years. As with many PC components, scanners have been subjected to price wars so fierce they drove at least one major player, Storm Technology, into bankruptcy. In addition to lower prices, today’s scanners are also more powerful and easier to use than ever before. In this article, we will discuss the hardware behind today’s scanners, the software that runs them, and a few trends computer users can expect to see in the future.

 Types Of Scanners. Many types of scanners are available, but only a few have met with commercial success. Each type of scanner has different strengths and weaknesses, and understanding the differences is important if you’re in the market to buy one.

Flatbed scanners. In the consumer market, and even the business market, flatbed scanners are the most popular models. These are the large rectangular boxes you see sitting on nearly everyone’s computer desks. Although most models look physically similar, their innards can vary wildly. This entire article focuses on flatbed designs, so we will discuss the specifics later. There are, however, a few basic strong points and drawbacks inherent to these models worth mentioning here.

Because of their design, few flatbed scanners are able to scan slides or transparencies. Light must pass through these media types to capture the color fidelity they provide, and typical flatbeds simply cannot take advantage of the improved detail a slide offers. On the bright side, flatbed scanners are better than any other scanner types for capturing images of items that are not flat. A page from an open book is the best example because most other scanners would require you to tear the page out before they could scan it.

Sheetfed scanners. These models are another popular component for desktop users because of their compact size. Typical models have a slot where you insert the paper and a motor draws the paper past the scanner’s stationary sensors to capture the document. Because the document moves instead of the scanner, these units are physically smaller than flatbed models.

Unfortunately, you can’t use most sheetfed models to scan anything other than flat pieces of paper, although a few designs exist that let you detach the scanner itself for use as a handheld scanner. In this configuration, the motor that normally draws paper through the unit instead pulls the scanner smoothly across the surface of a book page while you try to hold the unit steady.

Some flatbed scanners can accept a sheet feeder as an add-on module. This lets you place a stack of documents in the sheet-feeder bin, and then the feeder pulls the sheets individually onto the flatbed for scanning.

Handheld scanners. With most handheld scanners, you are the motor. Instead of using complicated hardware to smoothly scan a page, you must physically drag a handheld scanner across a document.

Most handheld models are very small, scanning in swaths five inches wide or less. Trying to smoothly pull the unit along a page is tedious, and today’s flatbed models are so inexpensive that you should only consider a handheld unit if you have special scanning needs that require one. Other than the pen-shaped models, chances are slim you’ll even find a traditional handheld model on the shelves.

Because handheld scanners only capture narrow strips of images, they come with special software that can “knit” the strips together into a complete image. This software has advanced to the point that it can take overlapping and skewed strips and align them properly to create a complete image of nearly any size.

Drum scanners. Another type of scanner that you probably will not see on the shelves is a drum scanner. This is not because they are inferior, but because they are high-end devices used by professionals. The best drum scanners sell for tens of thousands of dollars and have specifications that make a typical desktop flatbed scanner look like a toy.

Drum scanners use sensors called photomultiplier tubes to convert light signals into digital images. Photomultiplier tubes are much more capable than sensors used by consumer-level scanners, and they capture more color data at higher resolutions than most desktop models can achieve. The tubes sit in the middle of a glass cylinder. You mount the images on the cylinder, and it whirls around the photomultiplier tubes.

Other types. Many other scanners exist that serve a single purpose. There are tiny business-card scanners, film scanners, slide scanners, and even pen-sized scanners used to pick up individual lines of text from a book. On the other end of the spectrum are the MFDs (multifunction devices) that include scanners. These products typically let users scan, print, copy, and fax from the same machine.

A good scanner driver lets you adjust the scan before it tranfers to the PC. This driver includes a preview that dynamically updates, uses image filters, and provides tools for tweaking contrast, brightness, and colors.

 What’s Inside? Open the lid of a flatbed scanner and it looks like you just paid for a box full of air. Inside, however, are several advanced components designed to convert reflected light into a format a PC can understand. The components can differ, but their purpose is always the same.

All flatbed scanners have a glass plate where you can place the document you want to scan. As long as you keep the plate free of scratches and clean it with a solution that will not stain or cloud the glass, this scanner element will have the least effect on scan quality. Other components, as we’re about to point out, are more important.

CCD vs. CIS. A scanner’s CCD (charge-coupled device) and CIS (contact-image sensor) determine the horizontal optical resolution of the device. (We will talk about resolution later.) Most of the scanners you’ll find on the shelves are single-pass CCD or CIS models. This means the sensors and a white light source have to make one pass at a document to fully capture it.

Three-pass CCD models also are available, but these are slowly disappearing from the market. A three-pass scanner uses three light sources (red, green, and blue), and because of this, it must make three passes at the document to capture all the color data. CIS models use red, green, and blue LEDs (light-emitting diodes) to reflect light off an image, but they work simultaneously to create a white light source. This means all CIS models use a single-pass scanning method.

Single-pass scanners that employ CCD technology rely on a complicated series of mirrors and lenses to capture images. Light reflects off the image, hits mirrors, and reflects through lenses to the CCDs. The CCDs convert the light signals into the digital equivalents that show up on the PC.

The quality of the mirrors and lenses, along with their alignment, separates excellent flatbed scanners from simply adequate models. The number of CCDs packed onto the scan head defines the maximum optical resolution of the scanner. If there are 600 sensors per inch, the optical resolution is 600dpi (dots per inch). The scanner can never capture more pixels per inch than it has sensors.

Scanners using CIS technology currently cannot match the quality of top-notch CCD models, but they are catching up. CIS scanners do away with the elaborate optics used by CCD models in favor of an integrated analog-to-digital conversion system.

As we noted above, CIS scanners use LEDs instead of a fluorescent or cathode-ray tube to reflect light off a document. The CIS picks up the reflected light and sends it to a converter that translates it into digital format. Because light reflects directly from the image to the CIS, there is no need for mirrors or lenses, and CIS scanners can be much smaller and lighter than CCD scanners. Unfortunately, the technology has not matured fully, meaning high-end CIS scanners still cannot provide the quality of an excellent CCD design.

Stepper motor. A stepper motor moves the light source and optics so the scanner can capture the full document on the glass plate. This component determines the maximum vertical resolution of a scanner (discussed in more detail later).

Extras. In addition to the basic components, some flatbed models offer nice extras. Some have a row of buttons on the front that let you activate the scanner without using software. Sometimes the buttons let you automatically perform steps such as scanning a document and faxing it. With the right software, you can perform most of these functions via the PC, but the buttons are a handy extra.

Plus, some flatbed scanners come with special hardware that lets you scan slides or transparencies. Although the results will not be as good as using a device specifically designed for these media types, extra functionality is always nice.

Image editors can make good scans great by using impressive effects. Here, we modified an image using Adobe Photoshop. The image on the left is the original scan, the middle version includes simple adjustments, and the example on the right incorporates major changes.

 Deciphering Specifications. Scanner manufacturers throw a multitude of numbers and features at consumers in an attempt to sell their products, making it important for consumers to decipher what they are talking about. The two most important numbers to understand are the scanner’s resolution and color depth.

Resolution. Perhaps the most confusing and overrated scanner specification is its resolution. Two values, horizontal and vertical, define resolution, and are listed in specification tables as numbers such as 600 x 600 or 800 x 1,200. These numbers refer to the number of pixels the scanner can detect, which is also known as the scanner’s dpi. A value of 600dpi x 600dpi means that the manufacturer is claiming the scanner can detect 600 pixels per inch horizontally and 600 pixels per inch vertically.

The most important resolution to know is the scanner’s optical resolution. This is the maximum number of pixels per inch the scanner is physically capable of detecting horizontally. Most of the consumer-level flatbed scanners have true optical resolutions between 300dpi and 600dpi.

In many cases, the optical resolution appears as two disparate values, such as 600 x 1,200 or 1,200 x 2,400. This is misleading because only the first value is significant, and sometimes even that is inflated. The first number usually represents the scanner’s optical resolution (which also relates to the number of sensors on the CCD). And the second number is supposed to represent the fidelity of the stepper motor. The stepper motor moves in discrete steps, and a vertical-resolution value of 2,400dpi means the motor moves the scan head 2,400 steps per vertical inch. A vertical-scan value that exceeds the maximum horizontal resolution implies that interpolation is taking place, something we will discuss next.

Exaggerated interpolation. Many manufacturers cite an outrageously large maximum resolution value for their products, ranging from 2,400 x 2,400 to 24,000 x 24,000, and beyond. When interpolation takes place, special hardware or software is present that inserts extra pixels into the image to make it larger.

For example, a scan made at a 600 x 1,200 resolution without interpolation would result in a distorted image. There are only 600 horizontal pixels per inch when compared to 1,200 vertical pixels per inch. Interpolation uses algorithms to insert 600 extra horizontal pixels in the image, making it proportional.

The problem with interpolation is that adding detail to an image is impossible. You can only add more information, not more detail. Users who want a scanner so they can enlarge their wallet-sized photos into posters are in for a blurry surprise.

The only thing interpolation is good for is enlarging line art, which includes items such as pen sketches or other single-color documents. With these items, the “best guess” interpolated values work well because shades of color never factor into the equation.

These factors explain why you should always attempt to track down the true optical resolution of any scanner you buy. Then, while using a scanner, you should scan at or below its true optical resolution to avoid interpolation.

 Color. Years ago, the only way to get an inexpensive scanner was to buy a grayscale model. Grayscale scanners capture images only in black and white; color is out of the question. Now, with full-color scanners so prevalent and inexpensive, you’d be hard-pressed to even find a grayscale-only model.

A bit-depth rating shows the color-handling capabilities of a scanner. Most entry-level models support at least 24-bit color, and you will see other values such as 30-bit, 36-bit, and even 42-bit color. These values are divided into three channels: red, green, and blue.

A 30-bit scanner, for example, captures 10 bits per channel. Unless the specifications say differently, this same 30-bit scanner would also capture 10 bits of grayscale information. This bit depth tells us the number of discrete levels of color the scanner can detect. The number is always 2n, where “n” equals the number of bits. A 24-bit scanner detects 28 or 256 discrete levels per channel. A 30-bit model discerns among 1,024 levels, and a 36-bit scanner offers even greater precision, detecting 4,096 distinct color levels. The total number of distinct colors supported by the scanner is 2x, where “x” equals the overall bit depth.

Theoretically, a 24-bit scanner can capture 16,777,216 colors, a 30-bit scanner can detect 1,073,741,824 colors, and a 36-bit model distinguishes among a whopping 68,719,476,736 colors. These numbers never pan out in reality, but higher bit depths usually lead to more accurate scans.

When you see a scanner with bit values that are not evenly divisible by three (32-bit scanners, for example) the device uses these extra bits to detect extra grayscale levels. The 32-bit scanner will capture 1,024 levels (10 bits) of each primary color, and 4,096 levels (12 bits) of grayscale information. If the scanner is of high quality and has good software, these extra grayscale bits can provide more accurate scans of shadow zones or bright highlighted areas of an image.

Scanning this magazine cover at 300 x 1200 dpi without using interpolation resulted in distortion.
Understanding how the scanner uses the bits is as important as knowing what they are. Some scanners capture 36 bits internally but can transmit only 30-bit images to the PC. Once that 30-bit image is in the computer, you may load it into an image-editing program that supports only 24-bit color. This chain can extend further when you print the image on a 16-bit color printer, and each link represents a loss of image quality.

Try to find a scanner that transmits images at the same color depth at which it captures them. If colors must be flawless, use an image-editing program that supports extended-bit color.

And do not let high color bit claims mislead you. They are meaningless unless the scanner and your software can take full advantage of the extended bits. For example, a high-quality 30-bit color scanner will produce better scans than a low-quality 42-bit color scanner.

 Make The Connection. An important part of any scanner is the method you use to connect it to a PC. The cable that snakes from the back of the scanner to the back of the PC, along with any cards it is attached to, dictates how quickly information moves among the components. The three major options available are SCSI (Small Computer System Interface), USB (Universal Serial Bus), and parallel port connections.

If you demand absolute speed, you should consider a scanner that uses a SCSI connection. SCSI is different from the other options discussed here in that it requires the installation of a SCSI controller card to work. Some scanners come with the SCSI card, and others make you buy one separately. Installation requires physically seating the card into an empty slot on the motherboard and then progressing through a hardware setup phase, followed by a software installation phase.

SCSI connections are fast and require few resources from the computer’s CPU (central processing unit). Transfer rates of several megabytes per second are achievable through a standard connection. Unfortunately, the extra necessary SCSI hardware can add $100 or more to the price of a scanner, and the SCSI controller card consumes a slot on your motherboard that you may want back when you install other hardware down the road.

SCSI is a poor option for novices because of its installation hassles and is not recommended for people on a tight budget. Fortunately, users with newer computers can purchase a scanner that uses a USB interface and enjoy good transfer rates and a relatively simple installation for about the same price as a less-capable parallel port model. USB is slower than SCSI, offering transfer rates up to 12Mb (megabits) per second, but it is still plenty fast for most scanning applications. USB devices also are simple to install because Windows 98 and Windows Me recognize and install most USB devices automatically.

The most common and inexpensive option is a scanner with a parallel port connection. Even ancient PCs support this type of interface, and most users will not notice the speed penalties many parallel connections impose. Look at your computer manual to see if your PC can support ECP (Extended Capabilities Port) and/or EPP (Enhanced Parallel Port) standards. A parallel port scanner will work much better with these connections.

 Scanner Software. Now we know all about scanner hardware, but that is only half the story. The best scanner in the world is only as good as its software. There are more types of scanner software than we could ever hope to describe in the space of this article. Regardless, a few categories exist that every scanner user should know about.

OCR (optical character recognition) software, such as Caere's OmniPage Pro, is an indispensable scanning tool.
Drivers. A driver is a small program that allows a piece of hardware to communicate with the other components of a PC. Typically, you do not interact with these programs, and they run transparently in the background. Scanners are an exception, as any scanner driver worth its price lets you fiddle with a variety of parameters when the scanner is initialized. If you end up with a scanner that sends output directly to an image-editing program without letting you make some preliminary adjustments or take a preview scan, send the scanner back or ask the manufacturer to resolve the problem. Perhaps an updated driver is all you need.

Scanner programs often use a common driver called TWAIN to ensure compatibility with nearly any scanner. (TWAIN was not originally an acronym for anything, but has since become Technology Without An Interesting Name.) Usually, you will not even have to check whether any scanner you buy is TWAIN-compatible, but it doesn’t hurt to double-check things such as this. Keep in mind that a TWAIN-compatible scanner does not necessarily have a driver that can use all of the features supported by TWAIN. You’ll have to check the users manual or manufacturer’s Web site for more details about this.

Image editing. Of all the scanner software available, image-editing programs can have the most dramatic effects. They let you manipulate the digital images obtained through a scanner and allow you to apply anything from wild special effects to more practical color corrections. The best products have several filters that let novices perform complicated adjustments. Adobe Systems’ Photoshop (, for instance, has a filter that helps users remove scratch marks from scanned images.

The OCR movement. Scanners have obvious value when users need to capture a picture, but what about text documents? Because they take digital pictures of objects, scanners do not see text the way we do. It is comparable to taking a picture of a document written in pencil, and it’s impossible to use an eraser on the picture to edit the pencil marks.

The introduction of OCR (optical character recognition) software addressed this issue. When you send a scanned text document to an OCR program, the OCR engine attempts to translate the picture of text into real text that a word processor can understand and import so that users can actually manipulate the text. Different programs use different algorithms and methods to accomplish this, and the most recent programs offer fairly decent accuracy.

But consumer-level OCR packages still have some limitations. For one thing, they aren’t good at converting handwritten documents into typed text, although it won’t be long before this is possible. Less-capable packages also have trouble converting documents with extensive formatting. Scanning a form or another document using tables or boxes usually will result in a converted document full of garbage. This is because the OCR engine attempts to recognize the lines bounding the text as alphanumeric characters.

Some top-of-the-line OCR products, however, can convert a complex form without losing any formatting. With these programs, the original document appears on-screen with all the boxes and tables intact, and the text within these areas is editable.

Faxing and copying tools. The image-capturing capabilities of a scanner give it the ability to replace many other essential business tools. For example, software is available that lets a scanner work in conjunction with a modem to act as a fax machine. Another type of software works in unison with a printer to act as a makeshift photocopier. Simply scan the image, send it to the software program (where you can adjust the picture), and then send it to the printer.

 Here & Now. The all-in-one MFD units currently serve as another scanning alternative for consumers and small businesses. Instead of purchasing single-purpose flatbed scanners, consumers might consider investing in MFDs because they include integrated devices that scan, copy, fax, e-mail, and print with stunning clarity and speed.

We continue to see the most significant advances in the field of scanner software. Image-editing programs and OCR packages become more capable with each new release. Soon, software with hundreds of features that professionals rely on will be available with entry-level scanners, giving casual users more control than ever over their scanned documents.  

by Tracy Baker

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(NOTE: These pages are PDF (Portable Document Format) files. You will need Adobe Acrobat Reader to view these pages. Download Adobe Acrobat Reader)

Bit-Depth Table

Bit depth Bits per channel Levels per channel Theoretical maximum colors
24 8 256 16,777,216
30 10 1024 1,073,741,824
36 12 4096 68,719,476, 736

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© Copyright by Sandhills Publishing Company 2001. All rights reserved.