SCANNER
What is a scanner?
A computer scanner converts printed or transparent material into a digital image. Once you have a printed document or photo in digital form, you can modify it, print it, use it on a web page, or archive it on a durable storage medium like CD-R.
Two types of scanners are in common use today. Flatbed scanners usually come in letter or legal sizes, and they can scan printed material. Flatbed scanners can scan transparent media with an adapter called a "transparency adapter" or "light lid". Slide scanners (or film scanners) are designed specifically for 35mm slides or negatives.
How do you measure scanner performance?
To evaluate a scanner purchase, you need to know how the experts measure scanner performance. Resolution, dynamic range, and bit depth are the key performance components. You may also consider scanning speed and other special features of the scanner when making your purchase.
• Resolution
The resolution of a scanner determines the sharpness of the scan. Resolution is usually measured in dots per inch, or dpi. In principle, a scanner with a given dpi resolution should be able to resolve features with size approximately twice the distance between dots. For example, a scanner with a maximum resolution of 600 dpi should be able to resolve features of 1/300th of an inch. Typical resolutions for consumer flatbed scanners are 600 dpi to 2400 dpi, and typical resolutions for slide scanners are 2800 dpi to 4000 dpi.
• Dynamic Range
Scanners inherited the concept of dynamic range from the photographic film industry. Scanners that perfectly reproduce white are said to have a minimum optical density, or Dmin, of 0.0. Scanners that perfectly reproduce black have a maximum optical density, or Dmax, of 4.0. When you read the specifications for a scanner, you may find the minimum or maximum optical densities listed explicitly, or the manufacturer may report the difference between the two numbers as the dynamic range (sometimes symbolized as Dmax-min). Typical dynamic range for consumer scanners is about 2.5-3.4, with dedicated slide scanners at the high end. For comparison, the dynamic range of good quality black and white print film is about 2.8.
• Bit depth
The internal electronics in a scanner convert the color information to digital bits. This process is called analog-to-digital (or A/D) conversion, and the quality of the A/D conversion can affect the final digital output. Most modern scanners feature 48 bit A/D conversion, or 16 bits for each color.
The bit depth and dynamic range give a rough idea of the image quality of the scanner. Scanners with wide dynamic resolution and high bit depth should be capable of more precise color scanning. They don't tell the whole story, of course, since a scanner may have high performance numbers and still have problems that make it a poor choice for your application. But they provide a performance baseline for comparison between similar scanners.
• Speed
Scanning speed varies due to several factors. The physical construction of the scanner, the quality of the electronics, and the interface to the computer all contribute to scanning speed. Unfortunately, manufacturers have no standard method for reporting the scanning speed. The best way to compare scan speed is to scan a reference image at various resolutions, and report the time for each.
• Special Features
Some scanners have special features that may be particularly useful for your needs. Many flatbed scanners include transparency adapters for scanning slides, film negatives and other transparent media. Automatic document feeders (ADFs) are mechanical feed devices that allow you to rapidly scan loose paper documents. You can also get oversized scanners for scanning 11 inch by 17 inch tabloid sized documents and larger, although large format scanners cost a lot more than typical consumer units.
If you need special features, make sure to get them bundled with the scanner at the time of purchase. Most companies offer versions of their scanners with extra accessories included. If you decide to add an auto document feeder or transparency adapter after purchase, you may find that the accessories cost a lot more when purchased separately.
How does a scanner connect to your computer?
Most consumer scanners use the Universal Serial Bus (USB) to connect to your computer. Because of the speed limitations of older USB ports, the latest scanners use the USB 2.0 standard. Fortunately, they are reverse-compatible with older USB ports, so just about any computer manufactured since 1998 should be able to use the full range of modern consumer scanners.
Some scanners use the IEEE1394 ("Firewire") interface. These scanners work particularly well with Apple Macintosh computers and their integrated Firewire ports, but you can also use them with PCs with integrated Firewire or a Firewire upgrade card.
High-end large format or slide scanners may require a SCSI interface. You can add a SCSI interface to your PC or Mac with an expansion card for about $50.
What drivers does a scanner require to function?
Scanners communicate with your computer via drivers supplied by the manufacturer. The most common drivers are called TWAIN drivers, referring to a Rudyard Kipling poem that includes the phrase, "and never the twain shall meet".
TWAIN is an industry-supported standard designed to allow your imaging software to communicate with your scanner. If your scanner works with your operating system, it will include the required TWAIN drivers.
Recently Microsoft introduced native scanning support in Windows Millenium and Windows XP. These built-in drivers allow your scanner to appear as a device in your My Computer window, and you can double-click the scanner to start scanning. This service is in addition to the vendor-supplied TWAIN drivers.
What resolution and other settings should you use when scanning?
When you scan a photo or document, the scanner's TWAIN driver will present a number of configurable options for the scan. Unfortunately, these many options can be confusing for the novice. Although every TWAIN driver is different, here are a few general tips you can use to pick good settings for your scan.
• Resolution
Choose a scanner resolution high enough to capture as much detail as you need, but low enough that you don't overwhelm your computer. If you scan everything at 1200 or 2400 dpi, you will slow down your computer with unnecessary image data. Here is a table with suggested resolutions for different media, and the amount of RAM memory your computer will require to scan them. As you experiment with your scanner computer, you will probably want annotate this table with your own settings that work best for your system.
Suggested Scan Resolutions for Different Media
Media
Type Typical
Size Best
Resolutions Color
Depth Memory
Usage
Color Photo
Print 5 inches by
7 inches 200 dpi to
600 dpi 24-bit
color 6 - 50
megabytes
Black and White
Photo Print 5 inches by
7 inches 200 dpi to
600 dpi 8-bit
grayscale 1.5 - 12
megabytes
Color Document or
Magazine Photo 8.5 inches by
11 inches 150 dpi to
300 dpi 24-bit
color 8 - 32
megabytes
High-quality magazine or
art book photo 8.5 inches by
11 inches 200 dpi to
600 dpi 24-bit
color 15 - 60
megabytes
Newspaper article or
black and white text 8.5 inches by
11 inches 90 dpi to
200 dpi 8-bit
grayscale 1 - 4
megabytes
Color slide or
film negative 1.5 inches by
1.5 inches 600 dpi to
2400 dpi 24-bit
color 3.2 - 13
megabytes
• Color Depth
For all color documents or photos, use the 24-bit color setting in your scanner software. Some scanner drivers will include higher color settings (for example, 36-bit or 48-bit), but most photo editing software does not support these higher color depths.
For black and white originals, or any original where you don't need the color information, use the 8-bit grayscale or "black and white photo" option in your scanning software. You will save memory and storage space, but still get a great looking scan.
• Color Correction, Sharpness Enhancement, Descreening
Many scanner drivers include a color correction option that will automatically adjust the brightness, contrast and color saturation of the final image. You need to experiment with this setting to see if it makes sense for your source material. If it works for you, feel free to use it, but turn it off if you don't like the results. You can always fine-tune the scanned image in your photo-editing software.
The same advice goes for the sharpness enhancement feature (or "unsharp mask") found in most scanner drivers. Use it if you like the results, but feel free to turn it off. Your photo-editing software can probably do a better job than the scanner software.
Descreening attempts to convert an image made up of discrete dots (such as a halftoned newspaper photo) into a continuous-tone image. Most scanner drivers accomplish this task by overscanning the image: scanning the same area several times, each slightly offset from the last, and combining the results to create the final image. Some scanners do a good job descreening, so you can use this option if you want to get a better looking scan of a halftoned image. But descreening can significantly increase the time required for the scan.
How do you convert a document to editable text?
A scanner can convert any printed material into a digital image. For some tasks, like retouching photos or archiving old documents, a digital image may be all you need. For other tasks, you may need to take the scanned text and convert it to an editable format for word processing or page layout. This process is a called Optical Character Recognition, or OCR. Most scanners come with a very basic OCR utility, but if you have serious OCR needs you will probably want to get a package like Scansoft's Omnipage, Adobe Acrobat Capture, or ABBYY FineReader.
Virtual Drum Principle
The Flextight concept
Most people who know the Imacon Flextight scanner will think that the Flextight concept is identical with the flexible original holder. And it is correct that this ground-breaking technology is a very important part of the Flextight concept and it is correct that Imacon has done a lot of promotion to emphasize that.
But the Flextight concept is much more; it implies both some new ideas and some very simple ways of implementing things in a more clever way. But let us start with the holder and then go through all the smart parts of the concept.
The Flextight holder
What do we want to achieve with an original holder? We want to be able to keep the original in perfect focus across the scan line. We want to avoid Newton rings. We want to have as few surfaces in the light path as possible to clean and to break the light waves. Finally we made it as easy as possible to mount the original in the holder.
All this we have achieved in the Flextight original holder, which is glass free, with a magnetic rubber overlay, which makes mounting a very easy task. The perfect focus across the scan line is achieved by bending the original around a virtual drum. When you bend a media in one direction it becomes perfectly straight and stable in the other direction.
The holder concept makes it possible to take the holder from the scanner, after preview, and place it on a light table for comparing and adjusting the scanner preview. When finished the holder is easily put into the scanner for final scanning. This feature is not available on most of the competitive scanners.
The holder concept makes it possible to take the holder from the scanner, after preview, and place it on a light table for comparing and adjusting the scanner preview. When finished the holder is easily put into the scanner for final scanning. This feature is not available on most of the competitive scanners
Focal Length
Why is the scanner so high? It is because we want to have a long focal length. By having a long focal length we only utilize the lens where it is best, which means less distortion and therefore fewer corrections.
The glass free optical path
By building the scanner like we did we have actually achieved a glass free optical path. This means that we do not have to correct for unwanted effects like chromatic aberration or diffusion caused by glass plates, prisms or mirrors. It also means that less mechanics are to be adjusted and finally that less surfaces are to be kept clean.
CCD facing downwards
A simple but obvious thing is to place the CCD face down, as it is unlikely that dust gathers on the light sensitive cells in this position.
The direct analogue to digital conversion
When the analogue signal produced in the CCD is to be converted to a digital signal it has to move from the CCD to the AD converter. As the analogue signal is a low tension electric current it is very sensitive to electrical noise, which very likely is produced by other electrical units in and around the scanner.
Imacon has therefore decided to place the AD converters directly on the CCD board to keep the travelling distance as short as possible. And to lower the risk of noise we have even decided to have one AD converter for each color channel instead of sending the signals through a multi-plexer, which has the potential risk of adding more noise
Keeping heat out/down
Every time the temperature raises with 10° Celsius the electrical noise in the sensor doubles. This means that it is very important to prevent the sensor from heating up. The most obvious source of heat is the power supply. By removing that from the cabinet and placing it outside the scanner one big problem is eliminated. A second thing is the light tube. This light tube is a cold cathode tube, which produces very little infrared waves and therefore produces very little heat.
The sensor itself is an electronic device that heats up when the clock frequency is speeded up. This is usually done to make fast previews. The sensor samples full information even at preview scans, so every cell has to be emptied to produce even a low-resolution preview
Imacon has found a way to add the information in the sensor together directly, which means that we do not have to speed up the readout of the sensor, therefore not producing the heat, which is common in this process. At the top of the line scanner Imacon has even chosen to cool the sensor actively, which means that an electronic device is placed directly on the CCD to cool it down. This will increase signal to noise ration with 1 to 2 bits.
Stable metal construction
To keep the scanner stiff and stable all mechanic parts as well as the cabinet are made out of metal.
A. Laser Scanner Basic Principles
The synchronized scanning geometry developed at NRC is based on a doubled sided mirror which is used to project and detect a focused or collimated laser beam.
The source used is a laser, which is typically coupled to an optical fiber. The scanning mirror and a fixed mirror are used to project the laser beam on the scene. The scattered light is collected through the same scanning mirror used for the projection and focused to a linear CCD (shown on the left side of the figure). Note that the CCD is tilted (Scheimpflug condition) to compensate for defocusing at the detection.
With careful optical design the divergence of the laser beam can be made to match the resolving element field of view of the CCD linear array. In such conditions the parameters of the focused laser beam are kept constant over a large depth of view. This property combined with the Scheimpflug condition allows the 3-D digitizing from a very short distance (10's of cm) to a large distance (10 meters) without refocusing or processing algorithm modifications.
Essentially the configuration illustrated on this figure is a profile measurement device. A second scanning mirror (not shown on this illustration) can be used to deflect orthogonally both the projected and the reflected laser light. The whole camera arrangement can be mechanically translated by commercially available gantry linear positioning device or by rotary table.
The Large Field of View 3-D laser scanner shown below uses two orthogonal gloves to address a 4000 pixel by 4000 pixel field of view. The optical configuration allows the 3-D recordings from 50 cm to 10 m from the scanner. It uses a linear CCD array as a position sensor. The minimum element of resolution of the CCD corresponds to a resolution in depth of 100 microns at 50 cm, and approximately increases as the square of the distance. For example, at 10 m the depth resolution is 2 cm.
For heritage recording applications in the field, the scanner can be mounted on a conventional tripod. To record high or large objects, it can also be mounted on a telescoping tripod which extends to a height of 10 meters. For these applications, the scanner and a video camera is mounted on a remote controlled pan and tilt unit. This enables to operator to control the point of view of the scanner from the control system (see photos below).
3D digitizer systems
For the accurate localization of brain activity you need to know the exact positions of the electrodes at the scalp. To measure those positions a digitizer device can be used that gives you those coordinates.
For this purpose ANT offers two different kinds of devices:
• Polhemus Fastrak - system can be operated by the EETrak software
• ELGuide electrode positioning system
Quick comparison
Feature Fastrak ELGuide
Measurement principle Magnetic fields Ultrasonic pulses
Measurement resolution 0.005 mm 0.1 mm
Measurement accuracy approx. 0.76 mm approx. 0.8 mm
Reposition accuracy - approx. 1.2°
Software EETrak
ELPos
Operating system Windows 98, NT
and 2000 Windows 95, 98
and NT
read more
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3SPACE FASTRAK
This award-winning system of Polhemus Inc. is a highly accurate, low-latency 3D motion tracking and digitizing system. FASTRAK can track up to four receivers at ranges of up to 10 feet. The perfect solution for the position/orientation measuring requirements of applications and environments. Ideal for head tracking, hand tracking, instrument tracking, biomechanical analysis, graphic and cursor control, stereotaxic localization, telerobotics, digitizing and pointing.
The 3SPACE FASTRAK accurately computes the position and orientation of a tiny receiver as it moves through space. This device virtually eliminates the problem of latency as it provides dynamic, real time six degree-of-freedom measurement of position (X, Y, and Z Cartesian coordinates) and orientation (azimuth, elevation, and roll), and it is the most accurate electromagnetic tracking system available.
FASTRAK is the perfect solution for interfacing with Virtual Reality environments and controlling simulator projectors or other applications where real time response is critical. It is also ideal for measuring range of motion or limb rotation in biomedical research. It is a fast, accurate, easy to use, and effective method of capturing motion data on any non-metallic object.
The FASTRAK system utilizes a single transmitter and can accept data from up to four receivers. The use of advanced digital signal processing (DSP) technology provides an update rate of 120 Hz (with a single receiver) and a remarkable 4ms latency. The data are then transmitted over a high speed RS-232 interface at up to 115.2K baud or over an even faster optional IEEE-488 interface at up to 100K bytes/sec. If your application requires using the system in close proximity to a CRT, FASTRAK has special circuitry to allow you to synchronize with it for improved performance.
And because FASTRAK uses patented low-frequency magnetic transducing technology, there's no need to worry about maintaining a clear line-of-sight between receiver and transmitter. Polhemus has eliminated the problem of signal blocking and interference that distorts sonic or laser devices.
Components
The 3SPACE FASTRAK system includes a System Electronics Unit (SEU), a power supply, one receiver and one transmitter. You can expand the system's capabilities simply by adding up to three additional receivers. FASTRAK is also available as a board-level product for OEM/VARs.
System Electronics Unit: Contains the hardware and software necessary to generate and sense the magnetic fields, compute position and orientation, and interface with the host computer via an RS-232 port (IEEE-488 and RS-422 are optional).
Transmitter: The transmitter is a triad of electromagnetic coils, enclosed in a plastic shell, that emits the magnetic fields. The transmitter is the system's reference frame for receiver measurements.
Receiver: The receiver is a small triad of electromagnetic coils, enclosed in a plastic shell, that detects the magnetic fields emitted by the transmitter. The receiver is a lightweight cube whose position and orientation is precisely measured as it is moved. The receiver is completely passive, having no active voltage applied to it.
Stylus (Optional): The stylus is a pencil-like device that contains a triad of electromagnetic coils, and is used for digitizing objects, drawing in three-dimensional space, or collecting contours of objects. The stylus is available in 3" or 8" lengths, with either a sharp or round nib.
Features
Real Time: Virtually no latency. Digital Signal Processing (DSP) technology provides 4ms latency updated at 120 Hz. And data is transmitted to the host at up to 100K bytes/sec.
Improved Accuracy and Resolution: Accuracy of 0.03" RMS with a resolution of 0.0002 in./in. makes this the most precise device of its kind.
Range: Standard range is up to 10 feet. Operation over a range of up to 30 feet is now possible with the optional LONG RANGER transmitter.
Multiple Receiver Operation: Permits measurement of up to 4 receivers on a single system and up to 32 receivers at a time, utilizing eight multiplexed systems.
Reliable: From the pioneer in position/orientation measuring devices in business since 1970. Factory calibrated, never needs adjustment.
Multiple Output Formats: Position in Cartesian coordinates (inches or centimeters); orientation in direction cosines, Euler angles or Quaternions.
Specifications
Position Coverage: The system will provide the specified performance when the receivers are within 30 inches of the transmitter. Operation over a range of up to 10 feet is possible with slightly reduced performance.
Latency: 4 milliseconds.
Update Rate: 120 updates/seconds divided by the number of receivers.
Interface: RS-232 with selectable baud rates up to 115.2K baud (optional RS-422). IEEE-488 at up to 100K bytes/sec; ASCII or Binary format.
Static Accuracy: 0.03" RMS for the X, Y, or Z position; 0.15 degrees RMS for receiver = orientation.
Resolution: 0.0002 inches per inch of transmitter and receiver separation; 0.025 degrees orientation.
Range: Up to 10 feet with standard transmitter. Up to 30 feet with LONG RANGER transmitter.
Multiple Systems: Up to 8 systems can be frequency multiplexed with no change in update rate.
CRT Interference Rejection: Provided by means of an external cable and sensor.
Angular Coverage: The receivers are all-attitude.
Operating Environment: Large metallic objects, such as desks or cabinets, located near the transmitter or receiver, may adversely affect the performance of the system.
Operating Temperature:10 C to 40 C at a relative humidity of 10% to 95%, noncondensing.
Physical Characteristics: SEU - 11.0" L x 11.4" W x 3.6" H; Power Supply - 7.0" L x 3.7Y W x 2.2Y H; Transmitter - 2.3" L x 2.2" W x 2.2" H; Receiver - 0.9" L x 1.1" W x 0.6" H
Power Requirements:25 W, 90-250 VAC, 38-65 Hz
Regulations: Meets FCC, CSA, UL, and CE Requirements.
Specifications subject to change without notice
Software
To operate the Fastrak device you need software, for instance EETrak.
ELGuide digitizer
When using computer-assisted analytical methods for the evaluation of EEG data, the accuracy of the positional measurement of the EEG electrodes on the patient's head has a direct influence on the consistency of the results. ELGuide was developed in order to replace the previous complicated and mostly inaccurate methods for determining EEG electrode positions with a simple and highly precise procedure.
Applications
• Determining the 3D coordinates of electrodes already applied to the patient's head.
• Positioning the electrodes after specification of the exact coordinates of the measuring point.
• Sensing the cranial shape of the patient in 3D coordinates for correlation with data from imaging methods.
A pointer is used to determine the electrode positions and for sensing the cranial shape.
Electrode positions already measured can be used as a template for later examinations. By this means, the electrodes can again be applied to exactly the same positions in only a short time. The target coordinates for repositioning can also be taken from a freely definable template file which can be created with an editor.
Fixation of the patient's head during the measurement is not necessary with ELGuide. The operation of the software is learned very easily and quickly.
ELGuide improves the results of your EEG measurements and you obtain by simple means important data for accurate analysis of your examinations.
A template can be selected easily before a measurement. Pre-auricular points are acquired to transform the measured position to the nasion-ear coordinate system.
