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If you work with hard copy documents, you're going to need a document scanner a lot. Document scanners are of three types-flatbed, hand-held and sheet-fed. Of these, flatbed scanners work in a way that requires users to place the scannable document on the glass pane inside the device. Within the flatbed scanner lies an image sensor which moves down the page and copies its contents.

Hand-held scanners are used manually by the user. Here, the user must manually move the sensor over the length of the document so that it is scanned.

A sheet-fed scanner is just like a printer in the sense that it glides the sheet of paper over the sensor to get a scanned image of the page.

The CCD Sensor: Usually, scanners use a charge-coupled device (CCD) or a Contact Image Sensor (CIS) for its image sensor. However, drum scanners work with a photomultiplier tube as its image sensor. The image sensor of most document scanners comprises a range of charge-coupled devices. This refers to the device responsible for the movement of electrical charge from inside the device to a situation where it can be manipulated or converted into digital values.

This CCD range works through photo-sensitive diodes that convert light they catch into an electric signal carrying the same data. In flatbed scanners, the CCD array has a light within the scanner that brightens up the page and makes it easier for the array to read. Usually, sensors work in a three color mode - RGB. This means that the document's image will be separated into these three images - red, green and blue - by a lens before they are replaced when they can be seen on a computer.

The CIS Sensor: Here, the contact image sensor is different. It contains hundreds of RGB LED lights. When in combination, they become white light. To scan a document, the sensor is placed really close to the document with a glass pane separating them. When the document is scanned, it throws up white light. At this point, sensors pick up the image of the document.

Transfer to the computer: The scanning process ends with the scanned image being transferred to the attached computer. A document scanner can be attached to a computer by way of a USB, Small Computer System Interface, a group of computers connected to a common scanner or by FireWire. The language scanners understand and read is called TWAIN. These language drivers are usually accompanied by programs like PhotoShop and form part of the installation CD which comes with the scanner you buy.

Once the image is available on the computer, it can be put into different file types such as PDF for document viewing, or PNG which shows an uncompressed image with very high quality but which uses a lot of memory space, and JPG, another file compression tool that reduces the size of image files and consequently its image quality.

So, if you need a document scanner, choose wisely from the above types and ensure that you get the most out of it.

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Important Facts About Scanners

A scanner is a device that has succeeded telephotography. These days, there are different scanners being sold in the market.

A scanner is an input device that:
- scans images
- prints text
- prints handwriting
- prints an object and converts it to digital images

Kinds of Scanners
- Drum scanners have rotating drums with a one-photo detector for a standard speed of 60 or 120 rpm. Using a telephone voice line to receptor, they can send linear analog AM signals. It prints the proportional intensity on special paper synchronously. This was used from the 1920s to the 1990s.

Color photos were sent as three separated and repeated RGB filtered images. This was only used occasionally because of the transmission cost.

- Desktop scanners or flatbed scanners found in offices are common examples of scanners. Documents are placed on the glass window for scanning.
- Handheld scanners are devices moved by hand. It evolves from text scanning to 3D scanners which are used for:
• industrial design
• reverse engineering
• test measurement
• orthotics
• gaming and more

These mechanically-driven scanners are used for large-format documents. Flatbed designs are not practical.

What these scanners use:
1. Modern scanners use image sensors such as:
• Charge-coupled device (CCD) or
• Contact Image Sensor (CIS)
2. Older drum scanners use a photomultiplier tube as its image sensor.
3. Rotary Scanner uses a CCD selection instead of a photomultiplier. Rotary Scanner is used for express document scanning. This is another type of drum scanner.
4. Planetary Scanner takes photographs of books and documents.
5. 3D Scanners are used for producing three-dimensional models of objects.

Other category of scanners:
Digital Camera Scanners
This is a reprographic camera. It becomes an attractive alternative to ordinary scanners. The disadvantages of this scanner are:
• distortion
• reflection
• shadows
• low contrast

Some of the advantages are:
• speed
• portability
• gentle digitizing of thick documents without damaging the book spine

The new scanning evolution combines 3D scanners with digital cameras. This is to make:
- full color
- photo realistic 3D model objects

Flatbed Scanners possess a glass pane with a bright light. These bright lights illuminate the pane and move optical selection which may be CCD or CIS.

These lights are usually:
• xenon or
• cold cathode fluorescent

Color scanners usually have three rows of sensors with:
• red
• green and
• blue filters

The images are scanned by placing images face down on the glass. An opaque cover is lowered over it so as to cover ambient light. The sensor selection will move over the pane in order to read the entire copy. Due to the reflecting light, the image becomes visible to the charge-coupled device. See-through images do not work this way. It needs special accessories in order to light them up from the upper side.

Scanning is only a part of the process. In order to make the scanned object useful, it must be transferred to an application running on the computer.

There are two basic issues with this:
1. the physical connection of the scanner to the computer and
2. the information retrieval of the application from the scanner
The Physical Connection of the Scanner to the Computer
The amount of data gathered by a scanner can be very large.
For example: A 600 DPI 9" x 11" uncompressed 24-bit image. It consumes about 100 megabytes. With this, uncompressed data is transferred and stored on the computer. The latest scanners can gather this volume of data in a matter of seconds. It makes a desirably fast connection.

The Four Connections used by Scanner
1. Parallel
2. Small Computer System Interface (SCSI)
3. Universal Serial Bus (USB)
4. Fire Wire

Application Programming Interface
An application must be able to communicate with scanners such as Adobe Photoshop. There are different scanners and each scanner has different protocols. To simplify application for programming, Application Programming Interfaces were made. Hence, API gives a uniform interface to the scanner. The application does not require knowing the specific details of the scanner so as to access it directly.

In reality, there are problems with an application communicating with scanners. Maybe the application or the manufacturer of the scanner has made a mistake in their implementation of API.

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Why can photomultiplier tubes not be used with infrared radiation?

Why can photomultiplier tubes not be used with infrared radiation?

Short Answer: A PM tube doesn't operate in that wavelength.

A PM tube detects light which is generated from a scintillator. The scintillators are typically designed to scintillate as a result of a reaction with ionizing radiation.

Long Answer:

Photomultiplier Tubes

The purpose of the photomultiplier tube is to detect the scintillations and to provide an output signal proportional to the amount of scintillations. In doing this, photomultiplier tubes can provide amplifications of 1 E6 and higher.

Construction

Construction details vary from design to design; however, all photomultipliers have typical components. These common components are: the photocathode, the dynode assembly, an anode, voltage divider network, and shell. These components perform as follows:

• Photocathode - made of antimony - cesium composite. The purpose of the photocathode is to convert the light photons to electrons (called photoelectrons).

• Dynode Assembly - A series of electrodes used to amplify the signal. Each successive dynode has a higher voltage potential. The voltage gradient along the tube accelerates the electrons towards the anode. This works as follows: the photoelectron strikes the first dynode freeing one or more electrons. These electrons are drawn towards the second dynode. At the second dynode each electron frees one or more additional electrons. This process continues until the electron cascade reaches the anode. Through this process, the initial photoelectron is amplified, up to 106 times and higher. For an amplification of 106 an average of 4 electrons is freed by each incident electron reacting with each dynode (10 dynodes - 410 106).

• Anode - The anode collects the electrons and generates an output pulse.

• Voltage Divider Network - Splits the high voltage supply into the various potentials required by the dynodes.

• Shell - Supports the other components and seals the tube from stray light and stray electric/magnetic fields.

Output

The photomultiplier tube provides an output pulse which is proportional to the incident photons. The size of the pulse is a function of the energy of the light photon, and of the electron multiplication. Varying the HV to the photomultiplier varies the pulse height.

It is possible for stray electrons to be amplified by the dynode, creating an output pulse while no photon entered the tube. Those electrons can be spontaneously emitted from the photocathode or by the dynodes themselves. This output signal is commonly called dark current. Dark current increases with photomultiplier tube temperature, hence, temperature changes may cause the detector to "drift."

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