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Concepts in Digital Imaging Technology

Interactive Java & Flash Tutorials

We have constructed several interactive Java tutorials to help demonstrate many of the complex topics in photography through the microscope utilizing digital imaging technology. Use the links below to navigate to specific tutorials of interest.

Avalanche Diodes - An avalanche photodiode is a silicon-based semiconductor containing a pn junction consisting of a positively doped p region and a negatively doped n region sandwiching an area of neutral charge termed the depletion region. These diodes provide gain by the generation of electron-hole pairs from an energetic electron that creates an "avalanche" of electrons in the substrate.

Electron-Bombarded CCDs - Electron-bombarded charge-coupled devices (CCDs) are a relatively new development in which photons are detected by a photocathode in a manner similar to an image intensifier. The high-energy electrons that strike the CCD generate multiple charges resulting in a modest gain of a few hundred.

Binning - Discover how clock signals controlling a CCD can be used to combine integrated charge from adjacent pixels to improve signal-to-noise ratios and increase the readout frame rate.

CCD Blooming - Under conditions where a CCD is exposed to very high intensity illumination, it is possible to exhaust the storage capacity of the CCD wells, a condition known as blooming. When this occurs, excess charge will overflow into adjacent CCD photodiode wells resulting in a corrupted image near the blooming site. This tutorial explores the operation of a lateral overflow drain to prevent CCD blooming.

CCD Clocking Schemes - Charge transfer through CCD shift registers occurs after integration to relocate accumulated charge information to the sense amplifier, which is physically separated from the parallel pixel array. This tutorial explores several clocking schemes that are utilized to transfer charge from the collection gates to the output node.

Building A Charge-Coupled Device - Explore the steps utilized in the construction of a charge-coupled device (CCD) as a portion of an individual pixel gate is fabricated on a silicon wafer simultaneously with thousands or even millions of neighboring elements.

Building A Charge-Coupled Device (Version Number 2) - A second examination of the steps utilized in the construction of a charge-coupled device (CCD). Like the first version this tutorial examines the fabrication of a CCD as a portion of an individual pixel gate is fabricated on a silicon wafer simultaneously with thousands or even millions of neighboring elements.

CCD Operation - Explore the operation of a charge-coupled device (CCD) imaging semiconductor with this interactive Flash tutorial. Modern CCDs consist of a light-sensitive sandwich of insulating silicon dioxide positioned beneath an array of photodiodes and above an array of metal electrodes.

Electronic Shutters - Electronic shutters are employed in charge-coupled devices (CCDs) to control integration time (exposure) of the photodiode array and reduce smear when capturing moving objects in the microscope. Investigate the operation of an electronic shutter in controlling exposure using this interactive Java tutorial.

Frame-Transfer CCD Operation - Designed to operate fast and efficiently without a shutter or synchronized strobe, frame-transfer CCDs exhibit higher frame rates than full-frame designs. Explore image acquisition and transfer in frame-transfer charge-coupled devices with this interactive Java tutorial.

Full-Frame CCD Operation - Full-frame charge-coupled devices have the simplest architecture and are the easiest devices to build and operate. These devices feature high-density pixel arrays capable of producing digital images with the highest resolution currently available.

Interline CCD Operation - Interline charge-coupled device architecture is designed to compensate for many of the shortcomings of frame-transfer CCDs. These devices are composed of a hybrid structure incorporating a separate photodiode and a CCD storage region, protected with a mask structure, into each pixel element.

Interaction of Photons with Silicon - In a charge-coupled device (CCD) incident light must first pass through a silicon nitride passivation coating as well as several thin films of silicon dioxide and polysilicon gate structures before being absorbed into the silicon substrate. This interactive tutorial explores the interaction of photons with silicon as a function of wavelength.

Microlens Arrays - Microlens arrays (also referred to as microlenticular arrays or lenslet arrays) are used to increase the optical fill factor in CCDs, such as interline devices, that suffer from reduced aperture due to metal shielding. These tiny lens systems serve to focus and concentrate light onto the photodiode surface instead of allowing it to fall on non-photosensitive areas of the device, where it is lost from the imaging information collected by the CCD.

Photomultiplier Tubes - Photomultiplier tubes, useful for light detection of very weak signals, are photoemissive devices in which the absorption of a photon results in the emission of an electron. These detectors work by amplifying the electrons generated by a photocathode exposed to a photon flux. Explore electron amplification in photomultiplier tubes with this interactive Flash tutorial.

Channel Photomultipliers - Channel photomultipliers represent a new design that incorporates a unique detector having a semitransparent photocathode deposited onto the inner surface of the entrance window. Photoelectrons released by the photocathode enter a narrow and curved semiconductive channel. Each time an electron impacts the inner wall of the channel, multiple secondary electrons are emitted. These ejected photoelectrons have trajectories angled at the next bend in the channel wall (simulating a dynode chain), which in turn emits a larger quantity of electrons angled at the next bend in the channel. The effect occurs repeatedly, leading to an avalanche effect, with a gain exceeding 100 million.

Side-On Photomultiplier Tubes - In the side-on photomultiplier tube design, photons impact an internal photocathode and eject electrons from the front face. These ejected photoelectrons have trajectories angled at the first dynode, which in turn emits a larger quantity of electrons angled at the second dynode (and so on).

Proximity-Focused Image Intensifiers - Image intensifiers were developed for military use to enhance our night vision and are often referred to as wafer tubes or proximity-focused intensifiers. They have a flat photocathode separated by a small gap on the input side of a micro-channel plate (MCP) electron multiplier and a phosphorescent output screen on the reverse side of the MCP.

Contributing Authors

Mortimer Abramowitz - Olympus America, Inc., Two Corporate Center Drive., Melville, New York, 11747.

Kenneth R. Spring - Scientific Consultant, Lusby, Maryland, 20657.

John C. Long, Kirill I. Tchourioukanov, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.


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