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Glossary Of Terms

Accumulation

Accumulation is the process by which data that have been acquired from a number of similar scans are added together in computer memory.

Acid Etching

A technique that uniformly thins CCDs to approximately 10 µm so that an image can be focused on the back of the parallel register (where there is no gate structure)

Acquisition

An Acquisition is taken to be the complete data capture process.

Adapter

A device that allows a CCD camera to be attached to a variety of scientific instruments or lenses. Also referred to as mount adapter and lens mount adapter.

Analog

A scheme for representing data via continuous amplitudes.

Analog-to-Digital (A/D) Converter

Analog-to-digital converter. In a CCD camera system, the electronic circuitry that converts the analog information (continuous amplitudes) acquired by the detector into the digital data (quantified, discrete steps) used for image display.

Analog-to-Digital Unit

A number representing a CCD's output. The relationship between the ADUs generated and the number of electrons acquired on the CCD is defined by the system gain. Intensities given in ADUs provide a convenient method for comparing images and data generated by different cameras.

Angstrom

A unit of measure equal to 0.1 nanometers.

Arc Lamp

Often used as a light source on a microscope, an electric light in which a current traverses a gas between two incandescent electrodes and generates an arc that produces light. Arc lamps have a limited lifetime.

Background

Background is a data acquisition made in darkness. It is made up of fixed pattern noise, and any signal due to dark current.

Back-Illuminated (Back-Thinned) CCD

A CCD that has been uniformly reduced to a thickness of approximately 10 µm so that an image can be focused on the back of the parallel register (where there is no gate structure). Thinned CCDs exhibit high sensitivity to photons ranging from the soft x-ray to the near-infrared regions of the spectrum. Since light is hitting the silicon directly instead of passing through the gate structure, sensitivity to blue light is particularly good. Many back-illuminated CCDs also have ultraviolet coatings that "down convert" UV light into the visible portion of the spectrum, further increasing QE.

Bias

In a CCD camera system, the minimum intensity required for each exposure (equivalent to performing a zero-second exposure with the shutter closed). Without adding any light, the bias allows charge to be read out on the CCD while raising the intensity level high enough to ensure that the camera does not deliver a negative number to the A/D converter. (The A/D converter only works in the set of positive numbers and has no instructions for processing negative numbers.) The bias, which is not user selectable, is set at the factory and remains stable over the lifetime of the camera system.

Binned Readout

Within a CCD, the process of moving charge that has been binned to an output amplifier for conversion to an image.

Binning

Binning is a process that allows charge from two or more pixels to be combined on the CCD-chip prior to readout. Summing charge on the CCD and doing a single readout results in better noise performance than reading out several pixels and then summing them in the computer memory. This is is is because each act of reading out contributes to noise (see NOISE).The two main variants of the binning process are:

• VERTICAL BINNING
• HORIZONTAL BINNING

These variations are individually described in the pages that follow.In addition there are several binning patterns that tailor the main binning variants to typical application usage.

Vertical Binning

In Vertical Binning, charge from two or more rows of the CCD-chip is moved down into the shift register before the charge is read out. The number of rows shifted depends on the binning pattern you have selected.Thus, for each column of the CCD-chip, charge from two or more vertical elements is "summed" into the corresponding element of the shift register.The charge from each of the pixels in the shift register is then shifted horizontally to the output node of the amplifier and read out.Variants of Vertical Binning are used to affect a variety of binning patterns. These are namely, Single-Track, Multi-Track & Full Vertical Binning (FVB) and these are described as follows:

• Single-Track: Charge is vertically binned and read out from a number of complete, adjacent rows of pixels on the CCD-chip. The rows form a single track across the full width of the CCD-chip.A value is taken for each column in the track.
• Multi-Track: Multi-Track mode differs from Single-Track in that you may now define two or more tracks from which to read out charge. In processing terms, each track is treated as in Single-Track above.
• Full Vertical Binning (FVB): Charge from each complete column of pixels on the CCD is moved down and summed into the shift register and the charge is then shifted horizontally one pixel at a time from the shift register into the output node. In effect a value is read out for each complete column of the CCD-chip.

The example below illustrates readout of data from adjacent tracks, each track comprising two binned rows of the CCD-chip:

1. Exposure to light causes a pattern of charge (an electronic image) to build up on the frame (or "image area") of the CCD-chip.

2. Charge in the frame is shifted vertically by one row, so that the bottom row of charge moves down into the shift register.

3. Charge in the frame is shifted vertically by a further row, so that the next row of charge moves down into the shift register, which now contains charge from two rows - i.e. the charge is vertically binned.

4. Charge in the shift register is moved horizontally by one pixel, so that charge on the endmost pixel of the shift register is moved into the output node of the amplifier.

5. The charge in the output node of the amplifier is passed to the analog-to-digital converter and is read out.

6. Steps 4 and 5 are repeated until the shift register is empty. The process is repeated from Step 2 until the whole frame is read out.

Shifting the charge horizontally from several pixels at a time into the output node is known as horizontal binning. Horizontal binning in combination with vertical binning allows you to define so-called superpixels that in Image Display Mode represent as a single picture element charge that has been binned from a group of pixels.For example, charge that is binned vertically from two rows and horizontally from two pixels before being read out is displayed as a superpixel of dimensions 2 x 2 pixels. On the one hand, superpixels (by comparison with single pixels) result in a more coarsely defined image when the data are displayed in Image Display Mode; on the other hand, superpixels offer the advantages of summing data on-chip prior to readout.In the following example, where each superpixel is of dimensions 2x2 pixels, charge from two rows is first binned vertically into the shift register; then charge from two pixels of the shift register is binned horizontally into the output node of the amplifier.The effect of the combined binning processes is a summed charge equating to a 2x2 "superpixel".Since this example initially involves binning charge from two rows, the process begins in the same way as the previous example (see Steps 1 - 4 of Vertical Binning of Two Rows),then horizontal binning begins.

Horizontal Binning (Creating Superpixels)

4. Charge from two rows has already been vertically binned into the shift register (see "Vertical Binning of Two Rows", previously). Now charge in the shift register is moved horizontally by one pixel, so that charge on the endmost pixel of the shift register is moved into the output node of the amplifier.)

5. Charge in the shift register is again moved horizontally, so that the output node of the amplifier now contains charge from two pixels of the shift register - i.e. the charge has been horizontally binned.

6. The charge in the output node of the amplifier is passed to the analog-to-digital converter and is read out.

7. Steps 4 to 6 are repeated until the shift register is empty. The process is repeated from Step 2 (again, see Vertical Binning of Two Rows) until the whole frame is read out.

Binning Factor

The number of pixels to be combined on a CCD during binning. A binning factor of 2x2 means that the pixels in two rows and two columns (a total of four pixels) are combined for CCD readout.

Bit Depth

The number of bits (smallest unit of information in a notation using the binary system) that are digitized by a system's A/D converter.

BNC Connector

A connector used to couple coaxial cables to high-frequency electronic equipment, such as a video monitor.

Bulb Mode

A type of exposure in which a trigger signal from an external source controls both the start and end of the exposure.

Camera Electronics Unit (CEU)

In some digital camera systems, a separate hardware entity that manages the transfer of raw CCD data from the camera to the camera controller, and from the camera controller to the host computer.

Charge

.In CCD imaging technology, a measure of the number of electrons confined by a pixel.

Charge-Coupled Device

A silicon-based semiconductor chip bearing a two-dimensional matrix of photo-sensors, or pixels, referred to as the image area. The pixels can be considered as being arranged in rows or columns. A typical CCD-chip may comprise 256 rows and 1024 columns, or 578 rows and 385 columns.

The CCD in a detector is a scientific slow scanning device, in contrast to a fast scan CCD used in video cameras to capture moving images.

An example of a typical layout is shown here:

The shift register runs below and parallel to the light collecting rows. It has the same number of pixels as a light-collecting row, but is itself masked, so that no light can fall on it. When light falls on an element, electrons (photoelectrons) are produced and (in normal operation), these electrons are confined to their respective elements.Thus, if an image (or any light pattern) is projected on to the array, a corresponding charge pattern will be produced. To capture the image pattern into computer memory, the charge pattern must be transferred off the chip, and this is accomplished by making use of a series of horizontal (i.e. parallel to the rows/shift register) transparent electrodes that cover the array.By suitable "clocking", these electrodes can be used to shift (transfer) the entire charge pattern, one row at a time, down into the shift register. The shift register also has a series of electrodes (which are vertical, i.e. parallel to the columns) which are used to transfer the charge packets, one element at a time, into the output node of the "on-chip" amplifier. The output of the amplifier feeds the analog-to-digital (A/D) converter, which in turn converts each charge packet into a 16-bit binary number.

Charge Smearing

The ability of a CCD to transfer the charge in each individual pixel to the next pixel without any loss in the charge during the transfer. Scientific-grade CCDs typically have a charge-transfer efficiency (CTE) of 99.9998%, where 100% is perfect. Also refers to the process by which the electrons in one potential well are moved to an adjacent well.

Charge Transfer

Residual charge left behind in potential wells when an image is shifted within a CCD.

C-Mount

A standard screw-in lens mount common to many scientific instruments. (Thread of lens and lens mount: 1-inch diameter, 32 threads/inch. Back focal length: 17.52 mm.)

Convolution

In digital imaging, the replacement of each pixel's gray level with a new value that has been adjusted to take into account the values of neighboring pixels. The degree to which the image is either smoothed or sharpened depends upon the specific calculations performed.

Cooling

The CCD is cooled using a thermoelectric (TE) cooler. TE coolers are small, electrically powered devices with no moving parts, making them reliable and convenient. A TE cooler is actually a heat pump i.e. it achieves a temperature difference by transferring heat from its "cold side" (the CCD-chip) to its "hot side" (the built-in heat sink). Therefore the minimum absolute operating temperature of the CCD depends on the temperature of the heat sink. Andor's vacuum design means that we can achieve a maximum temperature difference of over 110°C performance unrivalled by other systems. The maximum temperature difference that a TE device can attain is dependent on the following factors:

• Heat load created by the CCD
• Number of cooling stages of the TE cooler
• Operating current

The heat that builds up on the heat sink must be removed and this can be done in one of two ways:

• AIR COOLING: A small built-in fan forces air over the heat sink. Air cooling is the most convenient method of cooling, but it will not achieve as low an operating temperature as water cooling (see below). Even with a fan a heat sink typically needs to be 10°C hotter than the air (room) temperature to transfer heat efficiently to the surrounding air. Therefore the minimum CCD temperature that can be achieved will be dependent on the room temperature.
• WATER COOLING: External water is circulated through the heat sink using the water connectors on the top of the head. A flow of water through the heat sink removes heat very efficiently, since the heat sink is never more than 1°C hotter than the water.

With this type of cooling, the minimum temperature of the CCD will be dependent only on the water temperature, and NOT on the room temperature. Water cooling, either chilled though a refrigeration process or re-circulated (which is water forced air cooled then pumped) allows lower minimum operating temperatures than air cooling.However, there is a very important point relating to water cooling. If the water temperature is lower than the dew point of the room, condensation will occur on the heat sink, the water taps and other metal parts of the head. This will quickly destroy the head and must never be allowed to happen. However this is not an issue when using a recirculator which eliminates the dew point problem. Whichever method is being used, it is not desirable for the operating temperature of the CCD simply to be dependant on or vary with the heat sink temperature. Therefore a temperature sensor on the CCD, combined with a feedback circuit that controls the operating current of the cooler, allows stabilisation of the CCD to any desired temperature within the cooler operating range. As well as a choice of cooling method there is also a choice of performance versus compactness. All Andor CCD systems support both cooling options.

Counts

Counts refer to the digitization by the A/D conversion and are the basic unit in which data are displayed and processed. Depending on the particular version of the detection device, one count may, for example, be equated with a charge of 10 photoelectrons on a pixel of the CCD.

Correlated, Double-Sampling Readout

A sampling technique used to achieve higher precision in CCD readout. The sampling circuit is reset to a predetermined reference level and then the actual pixel voltage is sampled in order to find the difference between the two. Using the resulting correlation minimizes read noise, especially in ultra-low-noise cameras.

Cropped Sensor Mode

Specialized readout mode for achieving sub-millisecond temporal resolution from EMCCD and CCD cameras. Cropped Sensor Mode is a user-defined ‘sub-array’ size from within the full image sensor area, such that it encompasses the region of the image where change is rapidly occurring). The sensor subsequently “imagines” that it is of this smaller defined array size, achieved through software executing special readout patterns, and reads out at a proportionally faster frame rate.

Dark Current

The charge accumulated within a well, in the absence of light. Also the background current that flows in a charge-coupled device or image intensifier of a camera system. Cooling the photodetector's primary imaging surface (i.e., the CCD's photoconductor or the image intensifier's photocathode) can reduce or eliminate dark current.

Dark Noise

The statistical variation of the dark current, equal to the square root of the dark current. Dark current can be subtracted from an image, while dark noise remains.

Dark Signal

Dark signal, a charge usually expressed as a number of electrons, is produced by the flow of dark current during the exposure time. All CCDs produce a dark current, an actual current that is measurable in (typically tenths of) milliamps per pixel. The dark signal adds to your measured signal level, and increases the amount of noise in the measured signal.Since the dark signal varies with temperature, it can cause background values to increase over time. It also sets a limit on the useful exposure time. Reducing the temperature of the CCD reduces dark signal (typically, for every 7°C that temperature falls, dark signal halves). CCD readout noise is low, and so as not to compromise this by shot noise from the dark signal, it is important to cool the detector to reduce the dark signal. If you are using an exposure time of less than a few seconds, cooling the detector below 0°C will generally remove most of the shot noise caused by dark signal.

Deep-Depletion CCD

A CCD that has been designed to provide enhanced near-infrared and high-energy X-ray sensitivity. These devices utilize a bias voltage applied to a thick layer of high-resistivity silicon in order to produce a deeper depletion region (active photosensitive area) than that of conventional CCDs. This architecture allows longer-wavelength photons to interact within the layer as opposed to merely penetrate it.

Detection Limit

The Detection Limit is a measure of the smallest signal that can be detected in a single readout. The smallest signal is defined as the signal whose level is equal to the noise accompanying that signal, i.e. a signal to noise ratio (S/N) of unity. Sources of noise are:

• Shot noise of the signal itself
• Shot noise of any dark signal
• Readout noise

If the signal is small, we can ignore its shot noise. Furthermore, if a suitably low operating temperature and short exposure time can be achieved, the lowest detection limit will equal the readout noise.

Diffraction Grating

An optical device used to disperse light into a spectrum. It is ruled with closely-spaced (typically several thousand per cm) fine parallel grooves that produce interference patterns in a way that separates all the components of the incoming light. A diffraction grating can be used as the main dispersing element in a spectrograph. The diffraction pattern produced by the grating is described by the equation:m (λ) = d sin (θ), where:

• m is the order number
• λ is a selected wavelength
• d is the spacing of the grooves
• θ is the angle of incidence of light

A transmission grating has grooves ruled onto a transparent material, such as glass or Perspex, so that a beam of light passed through the grating is partly split into sets, or orders, with spectra on either side of it; the blue light is diffracted the least and the red light the most in each order. The orders of spectra increase in dispersion and faintness with distance from the direct beam. A reflection grating has grooves ruled onto a reflective coating on a surface that may be plane or concave, the latter being able to focus light. Its advantage over a transmission grating is that it produces a spectrum extending from ultraviolet to infrared, since the light doesn't pass through the grating material.

Digital

A scheme for representing data via quantized, discrete steps.

Digitize

To convert data (or images) into a digital format.

Dynamic Range

The ratio of the maximum (brightest) to minimum (darkest) signal levels present in an image. For instance, a true 12-bit digital camera is capable of providing a dynamic range of 4096:1.

Echelle Grating

A diffraction grating in which the grooves are quite widely spaced and have a zigzag or step-like cross-sectional profile. The name is French for ladder. The light to be dispersed is made to fall on the grating at right angles to the faces of the grooves. This has the effect of producing a series of overlapping spectra with a high degree of resolution. A second, low-dispersion grating, or a prism, arranged perpendicular to the èchelle, serves to separate out the overlapping spectra.

EM Radiation

Electromagnetic Waves are produced by the motion of electrically charged particles. These waves are also called Electromagnetic Radiation because they radiate from the electrically charged particles. They travel through empty space as well as through air and other substances. Electromagnetic radiation can be defined by wavelength, frequency, energy, e.g.:

By measuring one of the characteristics of radiation, we can determine the other, i.e.: Frequency = Speed of light / wavelength (λ)

Energy = Constant x Frequency (f)

E = mc²

EM Spectrum

As Electromagnetic Radiation is spread across many wavelengths, the term Electromagnetic Spectrum is used to describe the wavelength areas. This includes radio waves, microwaves, infrared, visible light, ultraviolet, x rays, gamma rays, and other electromagnetic radiation of long and short wavelengths. As you can see from the figure below (which shows the approximate wavelengths, frequencies, and energies for selected regions of the electromagnetic spectrum) there is, for example, no definitive boundary between where Radio Waves change to Microwaves. Rather, the various portions of the spectrum blend into one another, i.e.:

Wavelength (and its dependent properties, frequency and energy) is the fundamental differentiator between any type of electromagnetic radiation, whether it be long wavelength Radio Waves or short wavelength X-Rays.

If all electromagnetic radiation is fundamentally the same thing, you might ask, "Why don't we see radio waves like we see light?" or "Why do we need special infrared light bulbs to heat things up?" Although all portions of the electromagnetic spectrum are governed by the same laws, their different wavelengths and different energies allow them to have different effects on matter. Radio waves, for example, have such a long wave length and low energy that our eyes can't detect them and they pass through our bodies. It takes a metal antenna with special electronics to capture and amplify radio waves. Infrared radiation is made up of wavelengths that are easily absorbed by matter and turned into heat. X-rays are radiation of wavelengths that can pass through soft tissue but are stopped by bone.

EMCCD Camera

Advances in sensor technology have led to the development on a new generation of ultra-sensitive, low light CCDs.At the heart of your detector is the latest Electron Multiplying Charged Coupled Device (EMCCD), a revolutionary technology, capable of single photon detection.An EMCCD, is a silicon-based semiconductor chip bearing a two-dimensional matrix of photo-sensors, or pixels. This matrix is usually referred to as the image area. The pixels are often described as being arranged in rows and columns - the rows running horizontally and the columns vertically. The EMCCD in the iXon^EM+ detector is identical in structure to a conventional CCD but with the shift register extended to include an additional section, the Gain Register.

Etaloning

In a standard back-illuminated CCD, reflections between the parallel front and back surfaces of the device that lead to unwanted fringes of constructive and destructive interference. This resonant effect causes the device to become semi-transparent in the near-infrared region of the spectrum.

Exposure Time

The length of time that a CCD is accumulating charge.

Faceplate

In a CCD camera system, the front surface of the camera head, which often incorporates a window. The faceplate is sometimes used to support a target, or mount the camera head.

Fiberoptic-Coupled (Bonded) CCD

A CCD with a coherent fiberoptic bundle bonded to the CCD's imaging surface. The fibreoptic bundle is used to transfer an image source to the imaging surface of the CCD.

Fiberoptic

Fiberoptics, also referred to as optical fiber, refers to the medium and the technology, which is used in the transmission of information in the form of light pulses through very fine, flexible glass or plastic fibers (which are usually about 120µm diameter).

Fill Factor

A term that relates to the light-gathering area of a CCD. For instance, a CCD with 90% fill factor has an imaging array in which 10% of each pixel's area is insensitive to light.

F-Mount

A standard lens mount common to many scientific instruments. (Back focal length: 46.5 mm.)

Fps

Frames per second.

Frame

One image moved from a CCD in a full parallel shift.

Frame Buffer

In a digital imaging system, the hardware in which the frame memory (RAM that stores full frames of the image signal) resides.

Frame-Transfer CCD

A type of CCD used for quantitative electronic imaging. Frame-transfer CCDs divide the parallel register into two areas (arrays): an image array (for image collection) and a storage array (for image storage). After the image array is exposed to light, the electronic image is shifted to the storage array and read out. A frame-transfer CCD can operate without a shutter, running continuously at high rates.

Front-Illuminated CCD

A CCD in which the gate structure is located in front of the potential wells.

Full-Frame CCD

The simplest type of CCD. Full-frame CCDs use the entire active parallel register to expose photons and to integrate and transport charge. They utilize a shutter to control the exposure and block light during CCD readout, preventing charge smearing.

Full Well Capacity

The number of electrons that can be held in one potential well. It is assumed that all pixels on a CCD have the same well size and that each well can hold the same number of electrons.

Gain

Gain & Dynamic Range

As the gain is increased it will at some point begin to cause a decrease in Dynamic Range. This occurs when the gain equals the readout noise, in electrons. Therefore at 1MHz readout rates the Dynamic Range will be maintained at over 20,000:1 for gains up to about 20 times, while at 62.5kHz it will be maintained at over 43,000:1 for gains up to 6 times. If higher sensitivities (and hence higher gains) are required then there will have to be a trade off for Dynamic Range. To maintain as much Dynamic Range as possible it is advisable not to use a higher gain than is necessary to measure a signal.

Gain & Noise

The output from the gain register is fed into a conventional CCD output amplifier.

This amplifier, even in a scientific CCD, will have a readout noise of a few electrons rms and around 10 or 20 electrons rms at MHz readout rates. However this noise will effectively be reduced by the multiplication factor of the gain register which, when high enough, will achieve noise levels below 1 electron rms. So by using the gain you can effectively reduce the noise to insignificant levels at any readout speed.For example, the iXon^EM+ 87 has an s readout noise of a few to tens of electrons, depending on read out speed.Using gain will itself add some noise to a measured signal due to the statistical nature of the multiplication process. A similar effect exists in ICCDs and is referred to as the Noise Factor. The amount added is dependent on the signal level and the gain.If there is no gain, then there is no extra noise.At high gain (tens of times higher) it is calculated as the square root of N (where N is the signal in electrons). This will add to the shot noise of the signal to become the square root of 2N.So if the signal is large enough to be above the readout noise then there is probably no need for gain and it should be reduced or turned off. Conversely, if the signal is being lost in the readout noise then increasing the gain is the only way to detect it. If the gain is set high enough, then detection of single electron events will be possible. These events will appear on an image as a spike several hundred counts high. In Andor EMCCD cameras the gain is limited to a maximum of 255 times at -50ºC for standard systems. This is comparable to high end ICCDs.

Gain Register

In an EMCCD, a gain register is placed between the shift register and the output amplifier .The electrons are multiplied in the gain register by impact ionisation.

Gain Temperature dependence

The gain of an EMCCD system varies with temperature. The graph below shows how the gain multiplication increases as the temperature decreases. Curves are shown for various software gain settings and the figures are typical values. So if a system is operated at room temperature it will have reduced gain. Because of this temperature dependence it is recommended that the system is cooled, so that the temperature, and hence the gain, is stabilized.

So if a system is operated at room temperature it will have reduced gain. Because of this temperature dependence it is recommended that the system is cooled, so that the temperature, and hence the gain, is stabilized.

The graph below shows how the EM Gain setting on the software is related to the actual electron multiplication factor for various temperature settings (again the figures are typical):

Gate Structure

In a traditional CCD, the polysilicon structure located on the parallel register. Polysilicon gates are transparent at long wavelengths, but become opaque at wavelengths shorter than 400 nm.

Gating

In an ICCD camera system, the application of a voltage that switches the image intensifier on and off in very short intervals. Gating improves temporal resolution.

Gray Level

The brightness of a pixel in an image, expressed as an integer. Gray levels range from 0 (black) to 255 (white) for an 8-bit digital signal, and from 0 (black) to 4095 (white) for a 12-bit digital signal.

HCCD Camera

High-performance CCD camera. A CCD-based digital imaging system that utilizes advanced design features, such as low-noise electronics and cooled-detector technology, to optimize camera capabilities for scientific or industrial imaging applications.

High-Speed Framing

The process by which frames are read from a CCD at a rapid rate.

Host Computer

The primary or controlling computer for a digital camera.

ICCD Camera

Intensified CCD camera. A digital imaging (or spectroscopy) system that utilizes an image intensifier coupled to a CCD. These cameras offer high sensitivity in ultra-low-light-level conditions. Gating can be utilized to provide better temporal resolution.

iDus

Andor's USB CCD camera for Spectroscopy.For full specifications of the iDus, please refer to the Products section.

Image Array

The portion of a frame-transfer CCD that is exposed to light (and in which the image is collected). After the CCD is exposed, charge is shifted to the other half of the device, the storage array.

Image Averaging

A method of reducing random image noise by averaging a pixel's brightness throughout a series of successive frames.

Image Intensifier

An Image Intensifier is a device that amplifies the intensity of an image, not the size of the image.The device is small, typically 1 to 2 inches in diameter by about 1 inch thick. An image is projected on to the input window of the device and an intensified image appears on its exit window (usually a fiber optic plate). As well as amplifying, an image intensifier can rapidly be switched on and off, allowing it to be used as a very fast shutter.The image intensifier used in the system can be either a 2nd Generation (GEN II) or a 3rd Generation (GEN III) tube. This is a proximity-focused device that is compact and, more importantly, is easily gated to 2 ns time-scales.There are three major elements in an image intensifier which determine the performance of the device and they are as follows:

• The Photocathode
• The Microchannel Plate (MCP)
• The output Phosphor Screen

The illustration shows the layout of an Image Intensifier:

Indium Gallium Arsenide (InGaAs)

A material used in some detectors (e.g., PDAs) to provide higher QE in the near-infrared region of the spectrum.

Indium Tin Oxide

Indium tin oxide. A material used in some CCD gates to provide higher QE, particularly in the blue-green region of the spectrum.

Infrared (IR)

The region beyond the visible spectrum at the red end, typically greater than 770 nm.

Integration

The act of accumulating signal or charge on a CCD.

Interline Mask

Opaque strips that span an interline-transfer CCD and function as storage areas.

Interline-Transfer CCD

A type of CCD in which the parallel register is subdivided so that opaque strips span and mask the columns of pixels. The masks act as storage areas. When the CCD is exposed to light, the image accumulates in the exposed areas (photosites) of the parallel register. In the serial register, the entire image is under the interline mask when it shifts for CCD readout. It is possible to shift the integrated charge quickly (200 ns) under the storage areas.

iStar

Andor's ICCD camera for low-light spectroscopy applications requiring fast gating.For full specifications of the iStar, please refer to the Products section.

iXon^EM+

Andor's first commercially available, revolutionary EMCCD camera for ultra sensitive dynamic digital imaging.For full specifications of the iXon^EM+, please refer to the Products section.

Light

A form of electromagnetic radiation. Visible light (400 nm to 770 nm) can be perceived by the unaided human eye.

Linearity

In CCD imaging technology, precise linearity dictates that an object that is twice as bright as another object will appear exactly twice as bright in the resultant image.

Liquid Nitrogen (LN) Cooling

Cooling of the CCD by direct contact of liquid nitrogen to the CCD cold block. The CCD is not run at LN temperature (-200˚C) because the charge-transfer efficiency of CCD arrays actually begins to suffer at such a low temperature. Thus, operation is usually regulated between -120˚C and -60˚C in order to optimally decrease dark current without unduly compromising charge-transfer efficiency.

Luca^EM

Luca is shorthand for Last Universal Common Ancestor and is Andor's latest mid-market EMCCD camera.For full specifications of the Luca^EM, please refer to the Products section.

Mechelle

The Mechelle is Andor's spectrograph based on the èchelle grating principal and patented optical design, which gives extremely low cross-talk, equally spaced order separation and maximum resolution.

The Mechelle range of spectrographs offer a simultaneously recorded wavelength range from UV to NIR with very high resolution and no overlapping wavelengths.For full specifications of the Mechelle, please refer to the Products section.

Schematic representation of the Mechelle design

Megapixel

A term used to describe a CCD whose imaging array contains at least one million pixels.

Microchannel Plate (MCP)

Microchannel plate. One of the major components of an image intensifier. A slightly conductive glass substrate with millions of parallel traversing channels containing a secondary electron emitter on their inner walls. Each channel acts analogously to a standard photomultiplier device.

Microlenses

Small lenses that increase the fill factor of interline-transfer CCDs by redirecting incident light away from the masked columns (storage arrays) to the pixels' photosensitive areas.

Multi-Pinned-Phase (Inverted) Operation (MPP)

A mode that reduces the rate of dark current generation by a factor of 20 or more, relaxing CCD cooling requirements to the level where a thermoelectric cooler is sufficient for most applications.

Near Infrared (NIR)

The region of the spectrum from about 0.77 µm to about 3.0 µm.

Newton

The world's FIRST spectroscopic camera with Electron Multiplying CCD technology. EMCCD is an innovative approach to amplifying very low light signals above the read noise floor of the CCD, which would otherwise set the detection limit of the system. It incorporates the all solid-state electron multiplying technology based on the Impact Ionisation phenomenon in silicon.For full specifications of the Newton, please refer to the Products section.

Noise

Noise is a complex topic, the full exploration of which is beyond the scope of this glossary.Noise may, however, be broken down into two broad categories:

• PIXEL NOISE
• FIXED PATTERN NOISE

Pixel Noise

Let us first attempt to define pixel noise.Assume that a light signal is falling on a pixel of the CCD. If the charge on the pixel is read and the read process is repeated many times, the noise may be taken as the variation in the values read.The Root Mean Square (r.m.s.) of these variations is often used to express a value for noise. As a rule of thumb, the r.m.s. is four to six times smaller than the peak to peak variations in the count values read from the pixel. Pixel noise has three main constituents:

• Readout noise
• Shot noise from the dark signal
• Shot noise from the light signal itself

Shot noise cannot be removed because it is due to basic physical laws.Most simply defined, shot noise is the square root of the signal (or dark signal) measured in electrons. Readout noise (which in our detectors is, in any case, low) is due to the amplifier and electronics. It is independent of dark signal and signal levels; it is only very slightly dependent on temperature and it is present on every read, as a result of which it sets a limit on the best achievable noise performance. Shot noise from the dark signal is dependent on the exposure time and is very dependent on the temperature; shot noise from the signal is additionally dependent on the signal level itself.If either the signal or the dark signal falls to zero, their respective shot noise also falls to zero.The total pixel noise is not, however, simply the sum of the three main noise components (readout noise, shot noise from the dark signal, and shot noise from the signal). Rather, the Root Sum Square (r.s.s.) gives a reasonable approximation,thus:

Total = sqrt (readnoise²+ darkshot² + sigshot² )

where:

Total is the pixel noise

Readnoise is the readout noise

Darkshot is the shot noise of the dark signal

Sigshot is the shot noise of the signal

Fixed Pattern Noise

Fixed pattern noise (FPN) consists of the differences in count values read out from individual pixels, even if no light is falling on the detector.These differences remain constant from read to read. The differences are due in part to a variation in the dark signal produced by each pixel, and in part to small irregularities that arise during the fabrication of the CCD.Since fixed pattern noise is partly due to dark signal, it will change if the temperature changes, but because it is fixed, it can be completely removed from a measurement by background subtraction.

On-Chip Multiplication Gain

A technology that enables multiplication of charge (i.e electrons) collected in each pixel of the CCD’s active array. Secondary electrons are generated via an impact-ionization process that is initiated and sustained when higher-than-typical voltages are applied to an “extended” portion of the CCD’s serial register. Multiplying the signal above the read noise of the output amplifier enables ultra-low-light detection at high operation speeds. (Some CCD cameras with on-chip multiplication gain utilize two output amplifiers, an “on-chip multiplication gain” amplifier that allows the camera to be used for low-light, high-speed applications and a “traditional” amplifier for wide-dynamic-range applications.)

Opaque Mask

In CCD imaging technology, a light-impenetrable material that is used to shield selected parts of a photosensitive surface. Opaque masks are used in interline-transfer CCDs and frame-transfer CCDs.

Origin

In a CCD, the point located closest to the output node.

Output (Readout) Amplifier

A mechanism in the CCD that amplifies the electrons in the output node sufficiently to get the signal to the A/D converter. The output amplifier is the primary source of read noise.

Output Node

The location on a CCD (often a single pixel adjacent to the serial register) where charge is collected as a discrete picture element for CCD readout. Data enters the output node from the serial register and exits to the A/D converter.

Parallel (Vertical) Binning

The accumulation of multiple rows of charge in a CCD's serial register. The amount of charge shifted is defined by the user-specified binning factor.

Parallel Binning Factor

In the parallel register of a CCD, the number of pixels (in the parallel direction) to be shifted to the serial register, read out, and processed into an image. The binning factor is specified by the user in the imaging software prior to exposure of the CCD.

Parallel Direction

In a serial, parallel (s, p) coordinate system, the direction that begins at the origin and runs perpendicular to the serial register.

Parallel Offset

When defining an ROI in the parallel register of a CCD, the distance (in pixels) between the serial axis and the rectangular / square exposure area on the CCD.

Parallel (Vertical) Register

In a CCD, a large, square array that contains many potential wells (pixels). When the CCD is exposed to light, charge accumulates in the potential wells, which when shifted and read out, form an image.

Parallel Shift

In a CCD, columnar movement of charge from one or more pixels to an adjacent row. The movement continues until the number of pixels to be binned (specified by the user) are emptied into the serial register.

Parallel Size

In CCD imaging technology, the size of the ROI (in pixels) extending in the parallel direction.

Peltier Effect

The transfer of heat in the opposite direction of the current flow. By pumping current through a "Peltier stack" to a heat sink, heat can be removed from a CCD. The heat sink is cooled by circulating liquid or air. The temperature must be regulated.

Phosphor

A chemical substance that fluoresces when excited by X-rays, an electron beam, or ultraviolet radiation. Phosphors are composed of rare earth oxides or halides (e.g., gadolinium, lanthanum, yttrium) and usually emit green light with decay times ranging from hundreds of nanoseconds to a few milliseconds.

Phosphor Screen

One of the major components of an image intensifier. Electrons exiting the microchannel plate (MCP) are accelerated by a constant voltage and strike the screen, where they are converted back into light photons for detection by a CCD.

Photocathode

One of the major components of an image intensifier. Coatings on the photocathode convert a portion of the incident light photons into electrons. Good QE is critical, as photons that are not captured by the photocathode are lost from the final signal produced by the intensifier.

Photodiode Array (PDA)

A linear array of discrete photodiodes on an integrated-circuit chip used in digital detection systems.

Photometry

The measurement of the properties of light, particularly (luminous) intensity.

Photon-Noise-Limited Operation

In CCD imaging technology, operating the detection system at levels of noise so low that photon (shot) noise is the dominant noise source.

Pixel

Picture element. The smallest element in a visual display.

Poisson Distribution

A probability function used to model the density of counts of a randomly occurring event obtained during a specified interval of time.

Potential Well

In a CCD, a discrete region within the device's imaging array where an incident photon may be trapped (a pixel).

Qualitative Analysis

In digital imaging, the process of evaluating the appearance of an acquired image.

Quantitative Analysis

In digital imaging, the process of measuring and comparing the intensity of light incident on individual pixels.

Quantum Efficiency (QE)

The measure of the effectiveness of an imager to produce electronic charge from incident photons. Especially important to perform low-light-level imaging.

Radiometry

The science of measuring electromagnetic radiation, often accomplished with a device called a radiometer.

RAMAN

Radiation effect used in spectroscopy named after Sir Chandrasekhara Venkata Raman. It is used in condensed matter physics and chemistry to study vibrational, rotational, and other low-frequency modes in a system. It relies on inelastic scattering (or Raman scattering) of monochromatic light, usually from a laser in the visible, near infrared, or near ultraviolet range.Phonons or other excitations in the system are absorbed or emitted by the laser light, resulting in the energy of the laser photons being shifted up or down. The shift in energy gives information about the phonon modes in the system.

Rayleigh Scattering

Rayleigh Scattering refers to the scattering of light from the molecules in the air, and can be extended to scattering from particles up to about a tenth of the wavelength of the light. It is Rayleigh scattering off the molecules of the air which gives us the blue sky. Named after Lord Rayleigh, who calculated the scattered intensity from dipole objects, much smaller than the wavelength, to be as shown in the diagram.

Rayleigh scattering can be considered to be elastic scattering since the photon energies of the scattered photons is not changed. Scattering in which the scattered photons have either a higher or lower photon energy is called Raman scattering. Usually this kind of scattering involves exciting some vibrational mode of the molecules, giving a lower scattered photon energy, or scattering off an excited vibrational state of a molecule which adds its vibrational energy to the incident photon.

Read (Pre-Amplifier) Noise

In CCD imaging technology, unwanted signal or disturbance that is generated by the on-chip output amplifier. The noise can be reduced to a few electrons by modifying operating conditions.

Read-Noise-Limited Operation

In CCD imaging technology, operating the detection system such that read noise exceeds photon (shot) noise. This is an undesirable condition in which the image data is limited by experimental restrictions or deficiencies in the camera design.

Readout

Readout is the process by which data are taken from the pixels of the CCD and stored in computer memory. The pixels, which are arranged in a single row, are read out individually in sequence.Readout involves amplifying the charge on each pixel into a voltage, performing an A/D conversion, and storing the data in computer memory.The time taken to perform this operation is known as the "read time".

Readout Sequence of a CCD

In the course of readout, charge is moved vertically into the shift register, and then horizontally from the shift register into the output node of the amplifier. The readout sequence illustrated below (which corresponds to the default setting of the Full Resolution Image binning pattern) allows data to be recorded for each individual element on the CCD-chip.Other binning patterns are achieved by summing charge in the shift register and/or the output node prior to readout (see Horizontal and Vertical Binning).

1. Exposure to light causes a pattern of charge (an electronic image) to build up on the frame (or "image area") of the CCD-chip.

2. Charge in the frame is shifted vertically by one row, so that the bottom row of charge moves into the shift register.

3. Charge in the shift register is moved horizontally by one pixel, so that charge on the endmost pixel of the shift register is moved into the output node of the amplifier.

4. The charge in the output node of the amplifier is passed to the analog-to-digital converter and is read out.

5. Steps 3 and 4 are repeated until the shift register is emptied of charge.

6. The frame is shifted vertically again, so that the next row of charge moves down into the shift register.The process is repeated from Step 3 until the whole frame is read out.

Readout Sequence of an EMCCD

1. Exposure to light causes a pattern of charge (an electronic image) to build up on the frame (or Image Area) of the EMCCD-chip.

2. Charge in the frame is shifted vertically by one row, so that the bottom row of charge moves into the shift register.

3. Charge in the shift register is moved horizontally by one pixel, so that charge on the endmost pixel of the shift register is moved into the Gain register.

4. Charge is shifted into the output node ofthe amplifier.

5. The charge in the output node of the amplifier is passed to the analog-to-digital converter and is read out.Steps 3 and 4 are repeated until the shift register is emptied of charge.

The frame is shifted vertically again, so that the next row of charge moves down into the shift register.The process is repeated from Step 3 until the whole frame is read out.In the course of readout, charge is moved vertically into the shift register, and then horizontally from the shift register into the output node of the amplifier. The simple readout sequence illustrated in the figure above (which corresponds to the default setting of the Full Resolution Image binning pattern) allows data to be recorded for each individual element on the CCD-chip.Other binning patterns are achieved by summing charge in the shift register and/or the output node prior to readout (see Binning).

Recirculation

A refrigeration process in which water forced-air is cooled then pumped back around a circuit. Used extensively in research & test laboratories.

Region Definition

The designation of a rectangular / square area on a CCD to be exposed as an image. The user defines the region by specifying coordinates in the serial, parallel (s, p) coordinate system.

Region of Interest (ROI)

Region of interest. A user-defined, rectangular area (a square is common) on a CCD that is exposed and processed as an image.

Resolution

A measure of how fine a detail can be detected, in terms of either space (spatial resolution), time (temporal resolution), or intensity.

Saturation

Saturation is the largest signal the CCD can measure. A signal is measured in terms of the amount of charge that has built up in the individual pixels on the CCD-chip. A number of factors determine the maximum amount of charge that the CCD can handle.

Scanning

The CCD is continually being "scanned" to prevent its becoming saturated with dark current (see dark signal). If the Scan is being used simply to "clean" the CCD (i.e. it is a Keep-Clean scan), the charge from the CCD is discarded.In an acquired scan, however, the charge undergoes A/D conversion and is acquired into computer memory so that it can be used for subsequent processing and display, i.e. it is readout (see section on Readout).

Scientific-Grade CCD

A high-performance CCD that offers fewer defects than commercial-grade CCDs. Scientific-grade CCDs produce better spatial resolution, have lower noise, and enable the user to accurately measure intensity differences between objects.

SDK

The AndorMCD Software Development Kit (SDK) gives the programmer access to the Andor range of CCD, ICCD and iStar cameras. The key element of SDK is the 32-bit dynamic link library "ATMCD32D.DLL" which can be used with a wide variety of programming environments including C, C++, Visual Basic and LabVIEW.

Serial (Horizontal) Binning

The accumulation of charge from two or more pixels of a CCD's serial register into the output node before the charge is shifted for CCD readout.

Serial Binning Factor

In the serial register of a CCD, the number of pixels (in the serial direction) to be shifted to the output node, read out, and processed into an image. The binning factor is specified by the user in the imaging software prior to exposure of the CCD.

Serial Direction

In a serial, parallel (s, p) coordinate system, the direction beginning from the origin and moving away from it in a direction parallel to the serial register.

Serial Offset

When defining an ROI in the parallel register of a CCD, the distance (in pixels) between the parallel axis and the user-defined, rectangular / square exposure area on the CCD.

Serial, Parallel (S, P) Coordinate System

In CCD imaging technology, a nomenclature based on the point of orientation located on the parallel register in the corner closest to the output node. Coordinates increase as the locations move away from this origin. s represents the serial coordinate, p represents the parallel coordinate.

Serial (Horizontal) Register

A row of pixels adjacent to the parallel register. When the CCD is exposed to light, the serial register receives charge from the parallel register and shifts it to the output node to form an image.

Serial Shift

In a CCD, the movement of charge (accumulated from the parallel register) to the output node. The charge moves pixel by pixel along the serial register. From the output node, the charge is processed as an image.

Serial Size

In CCD imaging technology, the size of the ROI (in pixels) extending in the serial direction.

Shamrock

Shamrock is the name of Andor's range of Czerny-Turner spectrographs which are available in 3 models, i.e.:

• Shamrock SR-303i
• Shamrock SR-163/SR-163i

For full specifications of the Shamrock range of spectrographs, please refer to the Products section.

Shift Register

The Shift Register usually consists of a single row of elements (or pixels) running parallel to and below the bottom row of light-gathering pixels (the image area) on the CCD-chip.The shift register is protected from light by an aluminum mask.The elements in the shift register have a greater capacity to store charge (a greater "well depth") than the other pixels on the CCD-chip.

Shot (Photon) Noise

Shot Noise is due to basic physical laws and cannot be removed. Any signal, whether it be a dark signal or a light signal, will have shot noise associated with it. It can be most simply defined as follows:

If the signal or dark signal = N electrons, the shot noise is the square root of N. You can do nothing about the shot noise of your signal, but by choosing minimum exposures and operating the CCD at suitably low temperatures, the dark signal, and hence the noise from the dark signal, can be reduced.

Signal-to-Noise Ratio (SNR)

The Signal to Noise Ratio (S/N) is the ratio between a given signal and the noise associated with that signal.Noise has a fixed component and a variable component (shot noise) which is the square root of the signal. Thus, the Signal to Noise Ratio usually increases (improves) as the signal increases.The maximum Signal to Noise Ratio is the ratio between the maximum signal (i.e. the saturation level) and the noise associated with that signal. At near saturation levels the dominant source of noise is the shot noise of the signal.

Silicon

A tetravalent, nonmetallic element used to fabricate CCDs.

Slow-Scan CCD

A CCD with special circuits that allow data readout at slower-than-standard rates in order to reduce read noise.

Spectrometer

An optical instrument that allows a user to view, record, and analyze a spectrum by rendering its component waves distinct and visible.

Spectrograph

An optical instrument that uses dispersed light from a source to produce a spectra. The elements that make up a spectrograph and their functions are as follows:

• Entrance aperture: Provides source to be imaged at exit plane
• Collimating optics: Produces collimated beam of light to dispersive element
• Grating: Grating or prism disperses light from source into its component wavelengths
• Focusing Optics: Focus light to imaging plane
• Exit plane: Where the detector sits

A typical layout is shown here:

Spectroscopy

The branch of science that deals with the theory and interpretation of spectra. Spectra are regions of the electromagnetic spectrum.

Storage Array

In a frame-transfer CCD or interline-transfer CCD, the portion of the parallel register that is covered with an opaque mask to provide temporary storage for collected charge.

Superpixels

A combination of vertical binning and horizontal binning. Horizontal binning involves shifting charge horizontally from several pixels at a time.Superpixels consist of two or more individual pixels that are binned and read out as one large pixel, i.e. the CCD effectively becomes a matrix of superpixels.

Subarray Readout

In a CCD, the process of moving charge from a user-defined, rectangular / square subregion of the array to an output amplifier for conversion to an image.

System Noise

In a CCD camera system, internally generated interference or a number of other factors can cause unwanted signal to appear in an image. Many noise sources exist, but three sources account for the majority of total system noise: dark noise, photon (shot) noise, and read noise.

Temporal Resolution

Temporal resolution is defined as the frequency at which images are recorded/captured in a specific place on the earth. The more frequently images are captured, the better the temporal resolution.

Time-Delay Integration (TDI)

An integration and CCD readout mode that allows the acquisition of long swaths of a moving image.

Thermoelectric Cooling

The process of pulling heat away from a CCD by using Peltier cooling devices.

Thinning

A process that uses acid etching to uniformly reduce the size of a CCD to approximately 10 µm so that an image can be focused on the back of the parallel register (where there is no gate structure).

Trigger

A signal (typically a TTL signal) that is transmitted in order to synchronize two or more instruments; something that acts like a mechanical initiator in setting up a process or reaction.

USB

Acronym for Universal Serial Bus, a standard computer interconnection system that was introduced in 1995 by Intel, Compaq, Microsoft and several other IT computer companies.

Ultraviolet (UV)

The region of the spectrum from about 400 nm (just beyond the violet in the visible spectrum) to about 40 angstroms (on the border of the x-ray region).

X-rays

X-rays are a form of electromagnetic radiation with a wavelength in the range of 10nm - 100pm (corresponding to frequencies in the range 30 PHz to 60 EHz).They were first discovered by accident in 1895 by a German scientist, Wilhelm Conrad Röntgen, who noticed them when experimenting with vacuum tubes and they aresometimes still called Röntgenstrahlen (Röntgen Rays) in German-speaking countries.Andor Technology manufactures a comprehensive range of CCD detection systems for a wide variety of X-ray applications.