Photonics and Optical Detectors terms glossary (A-C)

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.

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.

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.

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.

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.