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Home > Mall Dynamic > Unveiling The Marvels OF CCD Detectors: Illuminating The World OF Imaging Technology
Charge-coupled device (CCD) is a kind of highly sensitive photon detector. The CCD is divided into many small light-sensitive areas (called pixels) that can be used to build an image of the scene of interest. A photon falling within a region defined by a pixel will be converted into one (or more) electrons, and the number of electrons collected will be proportional to the intensity of the scene at each pixel. When the CCD is moved out, the number of electrons in each pixel can be measured and the scene can be reconstructed.
In fact, not every photon that lands on the detector will be detected and converted into an electrical pulse. The percentage of photons actually detected is called quantum efficiency (QE). For example, the QE of the human eye is about 20%, the QE of photographic film is about 10%, and the best CCDS can achieve more than 80% QE. Quantum efficiency will vary with wavelength.
CCD can have a wide wavelength range of about 400nm (blue) to about 1050nm (infrared), with a peak sensitivity of about 700nm. However, by using the so-called "backthinning" process, the wavelength range of the CCD can be extended down to shorter wavelengths, such as Extreme UV and X-rays.
The ability to correctly view bright and dim light sources in the same image is a very useful property of the detector. The difference between the brightest and darkest light sources that a detector can accurately see in the same image is called dynamic range. When light falls on the CCD, photons are converted into electrons. Therefore, the dynamic range of a CCD is usually discussed in terms of the minimum and maximum number of electrons that can be imaged. As more light falls on the CCD, more and more electrons are collected in the potential well, and eventually no more electrons will be held in the potential well, so the pixel is said to be saturated. For a typical scientific CCD, this can happen at around 150,000 electrons. The smallest signal that can be detected is not necessarily an electron (corresponding to a photon at a visible wavelength). In fact, there is a minimum amount of electron noise associated with the physical structure of the CCD, typically about 2-4 electrons per pixel. Therefore, the minimum signal that can be detected can be determined from this readout noise.
In the example above, the CCD has a dynamic range of 150,00:4 (with the higher noise level). But - this dynamic range also depends on the ability of the electronic device to fully digitize all of this dynamic range (for a discussion of electronic resolution, see CCD information in more detail).
In general, the eye is not a linear detector (except for very small changes in intensity) and has a logarithmic response. An important consideration in the detector is its ability to respond linearly to the image being viewed. We mean that if it detects 100 photons, it will convert them into 100 electrons (if we have 100% QE), and if it detects 10,000 photons, it will convert them into 10,000 electrons. In this case, we say that the detector has a linear response. Such a response is obviously very useful because no other processing of the image is required to determine the "true" intensity of the different objects in the image.
One of the most important aspects of CCD performance is its noise response. The noise performance of CCD has many contributions, which are briefly listed below:
Dark current - that is, noise produced by heat. At room temperature, the noise performance of a CCD can be as high as thousands of electrons per pixel per second. Therefore, in a few seconds the full trap capacity of each pixel will be reached and the CCD will be saturated. Dark current can be greatly reduced by cooling. For example, the noise performance of a CCD can be reduced from thousands of electrons at room temperature to only tens of electrons per second per pixel at -40 degrees Celsius. By cooling to temperatures below about -70 degrees Celsius, the dark current can actually be eliminated (basically less than one electron per second per pixel). The second way to reduce noise is to slightly change the CCD processing technique to produce a multi-pin phase (MPP) CCD.
Read noise - The ultimate noise limit of a CCD is the read noise. The readout noise results from the conversion of electrons in each pixel to the voltage at the CCD output node (typical values are about 4µV per electron). The size of this noise depends on the size of the output node. A great deal of effort has been made to reduce the readout noise of the CCD, as this noise value will ultimately determine the dynamic range and should be as low as possible, especially when detecting very weak sources, such as photons at X-ray energies, as in the XMM-Newton mission. Now, many CCDS typically have a noise value of 2-3 electron root-mean-square (RMS), but some companies have recently claimed a noise resolution of less than 1 electron RMS.
When a CCD is used as part of an astronomical imaging camera, other sources of noise must also be included, such as random (shot) noise present on the image itself and noise introduced by the camera's electronics. However, these sources of noise are discussed elsewhere.
The CCD itself consumes very little power. During the integration, only a small amount of current flows, and the CCD consumes only about 50mW. Although the CCD clock output, but can consume more power, but usually only a few watts or so. Of course, the electronics needed to operate the CCD and process the images consume more power.
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