An important consideration in a detector is its ability to respond linearly to any image it views. In such a situation, we say that the detector has a linear response.
Such a response is obviously very useful as there is no need for any additional processing on the image to determine the 'true' intensity of different objects in an image. Noise One of the most important aspects of CCD performance is its noise response.
There are a number of contributions to the noise performance of a CCD, these are briefly listed here: Dark current - i. At room temperature, the noise performance of a CCD can be as much as thousands of electrons per pixel per second. Consequently, the full well capacity of each pixel will be reached in a few seconds and the CCD will be saturated. Dark current can be massively reduced by cooling. For example, the noise performance of the CCD could be reduced from thousands of electrons at room temperature to only tens of electrons per pixel per second at degrees C.
By cooling down to temperatures below about degrees C dark current can be virtually eliminated substantially below one electron per pixel per second. This technique can reduce the dark current to very low levels a few hundred electrons per pixel per second at room temperature. Readout noise - the ultimate noise limit of the CCD is the readout noise.
The magnitude of this noise depends on the size of the output node. A large amount of effort has been dedicated to reducing the CCD readout noise, as this noise value will ultimately determine the dynamic range and should be as low as possible, particularly when detecting very faint sources for example, detecting photons at X ray energies such as in the XMM-Newton mission.
Noise values of electrons rms root mean square are now typical for many CCDs but some companies have recently claimed a noise resolution of under 1 electron rms. When the CCD is used as part of a camera for astronomical imaging, other sources of noise must also be included such as the random shot noise present on the image itself, along with noise introduced by the camera electronics.
However, these noise sources are discussed elsewhere. Power CCDs themselves consume very little power. During integration, only a very small current is flowing and the CCD consumes only 50mW or so. Whilst the CCD is being clocked out more power can be consumed but this is typically only several Watts or so. Of course, the electronics required to operate the CCD and process images can consume much more power.
Why are scientific CCDs so expensive? A Video camera using a CCD can be bought for as little as pounds. However, a scientific grade CCD may cost up to times this price, sometimes more. Some of the reasons why scientific CCDs are much more expensive than CCDs in consumer electronics are outlined below: Cost and complexity Scientific grade CCDs are much more expensive than the basic type of CCDs that are usually found in devices such as commercially available Video cameras.
Commercially available video cameras normally have a number of disadvantages that make them unsuitable for scientific use. While this model is an oversimplification we provide an in depth explanation below. Photons striking a silicon surface create free electrons through the photoelectric effect.
A simultaneous positive charge or holes are generated as well. If nothing is done the hole and the electrons will recombine and release energy in the form of heat. Small thermal fluctuations are very difficult to measure and it is thus preferable to gather electrons in the place they were generated and count them to create an image.
This is accomplished by positively biasing discrete areas to attract electrons generated while the photons strike the surface. The substrate of a CCD is made of silicon, but photons coming from above the gate strike the epitaxial layer — essentially silicon with different elements doped into it — and generate photoelectrons. The gate is held at a positive charge in relation to the rest of the device, which attracts the electrons.
The figure to the right shows how electrons are held in place and moved to where they can be quantified. The top black line represents the potential well for the electrons that are represented by the blue color and is low , or downhill , where the potential is high since opposites attract. Electrons are shifted in two directions on a CCD, called the parallel or serial direction. One parallel shift occurs from the right to the left shown at left. The serial shift is performed from top to bottom and directs the electron packets to the measurement electronics.
In the example to the left, the image is split up into 2 and then 4 different sections and read-out. The method of reading this voltage is called dual slope integration DSI and is used when the absolute lowest noise possible is required. Generally speaking, the faster a pixel is read, the more noise is introduced into the measurement.
If the gain of the measurement is known the ADU number for each pixel generated can be directly correlated to the number of electrons found in that pixel. All Spectral Instruments cameras come with a detailed test report showing the gain at a given readout speed. Thus, a bit camera can never show more than 65, ADUs in any given pixel. Scientific grade CCDs can generally hold anywhere from 70, to , electrons in any given pixel. At slow read speeds, i.
At higher read speeds i. This sacrifices, however, higher read noise for the extra dynamic range. Interline transfer uses alternating parallel strips, in which a portion of each pixel is masked to light, allowing for fast transfer without any charge smear. This however reduces the light sensitive area and makes the light collecting area of each pixel smaller. As these sensors have a thicker depletion region, they are no longer transparent to NIR wavelengths and are therefore able to generate charge, detecting each NIR photon.
This allows for nanosecond gating optimal for ultra-low exposure times. Figure 1: Schematic depicting charge transfer on a CCD. A Different numbers of photoelectrons accumulate on pixels within the sensor when it is exposed to light. Each row of electrons is shifted down a row using a positive voltage. B The electrons are shifted by spreading the positive voltage over neighboring pixels in the same column to transfer them to a new pixel. This will continue all the way down the sensr until they are transferred to the readout register.
C Those electrons that are on the bottom row are transferred into the readout register. D Once on the readout register, the electrons are shifted horizontally, column by column, via a positive charge until they reach the output node, where they are amplified and digitized.
This process is repeated until the whole sensor is clear of electrons. Then the sensor can be exposed to light again to acquire a new image. Figure 2: Schematic showing three common ways that photoelectrons can be transferred from the CCD.
A Full frame, in which the entire frame is light sensitive, and any charge accumulated must be vertically transferred down the sensor into the readout register. B Frame transfer, in which half of the sensor is masked light insensitive allowing for rapid charge shift. C Interline transfer, in which alternating strips of light sensitive and insensitive pixels are used to allow for rapid charge transfer without the risk of charge smear.
Figure 4: Deep-depletion CCDs are made of thicker silicon so are therefore able to detect NIR wavelengths which travel deeper into the silicon, unlike typical depletion CCDs which generates majority of signal from visible light. Figure 5: Schematic showing vapor-phase i. Step 1: Reagents and carrier gas are adsorbed onto the silicon substrate. Adsoprtion is when a solid captures molecules of a solute, liquid or gas to form a thin film.
Step 2: These elements then undergo nucleation onto the silicon substrate surface. Nucleation is the initial formation of a self-assembled surface. Step 3: Any unreacted products and carrier gas undergo desorption from the surface. Step 4: This process continues until a layer is formed. Image not to scale and adapted from M. Powell [1]. Find Out More.
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