CCD基础知识
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Signal-to-noi ratio
Signal-to-noi ratio (SNR) describes the quality of a measurement. In CCD imaging, SNR refers to the relative magnitude of the signal compared to the uncertainty in that signal on a per-pixel basis. Specifically, it is the ratio of the measured signal to the overall measured noi (frame-to-frame) at that pixel. High SNR is particularly important in applications requiring preci light measurement.
Photon noi 日记200字作文refers to the inherent natural variation of the incident photon flux. Photoelectrons collected by a CCD exhibit a Poisson distribution and have a square root relationship between signal and noi.
Read noi refers to the uncertainty introduced during the process of 个性有哪些quantifying the electronic signal on the CCD. The major component of readout noi aris from the on-chip preamplifier. -
Dark noi aris from the statistical variation of thermally generated electrons within the silicon layers comprising the CCD. Dark current describes the rate of generation of thermal electrons at a given CCD temperature. Dark noi, which also follows a Poisson relationship, is the square root of the number of thermal electrons generated within a given exposure. Cooling the CCD from room temperature to -25°C will reduce dark current by more than 100 times. In addition, many scientific-grade CCDs employ MPP technology to even further reduce dark current.
Taken together, the SNR for a CCD camera can be calculated from the following equation:
where:
I = Photon flux (photons/pixel/cond)
QE = Quantum efficiency
abac的四字词语有哪些t = Integration time (conds)
Nd = Dark current (electrons/pixel/c)
Nr = Read noi (electrons)平原
Under low-light-level conditions, read noi exceeds photon noi and the image data is said to be "read-noi limited". The integration time can be incread until photon noi exceeds both read noi and dark noi. At this point, the image data is said to be "photon limited".
An alternative means of raising the SNR is to u a technique known as 火柴棒数学题移动一根binning. Binning is the process of combining charge from adjacent pixels in a CCD during readout into a single "superpixel". Binning neighboring pixels on the CCD array may allow you to reach a photon-limited signal more quickly, albeit at the expen of spatial resolution.
Once you have determined acceptable values for SNR, integration time, and the degree to which you are prepared to bin pixels, the above equation can be solved for the minimum photon flux required. This is, therefore, the lowest light level that can be measured for given experimental conditions and camera specifications.
慈善家Visual impact of increasing SNR of a typical test pattern.
Binning 牺牲分辨率降低read noi
Binning is the process of combining charge from adjacent pixels in a CCD during readout.
This process is performed prior to digitization in the on-chip circuitry of the CCD by specialized control of the rial and parallel registers. The two primary benefits of binning are improved signal-to-noi ratio汽车电瓶亏电 (SNR) and the ability to increa frame rate, albeit at the expen of reduced spatial resolution.
To understand the process, consider the example of 2x2 binning shown below. As in normal operation, charge integrates in individual pixels while the CCD is expod to light. During the parallel readout, the charge from two rows of pixels, rather than a single row, is shifted into the rial register. Next, charge is shifted from the rial register, two pixels at a time, into the summing well. It then goes to the output amplifier where it is converted to a voltage before being transferred off-chip for further amplification and digitization. This procedure is iterated until the entire array has been read out. The result is that each readout event from the summing well contains the collected charge from four pixels on the CCD. It should be noted that Photometrics cameras have the ability to perform binning into any arbitrary MxN binned pixels (superpixels) through simple software control.
Since both the rial register and summing well accumulate charge from multiple pixels during binning, they must have sufficient capacity to prevent saturation. In high-performance CCDs, the rial register typically has a charge capacity double that of the p
arallel registers, and the summing well double that of the rial register. However, the specifications of the particular CCD being ud should be noted and understood before using the binning technique. This is especially true when working at high-illumination levels where saturation could lead to erroneous data collection.
The primary benefit of binning is higher SNR due to reduced read noi contributions. CCD read noi is added during each readout event and in normal operation, read noi will be added to each pixel. However, in binning mode, read noi is added to each superpixel, which has the combined signal from multiple pixels. In the ideal ca, this produces SNR improvement equal to the binning factors (4x in the above example). The figure below shows the effect of 2x2 binning for a four-pixel region. This example assumes that 10 photoelectrons have been collected in each pixel and the read noi is 10 electrons. If this region is read out in normal mode the SNR will be 1:1 and the signal will be lost in the noi. However, with 2x2 binning, the SNR becomes 4:1, which is sufficient to obrve this weak signal.
