CCD参数的基础知识

CCD

基础知识

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Signal-to-noise ratio

Signal-to-noise 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 noise

(frame-to-frame) at that pixel. High SNR is particularly important in applications requiring precise

light measurement.

Photon noise

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 noise.

Read noise

refers to the uncertainty i

ntroduced during the process of

quantifying

the electronic

signal on the CCD

. The major component of readout noise arises from the on-chip preamplifier. -

Dark noise

arises

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 noise, 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/second)

QE = Quantum efficiency

t = Integration time (seconds)

Nd = Dark current (electrons/pixel/sec)

Nr = Read noise (electrons)

Under low- light-level conditions,

read noise exceeds photon noise and the image data is said to be

.

The integration time can be increased until photon noise exceeds both read

noise and dark noise. At this point, the image data is said to be

.

An alternative means of raising the SNR is to use a technique known as

binning

. Binning is the

process of combining charge from adjacent pixels in a CCD during readout into a single

photon-limited signal more quickly, albeit at the expense 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.

Visual impact of increasing SNR of a typical test pattern.

Binning

牺牲分辨率降低

read noise

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 serial and parallel registers. The two primary benefits of binning are improved

signal-to-noise ratio (SNR) and the ability to increase frame rate, albeit at the expense 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 exposed to light. During the

parallel readout, the charge from two rows of pixels, rather than a single row, is shifted into the

serial register. Next, charge is shifted from the serial 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 serial register and summing well accumulate charge from multiple pixels during

binning, they must have sufficient capacity to prevent

saturation

. In high- performance CCDs, the

serial register typically has a charge capacity double that of the parallel registers, and the summing

well double that of the serial register. However, the specifications of the particular CCD being

used 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 noise contributions. CCD read

noise is added during each readout event and in normal operation, read noise will be added to each

pixel.

However, in binning mode, read noise is added to each superpixel, which has the combined

signal from multiple pixels. In the ideal case, 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 noise 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 noise. However, with 2x2 binning, the SNR becomes 4:1, which

is sufficient to observe this weak signal.

Unlike read noise, dark current noise is not reduced by binning since each pixel will contribute

dark current noise to the superpixel. To ensure that dark current noise does not lower SNR during

binning, it is essential that the CCD be cooled sufficiently to reduce the dark current noise to a

negligible level relative to the read noise.

One of the most common applications of binning is spectroscopy. In spectroscopic CCD systems,

the dispersed slit images lie along the CCD columns (perpendicular to the serial register) and the

resultant images are then binned along the columns. Binning thus provides marked increases in

SNR without any loss of spectral resolution. Spatial resolution is lost along the slit axis, but this

typically is not a concern.

Another use of binning is to increase frame rate. Since the slowest step in the readout sequence is

the digitization of a given pixel, binning can be used to increase the effective total frame rate of a

given system. Thus, highly binned, low-resolution images can be obtained when high speed is

required (such as in focusing) and full-frame, high- resolution images can be obtained when the

ultimate resolution is required. Because this can all be controlled via software, Photometrics

digital cameras are extremely flexible and can be used in a wide variety of analytical imaging

applications.

Full Well Capacity

Full well capacity

defines

the amount of charge an individual pixel can hold before saturating.

Saturation must be avoided in high- performance CCD imaging because it diminishes the

quantitative ability of the CCD and produces image smearing due to a phenomenon known as

blooming.

Full well is dependent upon the pixel size of the CCD, whether or not multi-pinned-phase (MPP)

mode is used, and the operating voltages used on the CCD

. Larger full wells are found on

large-pixel devices. MPP mode reduces full well since a large gate potential is not applied to the

CCD electrodes during integration. This has the intended effect of reducing dark current, but it can