Dark noise

Thermoelectrical cooling of the photomultiplier tube at about — 30°C reduces the dark noise current to a very low level. However, as the quantum efficiency of the S-20 type decreases as rapidly as the dark current in the red region, cooling brings only modest increases in the signal-to-noise ratio 23). 

Dark noise of the ion counter or analog electron multiplier (especially important for the latter, for which the 2c dark noise is typically 10-15 ions/second)... 

This noise is due to the thermal motion of the carriers (electrons) in the different resistors used in the photomultiplier. In general, the signal uncertainty caused by this source of noise is much lower than those generated by both dark noise and shot noise. 

Dark noise in photomultipliers is caused by (1) leakage current across insulating supports (2) field emission from electrodes (3) thermal emission from the photocathode and dynodes (4) positive ion feedback to the photocathode and (5) fluorescence from dynodes and insulator supports. Careful design can eliminate all but item (3). Associated with the photocurrent from the photocathode is shot noise. There is also shot noise from secondary emission in the multiplier structure. 

Noise associated with the detector elements themselves (dark or thermal noise) can be reduced by decreasing the temperature, typically from —20 to —70° C although cooling to liquid nitrogen temperatures essentially eliminates dark noise, it is typically impractical for applications outside of the laboratory. The theoretical value for relative signal-to-noise reduction is a factor of 2 improvement for every 6.3°C reduction in temperature, although this can be... 

For detection of very low light signals, it will be required to extend the number of delays, i.e. the period of "on target" signal integration. In such case the dark noise may be minimized by cooling the OMA. 

Each of the amplification stages is a potential source of the dark noise mentioned above (1 activation/100 s). The spontaneous activation of rhodopsin accounts for about 50% of the noise, and it can be estimated that a thermal activation event occurs on the average once every 2000 years (Burns and Baylor, 2001). This extremely low thermal activation rate is due to the covalently linked chromophore that provides extreme stability to the off state of the protein. 

Typical detectivity values as a function of wavelength for PbS photoconductive and various photovoltaic detectors. is a figure of merit defined as A /NEP, where A is the detector area and NEP is the noise-equivalent power, the rms radiant power in watts of a sinusoidally modulated input incident on the detector that gives rise to an rms signal equal to the rms dark noise in a 1-Hz bandwidth. Data from Hughes Aircraft Company. 

SNRrf scales with but is always less than SNR. As (Po aDaKC) increases to the point where it significantly exceeds 0, analyte shot noise will exceed dark noise (cr cr ), and Eq. (4.22) will revert to Eq. (4.16). As always, the best one can do is the analyte shot noise limit, when other noise sources become insignificant. 

As noted earlier, this is a rare case, resulting from a very weak signal or a high Ur (such as a photodiode array). Unlike the analyte, sample, or dark noise limits, SNRr is linear with t rather than... 

Dark Noise. Not the same as dark signal but is defined rigorously as the standard deviation of the signal obtained with the laser off. 

The statement that the dark noise equals is simple enough, but it does have some practical consequences. When the dark spectrum is subtracted from the Raman spectrum obtained with the same integration time, the noise on the spectrum will increase according to ... 

You should also explore the issue of background signal due to dark noise, scattered light or (possibly) luminescent impurities in the cuvette or water. 

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