Other sources of noise

There are several other sources of noise that are specific to lifetime imaging. In particular, noise related to the timing jitter of... 

There are other sources of noise, whose behavior cannot be described analytically. They are often principally due to the sample. A premier example is the variability of the measured reflectance of powdered solids. Since we do not have a rigorous ab initio theory of diffuse reflectance, we cannot create analytic expressions that describe the variation of the reflectance. Situations where the sample is unavoidably inhomogeneous will also fall into this category. In all such cases the nature of the noise will be unique to each situation and would have to be dealt with on a case-by-case basis. 

Thermal electric noise thermometry 1. Josephson junction point contact 2 Conventional amplifier 0.001-1 4-1400 Mean square voltage fluctuation Nyquist s law oc fegT Other sources of noise serious problem for T > 4 K... 

In many cases it may be impossible to separate fluctuations in ambient HO from background fluctuations or other sources of noise. The proper uncertainty in the net HO signal, averaged over any chosen time interval, is the standard error of the mean calculated from standard statistical formulas from the net data only. This calculation is independent of the applicability... 

Measurement accuracy. Our r.m.s. shot to shot fluctuations are 5%, thanks to the use of reference. This accuracy does not depend on the mode of operation (spatial resolution and choice of field polarizations). It is not as good, however, in multiplex CARS, or in dilute samples, or in the vicinity of narrow lines, because other sources of noise such as photoelectron statistics and laser frequency instabilities (however small) then play a major role. 

Many sources of noise can be overcome by careful design, such as the use of shielding to prevent pickup of random voltages, smoothing of power supplies to obviate the effect of main voltage variations, and the use of shock absorbers to reduce mechanical vibration. Other sources of noise can be avoided by simple precautions, such as avoiding defective components, poor contacts, leaky insulations, and temperature fluctuations which adversely affect the noise of ampliflers and photomultipliers (W8). 

Dark current and amplilier noise arc usually small compared with other. sources of noise in photometric and spcclrophotomctric iii.siruinents and become important only under conditions of low phoiocurrents where ihe lamp intensity or the transducer sensitivity is low. For example, such conditions are often encountered near the wavelength extremes for an insirumeiU. 

The two constraints that make direct observation of weak absorption signals impracticable are the presence of pink noise, which contains a preponderance of low-frequency power compared with white noise, and the enhancement of this and other sources of noise by the rectification process through which MMW signals are detected. Both predicate the use of some kind of modulation at a frequency greater than the noise comer frequency and the use of a phase-coherent detector to convert the resulting modulated signal to a DC level suitable for display or for post-detection computer processing. 

Mechanical transients can also cause variations in the flow. Other sources of noise are also possible, and will be investigated in future research. 

If there are other sources of noise, SNR, will decrease below the value predirted by the number of interacting quanta so that DQE will fall below tj. From the measurement of SNR it will appear that fewer X-rays have been used to form die image than has actually been die case and DQE is a measure of that apparent lack of efiiciency. In fact, the quantity, SNR which, in the absence of additional noise sources is just the number of X-rays detected, is known as the number of noise-equivalent quanta or NEQ. 

Although the 1/f noise is computed within the Detector Module, it is included in the simulated interferograms at the Add Noise Module along with the other sources of noise. 

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