Measurement of detector noise

The detector noise is defined as the maximum amplitude of the combined short- and long-term noise measured over a period of 10 minutes (the El9 committee recommends a period of 15 minutes). The detector must be connected to a column and mobile phase passed through it during measurement. The detector noise is obtained by constructing parallel lines embracing the maximum excursions of the recorder trace over the defined time period as shown in figure 5. The distance between the parallel lines measured in millivolts is taken as the measured noise (vj, and the noise level (N ) is calculated in the following manner. 

The responsivity (E) or specific detectivity (D ) and the noise equivalent power NEP (Wn), are often used to measure the sensitivity of a detector. The responsivity depends on the wavelength of the radiation and the temperature of the detector. The NEP, also called minimum detectable power, is the quotient of detector noise (N) divided by voltage responsivity (E). The D is the reciprocal of NEP, thus W = NIE and D = 1/Wn- A more sensitive detector has a smaller NEP and larger D, which results in less noise and a faster response time. 

The noise equivalent power (NEP) of an infrared detector is a measure of the noise generated by the detector and is given by ... 

The detector will also contribute to the noise in the measured CARS resonant signal. Depending on the type of device used in these experiments to digihze the dispersed CARS signal, the detector itself will have a noise component to contribute to the measured signal. The contribution of detector noise to the total noise in a CARS measurement is discussed by Snelling et al. 

Whereas, in principle, simple experiments with tracers and one for each solute (explained below) allow the determination of e, e, and the component specific H from the experimentally determined pt <-> the other model parameters cannot be simply extracted from the second moment. Dispersion (D x), liquid film mass transfer (kfiijn), diffusion inside the particles (Dapp,pore)> and adsorption kinetics (kads) contribute in a complex manner jointly to the overall band broadening as described by Ot < (Equation 6.136). Therefore, an independent determination of these four parameters is not possible from Equation 6.136 only. In principle, additional equations could be obtained from higher moments (Kucera, 1965 Kubin, 1965). However, as the effect of detector noise on the accuracy of the moment value strongly increases the higher the order of the moment, a meaningful measurement of the third, fourth, and fifth moments is practically impossible. Equation 6.136 is thus not directly suited for parameter determination, but... 

In many cases it is useful to determine how the detector noise depends upon frequency. A plot of noise voltage (or current) as a function of frequency is known as the noise spectrum. Because the various types of detector noise exhibit different dependencies upon frequency, measurement of the noise spectmm is useful in determining the mechanism giving rise to the noise. Furthermore, the noise spectrum and frequency response (i.e., signal spectrum) can be used to calculate the dependence of signal-to-noise ratio on frequency, thereby determining the dependence of D on frequency. 

The 3M Sound Detector SD-200 is a compact, lightweight sound level meter designed for measurement of workplace noise levels. Its intuitive design makes it easy to measure sound levels and determine the level of hearing protection that may be required. 

The multiplex advantage is important enough so that nearly all infrared spectrometers are now of the Fourier transform type Fourier transform instruments are much less common for the ultraviolet, visible, and near-infrared regions, however, because signal-to-noise limitations for spectral measurements with these types of radiation are seldom a result of detector noise but instead are due to shot noise and flicker noise associated with the source. In contrast to detector noise, the magnitudes of both shot and flicker noise increase as the radiant power of the signal increases. Furthermore, the total noise for all of the resolution elements in a Fourier transform measurement tends to be averaeed... 

Detectors. The function of the gc detector is to sense the presence of a constituent of the sample at the outlet of the column. Selectivity is the property that allows the detector to discriminate between constituents. Thus a detector selective to a particular compound type responds especially weU to compounds of that type, but not to other chemical species. The response is the signal strength generated by a given quantity of material. Sensitivity is a measure of the abiHty of the detector to register the presence of the component of interest. It is usually given as the quantity of material that can be detected having a response at twice the noise level of the detector. 

Detect 100% of photons Photon detected as a delta function Large number of pixels Time tag for each photon Measure photon wavelength Measure photon polarization No detector noise fr Up to 99% detected fr One electron for each photon fr Over 377 million pixels 0 No - framing detectors 0 No - provided by optics 0 No - provided by optics 0 Readout noise and dark current... 

Figure 21. Noise spectrum of detector amplifiers. Note that both axes have logarithmic scale. There are two main components of noise - the white noise which is present at all frequencies, and the 1// noise that is dominant at low frequencies. 1// noise has a fractal structure and is seen in many physical systems. The bandpass of a measurement decreases for slower readout, and the readout noise will correspondingly decrease. A limit to reduction in readout noise is reached at the knee of the noise spectrum (where white noise equals l/f noise) - reading slower than the frequency knee will not decrease readout noise.

---