Radiant noise

Elevated Flares See Flares for a general definition. The elevated flare, by the use of steam injection and effective tip design, operates as a smokeless combustion device. Flaring generally is of low luminosity up to about 20 % of maximum flaring load. Steam injection tends to introduce a source of noise to the operation, and a compromise between smoke elimination and noise is usually necessary. When adequately elevated (by means of a stack) this type of flare displays the best dispersion characteristics for malodorous and toxic combustion products. Visual and noise pollution often creates nuisance problems. Capital and operating costs tend to be high, and an appreciable plant area can be rendered unavailable for plant operations and equipment because of excessive radiant heat. 

Environmentally Responsive Work-stations (ERWs). Workers in open-office areas have direct, individual control over both the temperature and air-flow. Radiant heaters and vents are built directly into their furniture and are controlled by a panel on their desks, which also provides direct control of task lighting and of white noise levels (to mask out nearby noises). A motion sensor in each ERW turns it off when the worker leaves the space, and brings it back on when he or she returns. 

There are a variety of FPA detectors available that are sensitive in the NIR spectral region. The optimal choice of detectors depends on several factors desired wavelength range, whether the application will be laboratory based or part of a process environment, the sensitivity needed to adequately differentiate sample spectra and price. The figure of merit most often used to describe detector performance is specific detectivity or D, which is the inverse of noise equivalent power (NEP), normalized for detector area and unit bandwidth. NEP is defined as the radiant power that produces a signal-to-dark-current noise ratio of unity. 

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. 

In dispersive spectrometers, the Rayleigh radiation may produce stray radiation in the entire spectrum, the intensity of which may be higher than that of the Raman lines. Interferometers transform the Poisson distribution of the light quanta of the Rayleigh radiation into white noise, which overlays the entire Raman spectrum. Therefore, all types of spectrometers must have means to reduce the radiant power of the exciting radiation accompanying the Raman radiation. 

Comfort is influenced by temperature, humidity, air velocity, radiant heat, clothing, and work intensity. Psychological factors may mso influence comfort, but their discussion is beyond the scope of this handbook. The reader is referred to Chap. 42 of the HVAC Applications volume of the A.S.H.R.A.E. Handbooks for a full discussion of the control of noise, which must also be considered in air-conditioning design. Figure 5 in Chap. 8 of the HVAC Fundamentals volume of the A.S.H.R.A.E. Handbooks relates the variables of ambient temperature, dew point temperature (or humidity ratio) to comfort under clothing and activity conditions typical for office space occupancy. It also shows boundary values for ET, the effective temperature index. This index combines temperature and moisture conditions into a... 

The ideal transducer for electromagnetic radiation responds rapidly to low levels of radiant energy over a broad wavelength range. In addition, it produces an electrical signal that is easily amplified and has a low electrical noise level. Finally, it is... 

A continuous source and a continuously measuring detector here the signal is proportional to the mean radiant density of the source B/ve and the measurement time to, the shot noise Ns (being of) is proportional to the signal fJ/ve and to, and... 

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 7. = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

Furthermore, the radiant density of the D2 lamp in a large part of the spectrum is fairly low. Hence, the procedure limits the number of analytical lines which can be used and the number of elements that can be determined. As the spectral radiance of the D2 lamp is generally low as compared with that of a hollow cathode lamp, the latter must be operated at a low radiant output (low current), which means that detector noise limitations and poor detection limits are soon encoun-terd. Finally, as work is carried out with two primary radiation sources, which are difficult to align as they have to pass through the same zone of the atom reservoir, this may lead to further systematic errors. 

The spectral range of interest for AAS spans from the near infrared (852.1 nm for Cs) to the vacuum ultra violet (193.6 nm for As). A monochromator is a device which separates, isolates, and controls the intensity of a narrow region of the radiant energy which is transmitted to the detector (characterised by its spectral slit width). It consists of an entrance slit, a dispersing unit (a diffraction grating) and an exit slit. The greater the intensity of the radiation transmitted to the detector the lower the signal amplification required (which contributes to electronic noise). However, the entrance and exit slits must be of similar mechanical widths (aperture size) so that, in practice, a compromise must be chosen (slit width versus narrow... 

This graph summarizes the wavelength response of some semiconductors used as detectors for infrared radiation. The quantity D (X) is the signal to noise ratio for an incident radiant power density of 1 W/cm and a bandwidth of 1 Hz (60° field of view). The Ge, InAs, and InSb detectors are photovoltaics, while the HgCdTe series are photoconductive devices. The cutoff wavelength of the latter can be varied by adjusting the relative amounts of Hg, Cd,... 

The ideal transducer would have a high sensitivity, a high signal-to-noise ratio, and a constant response over a considerable range of wavelengths. In addition, it would exhibit a fast response time and a xero output signal in (he absence of illumination. Finally, the electrical signal produced by the ideal transducer would be directly proportional to Ihc radiant power f. Fhat is, S - kP (7-18)... 

One source of noise of ihls type is the slow drift in the radiant output of the source. This type of noise can be called source flicker noise (Section 5B 2). The effects of nuctuations in the intensity of a source can be minimized by the use of a constani-voUagc power supply or a feedback system in which the source intensity is maintained at a constant level. Modern double-beam spectrophotometers (Sections I3D-2and I3D-3) can also help cancel the effect of flicker noise. With many instruments, source flicker noise does not limit performance. 

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