Sharp-cut filters

Absorption spectra were measured on a Hitachi U-3500 spectrophotometer. Photoirradiation was carried out by using a USHIO 500-W super high-pressure mercury lamp. Mercury lines of 313, 365, 517 and 578 nm were isolated by passing the light through a combination of a Toshiba band-pass filter (UV-D33S) or sharp cut filter (Y-48, Y-52) and monochromator. 

Excitation of the target molecule has been found80 for Na + S02 and N02. and has been studied later on in more detail for Na + NOz.81 The identity of the excited molecular species could be demonstrated by measuring the lifetime of the transitions under question. A pulsed beam of sputtered sodium, the pulse width being 10 //sec, was employed and the time between the excitation of the target gas and the observation with the photomultiplier was varied. The variation of the emission intensity with the delay time gives the lifetime of the excited state. The spectral analysis was made with interference filters of 70 A bandwidth some measurements have been made with sharp-cut filters. 

Searle, D., 1985, Spectral factors in photodegradation activation spectra using the spectrograph and sharp cut filter techniques, 7th International Conference on Advances in Stabilization and Controlled Degradation of Polymers, Lucerne, May 22-24. 

Optical Filters. Various types of optical filters may be used to isolate certain wavelengths of light. There are narrow-bandpass filters, sharp-cut filters, and interference filters. The first two are usually made of glass and contain chemicals (dyes) that absorb all radiation except that desired to be passed. The sharp-cut filters absorb all radiation up to a specified wavelength, and pass radiation at longer wavelengths. 

Another problem was to choose the most efficient method of exciting salicylic acid, which has only one useful excitation band (at 308 nm). One approach was to use a special phosphor-lamp source emitting between 270 and 340 nm with a peak at 306 nm this gave good sensitivity for salicylic acid. Another approach was to use the somewhat weak 313 nm line emitted by a low-pressure mercury-arc lamp, selected with a combination primary filter consisting of the 7-54 filter along with a plastic filter used to cut out the 254-nm mercury line. This procedure essentially excited only the salicylic acid, not ASA. In both cases, a sharp-cut filter with 37% T at 465 nm was used to exclude any possible emission by ASA. 

In FT-Raman spectroscopy the radiation emerging from the sample contains not only the Raman scattering but also the extremely intense laser radiation used to produce it. If this were allowed to contribute to the interferogram, before Fourier transformation, the corresponding cosine wave would overwhelm those due to the Raman scattering. To avoid this, a sharp cut-off (interference) filter is inserted after the sample cell to remove 1064 nm (and lower wavelength) radiation. 

The filter and screen of the pyrometer shown ia Figure 9 require specific mention. From equation 21 it is evident that the observed radiation must be limited to a narrow bandwidth. Also, peak intensity does not occur at the same wavelength at different temperatures. The pyrometer is fitted with a filter (usually red) having a sharp cut-off, usually at 620 nm. The human eye is insensitive to fight of wavelength longer than 720 nm. The effective pyrometer wavelength is 655 nm. 

A detailed description of LES filtering is beyond the scope of this book (see, for example, Meneveau and Katz (2000) or Pope (2000)). However, the basic idea can be understood by considering a so-called sharp-spectral filter in wavenumber space. For this filter, a cut-off frequency kc in the inertial range of the turbulent energy spectrum is chosen (see Fig. 4.1), and a low-pass filter is applied to the Navier-Stokes equation to separate the... 

Instead of using the virtual impactor approach, North American air monitoring programs in the 1980s and later have adopted simpler reference methods that use the weighing of filters in the laboratory. The filters are obtained from samplers equipped with an inlet device that provides for a sharp cut-point in particle entry for samples of particles < 10 xm diameter or <2.5 [im diameter, which are operated over a fixed time period of 24 hours. The inlet fractionation is facilitated either by a carefully designed cyclone or by an impactor. The combination of the two samplers can give estimates of mass concentration for fine-particle and coarse-particle concentrations. 

A filter is required to pass a certain selected frequency band, or to stop a given band. The passband for a piezoelectric device is proportional to k2, where k is the appropriate coupling coefficient. The very low k value of about 0.1 for quartz only allows it to pass frequency bands of approximately 1% of the resonant frequency. However, the PZT ceramics, with k values of typically about 0.5, can readily pass bands up to approximately 10% of the resonant frequency. Quartz has a very high Qm (about 106) which results in a sharp cut-off to the passband. This, coupled with its very narrow passband, is the reason why the frequency of quartz oscillators is very well defined. In contrast PZT ceramics have Qm values in the range 102—103 and so are unsuited to applications demanding tightly specified frequency characteristics. 

A revision of the lighting practices in this tomb, for example the use of readily available, sharp-cut-off UV filters, must be undertaken to exclude possible photodegradation of pigments. At present, a system of fluorescent lights illuminates the walls from the base of each mural. This system is adequate now because of its infrequent use. However, the situation will be different if increased visitation is allowed. We are studying all these aspects of the conservation scheme with existing, well-documented samples taken from this tomb and other nearby tombs. 

This kind of noise is not very realistic, a true ideal filter is "real-time" impossible. However, such a spectrum can be approximated with a higher order filter with sharp cut-off characteristics. 

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