Absorption filters bandpass

Jot s method of incorporating absorptive filters into a lossless prototype yields a system whose poles lie on a curve specified by the reverberation time. An alternative method to obtain the same pole locus is to combine a bank of bandpass filters with a bank of comb filters, such that each comb filter processes a different frequency range. The feedback gain of each comb filter then determines the reverberation time for the corresponding frequency band. 

The precision of absorption measurements depends upon the degree of sophistication of the instrumentation and on the chemical species involved, both of which can affect the apparent validity of the Beer-Lambert law. Where a single absorbing species exhibiting a broad flat maximum is to be determined, adequate results can often be achieved with a simple filter-photometer. In the visible region, this technique is known as colorimetry. The inherent disadvantage of colorimetric procedures using simple filter instruments with a broad bandpass lies in the invalidation of the Beer-Lambert law and the lack of compatibility between results from different... 

Figure 1. Schematic of the apparatus. AC-absorption cell, BPF-bandpass filter, CdL-cadmium lamp, CM-capacitaiice manometer, D-frequency doubler, DG-three channel delay generator, DC-dye laser, EM-emergy monitor, Gl-gas inlet, HS-harmonic separator, HV-high voltage, PA-picoammeter, PD-photodiode, PM-photomultiplier, PL-photolysis laser, RC-reaction cell, SA-signal averager, T-chrottle, YL-Nd YAG laser, 7-54F-Corning 7-54 glass filter.

With broader-bandpass instruments, a didymium filter may be used to verify wavelength settings. This filter should show a minimum percent transmittance at 530 nm against an air blank (Figure 3-11). Because didymium has several absorption peaks, the setting should be verified grossly by... 

In the case of atomic absorption and atomic fluorescence the selectivity is thus already partly realized by the radiation source delivering the primary radiation, which in most cases is a line source (hollow cathode lamp, laser, etc.). Therefore, the spectral bandpass of the monochromator is not as critical as it is in atomic emission work. This is especially true for laser based methods, where in some cases of atomic fluorescence a filter is sufficient, or for laser induced ionization spectrometry where no spectral isolation is required at all. 

Whenever possible, essentially monochromatic light sources such as low- or medium-pressure mercury arcs equipped with bandpass filters or a monochromator (Figure 3.28), narrow-band photodiodes or lasers should be used for quantum yield determinations, because quantum yields can only be defined for monochromatic irradiation. This can be relaxed if the absorption spectrum of the actinometer is close to that of the sample (Section 3.9.6). One then assumes that the quantum yield is independent of the wavelength of irradiation. The stability of the light source over time is essential. Medium-pressure mercury arcs that have a stable output for many hours after a burn-in period of about 30 min are available. Xenon arcs tend to fluctuate abruptly when the arc between the electrodes jumps from one position to another. Intensity fluctuations of the light source in time can be monitored with photodiodes. This should be routinely done with pulsed lasers. 

When dedicated quantitative work is required in IR, simple filter instruments (also called photometers) are often used. These employ a narrow bandpass filter with a wavelength that corresponds to the absorption of the component being measured. This type of absorption is the more popular method for gas analysis, e.g. carbon monoxide, nitrogen dioxde and methane. It is also possible to have variable bandpass filters for use in scanning instruments. These are rugged and reliable dedicated analysers and commonly used in on-line systems. 

Some cases of refleetions in the IRF of a TCSPC system are shown in Fig. 7.27. Refleetions between two similar interferenee bandpass filters placed 2 cm from each other are shown in the upper left graph. The upper right graph shows a refleetion between the cathode of an MCP-PMT, and an interferenee filter placed 3.7 cm in front of it. The refleetion between an absorptive ND filter directly in front of the PMT and an interferenee filter 14 em in front of it are shown in the lower left graph. The undistorted IRF is shown in the lower right graph. 

The experimental setup to capture real-time images of the two-fluid mixing is similar to that used for p-PlV (Fig. 4). It also consists of an inverted microscope, but a continuous light source such as Hg arc lamp is used. The microscope filter cube, color filter assembly, is selected for the specific dye to be used as tracer. Fluorescein is the most common and inexpensive fluorescent dye with the absorption maxima of 490 nm and the emission maxima of 513 nm. For fluorescein, a FITC filter cube must be used. The standard FITC filter set from Chroma Technology consists of a 480 nm bandpass exciter filter, a 505 nm dichroic beam splitter, and a 535 nm bandpass emission filter. For another popular fluorophore, rhodomine 6G (absorption maxima of 530 run and emission maxima of 566 nm), a TRITC filter cube must be used. The standard TRITC filter set from Chroma Technology consists of a 540 run bandpass exciter filter, a 565 nm dichroic beam splitter, and a 605 nm bandpass emission filter. Higher quality filters... 

Shortpass, Longpass, Bandpass, Dichroic filter. Absorptive. 

It is important to scan at the wavelength of maximum absorption of the solute in the sorbent, where the absorption maximum for a compound adsorbed on silica gel particles is typically not the same as that for the same compound in an alcohol solution. Precise and sensitive determination requires that the absorption maximum be determined in situ on the plate. This is possible with an instrument equipped with sources of variable wavelength or monochromators. Narrow-bandpass filters are available for instruments based instead on filters. 

Fig. 14.5 A variety of typical filters, a An interference bandpass filter showing the typical mirrored surface of these types of filter b two square silvered glass neutral density filters and a round neutral density filter of a colloidal dispersion in glass c a series of round, coloured absorption cut-off filters, and d a pale blue glass heat filter e a Wratten filter of dyed gelatin, which is flexible and easily cut to shape f a polariser g a didymium absorption wavelength standard. The 5p UK coin, included top left to give some idea of scale, has a diameter of 18 mm...

Various alternatives to luc have received attention recently. Notably Renilla luciferase (7,31-33) as well as a number of red-shifted mutants of luc. Although the use of Renilla luciferase, which emits at 480 nm and requires a different substrate, may have utihty in two-color essays, strong absorption of the emission wavelength limits use for in vivo imaging. The development and use of red-shifted luc mutants is primarily aimed at reducing optical attenuation by tissue. However, given the bandpass filter effect of tissue chromophores (absorption of wavelengths <600 nm), the spectral components of even the native Iwc-mediate luminescence emitted in vivo are primarily in the red part of the spectrum (>600 nm) (34). 

The total transmittance of a multiple-stage filter is the product of the individual filter stage transmittances and has a sine function profile, shown in Fig. 8D. An LCTF based on the Lyot filter geometry having a maximum peak transmittance of 16%, a continuously tunable bandpass of 7.6 cm and a free spectral range of greater than 4600 cm" (500-650 nm) has been demonstrated [23]. However, a significant fraction of peak transmission loss in the Lyot LCTF is due to absorption by the polarizers and imperfect wave-plate action [24]. 

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