Filters absorption

Absorption filters, in contrast to interference filters, can be used at any angle of incidence and can thus be used in strongly converging or diverging beams. It should be noted that certain coloured glass filters can exhibit rather strong broad-band fluorescence when used to block UV light, which 

Neutral-density filters are used to reduce the intensity of a light beam in a well-defined way, e.g. for testing the linearity of an optical detector. Filters of this kind should have a constant attenuation over large spectral ranges to facilitate their use. Normally, semitransparent metal films of chromium or nickel on quartz substrates are used. A filter is characterized by its transmittance T, or optical density D. D and T are related according to 

a filter of optical density D = 1.0 has a transmittance T of 0.1 (10%). When two neutral density filters are combined the transmittances are multiplied while the optical densities are added. In Table 6.2 some pairs of D and T values are given. For high-power laser beams, bulk absorbing glasses or combinations of inclined quartz plates (Fresnel reflection) must be used [6.90]. 

Commonly available color filters have relatively wide spectral band-widths. Widths of 350-400 A are common, measured at a transmittance of one-half the maximum. The most suitable filter is that which has its maximum transmission at the desired wavelength and a small spectral bandwidth at 

FIGURE 3-2. Combination of two light filters to isolate a spectral region. 

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Medicine Single faucet activated carbon filter or whole-house tank-type activated absorption filter... 

Each of the techniques described above has unique strengths and weaknesses, and the optimum device structure for commercial full-color displays will also be heavily influenced by the ease with which it can be mass-produced. Currently full-color OLED displays have been manufactured commercially by using two of the above described techniques only, i.e., (a) side-by-side pixels deposited by high-precision shadow masking and (b) using white OLEDs and color absorption filters. 

As with prisms, there are other devices that have been historically used for dispersing or filtering electromagnetic radiation. These include interference filters and absorption filters. Both of these are used for monochromatic instruments or experiments and find little use compared to more versatile instruments. The interested reader is referred to earlier versions of instrumental analysis texts. 

The word monochromator is derived from the Latin language, mono meaning one and chromo meaning color. It is a device more sophisticated than an absorption filter that isolates the narrow band of wavelengths from visible and ultraviolet sources. 

In the laboratory, solutions of analytes that fluoresce are tested by measuring the intensity of the light emitted. The instrument for measuring fluorescence intensity is called a fluorometer. Inexpensive instruments used for routine work utilize absorption filters similar to what was described previously for absorption spectrophotometers (see Figure 8.2 and accompanying discussion) and are called filter fluorometers. Two such filters are needed—one to isolate the wavelength from the source to be absorbed, the wavelength... 

In multicomponent systems A"0 can be written as a sum of the individual absorption coefficients A ot = 2TA , where each AT,(A ) depends in a different way on the wavelength. If one or more of the components are fluorescent, their excitation spectra are mutually attenuated by absorption filters of the other compounds. This effect is included in Eqs. (8.27) and (8.28) so that examples like that of Figure 8.4 can be quantified. The two fluorescent components are monomeric an aggregated pyrene, Mi and Mn. The fluorescence spectra of these species are clearly different from each other but the absorption spectra overlap strongly. Thus the excitation spectrum of the minority component M is totally distorted by the Mi filter (absorption maxima of Mi appear as a minima in the excitation spectrum ofM see Figure 8.4, top). In transparent samples this effect can be reduced by dilution. However, this method is not very efficient in scattering media as can be seen by solving Eqs. (8.27 and 8.28) for bSd — 0. Only the limit d 0 will produce the desired relation where fluorescence intensity and absorption coefficient of the fluorophore are linearly proportional to each other in a multicomponent system. 

First, we do a survey checking 400 meters by 10 meters per filter. A filter suction sampler can reliably find explosive vapors (on dust) on the vegetation at 5 meters distances away from the source of the explosive, so one pass with a vehicle gives a 10-meter-wide check. The use of an absorption filter concentrates the explosive vapor. We suck at a liter per second continuously through the filter. 

This expression ignores the phase response of the absorptive filter, which has the effect... 

Figure 3.27 Reverberator constructed by associating a frequency dependent absorptive filter with each delay of a lossless FDN prototype filter [Jot and Chaigne, 1991].

Once a lossless prototype has been chosen, the absorptive filters and the correction filter need to be implemented based on a desired reverberation time curve. Jot has specified a simple solution using first order HR filters for the absorptive filters, whose transfer functions are written [Jot, 1992b] ... 

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. 

Reverberator constructed with frequency dependent absorptive filters 124... 

This is accomplished by starting with an energy conserving system whose impulse response is perceptually equivalent to stationary white noise. Jot calls this a reference filter, but we will also use the term lossless prototype. Jot chooses lossless prototypes from the class of unitary feedback systems. In order to effect a frequency dependent reverberation time, absorptive filters are associated with each delay in the system. This is done in a way that eliminates coloration in the late response, by guaranteeing the local uniformity of pole modulus. 

Figure 3.26 Associating an absorptive filter with a delay.

A consequence of incorporating the absorptive filters into the lossless prototype is that the frequency response envelope of the reverberator will no longer be flat. For exponentially decaying reverberation, the frequency response envelope is proportional to the reverberation time at all frequencies. We can compensate for this effect by associating a correction filter t(z) in series with the reference filter, whose squared magnitude is inversely proportional to the reverberation time [Jot, 1992b] ... 

A wavelength selector—interference and absorption filters or a monochromator (prisms and diffraction gratings)—to isolate the desired emission line. 

Absorption filters function to remove radiation by absorbing the radiation internally. This produces heat, which can change the filters dimensions and optical properties. 

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