Optical filters and detectors

An optical filter is essentially a piece of coloured glass. An extensive range of optical filters are readily available as off-the-shelf components. Alternatively, custom filters can be made at a higher cost. Four basic types of filter are used for optical sorting applications  

Low pass transmitting only below a certain wavelength (Fig. 6.10)  

Band-pass transmitting only within a band of wavelengths (Fig. 6.12)  

Combinations of the above in a single filter e.g., a double band-pass filter (Fig. 6.13). 

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In its simplest form the exciting system will consist of a source, illumination optics, filter and polariser, and the intensity-measuring system will consist of an analyser, collection optics, filter and detector. The source of ultraviolet light is usually a mercury or xenon arc and the filter on the source side of the specimen selects from its output a narrow band of wavelengths near the peak of the absorption spectrum of the particular fluorescent molecule in use. The illumination optics should transmit the light as a fairly parallel beam to the specimen, so that it can be given a... 

FT-Raman spectrometers with performance significantly higher than that shown by Hirschfeld and Chase. Although the basic design of these instruments has not changed dramatically over the ensuing 20 years, improvements in the optical filters and detectors have dramatically improved the signal-to-noise ratio of FT-Raman spectra. These and other components are discussed in the next section. 

Fig.8.1. Schematic representation of iuminescent sorter 1-funnei 2-vibrational feeder 3-frame 4-conveyer for concentrate 5-conveyer for waste 6-luminescence excitation source 7-collecting optics 8-optical filter 9-detector 10-air valve (Moskrousov and Lileev 1979)... 

As for the second condition, usually the spectral response, which is provided by the instrument manufacturers, is used. For the erythemal broadband detectors, it has been demonstrated that regular testing of their spectral sensitivity is needed, in addition and prior to their absolute calibration. This is because their spectral sensitivity is determined fiom a series of optical filters and other components (e.g. the phosphor layer) that may degrade with time or with environmental conditions (e.g. humidity), changing therefore its characteristics. 

The geometry or design of the detector, which also includes selection of optical filters and types of light detectors with their differing relative responses to light of various wavelengths. 

Most commercial fixed-wavelength UV detectors take advantage of the intense line source of 254-nm radiation in the low-pressure mercury arc lamp. The high intensity of the radiation provides excellent detectability for the small-aperture microvolume flow cells required in HPLC. Concentration of most of the radiation in a narrow-wavelength band places less demand on optical filters and enhances the linear range of the detector. 

A typical IR spectrometer consists of the following components radiation source, sampling area, monochromator (in a dispersive instrument), an interference filter or interferometer (in a non-dispersive instrument), a detector, and a recorder or data-handling system. The instrumentation requirements for the mid-infrared, the far-infrared, and the near-infrared regions are different. Most commercial dispersive infrared spectrometers are designed to operate in the mid-infrared region (4000-400 cm ). An FTIR spectrometer with proper radiation sources and detectors can cover the entire IR region. In this section, the types of radiation sources, optical systems, and detectors used in the IR spectrometer are discussed. 

Spectroscopic techniques are popular as a means of detection on chips. Examples include the determination of flavins and DNA by fluorescence. Spectrophotometric techniques are often used for biological samples . Mass spectrometry has also been used. Benetton et al. coupled electrospray ionisation MS to a chip while Sillon et al. developed a low cost mass spectrometer which incorporated the ionisation chamber, filter and detector on the chip. A fibre optic coupler has been developed as a detector. The dual optical fibre configuration (one transmitting, one receiving, (Eigure 10.5)) in the chip forms the microchannel as well as the detector itself and measures refractive index changes but can also be used to measure absorbance . 

PD = photodiode detector, SC = sample cell, HS = hot stage, S = lamp source, F = optical filter and P, A = polariser and analyser, respectively. (b) Enlarged view of sample cell construction. 

The flame photometric detector is the principal component in the determination of sulphur compounds for which it offers a selectivity of about five orders of magnitude with respect to hydrocarbons. The selective sulphur detection is based on the formation of electronically excited S2 molecules in a hydrogen-rich flame. These short-lived species revert to their ground state and emit characteristic molecular band spectra with peak wavelengths at 384 and 394 nm. This chemiluminescent radiation passes an optical filter and is monitored by a UV-sensitive photomultiplier. 

Figure 1 Scheme of the optical set up of the NDIR gas analyser. The box contains filters and detectors,... 

Secondly, NIR spectrometers are often used for practical or routine purposes. Spectrometers developed for such purposes should, therefore, ideally be (i) portable, small in size, and light in weight, (ii) capable of use under severe conditions such as at high or low temperatures, in dusty places, or with mechanical disturbances, and (iii) inexpensive. Since FT-NIR spectrometers cannot fiilly satisfy these conditions, various other types of spectrometers have been developed and used. A dispersive spectrometer with a multichannel detector can be used under the conditions with mechanical disturbances because it has no moving mechanism such a spectrometer can be installed, for example, on a rotary blender of powdered dmgs. On the other hand, spectrometers with optical filters and AOTF spectrometers, which are light in weight and inexpensive, are suitable for outdoor in-the-field type measurements. 

The responsivity includes all instmmental properties, such as the transmission characteristics of optical filters, the detector response, amplifier gain, etc. The term tiiy, Teff) is the Planck function corresponding to the effective instrument temperature. If the instrument and the detector are at the same temperature, Tj, then that temperature is the effective temperature. However, the detector and the rest of the instrument are often at different temperatures for example, the detector may... 

The NICMOS optics emissivity is one minus the product of the efficiencies of the NICMOS optical elements except for the shield window, filter and detector which are cold. Again this is a conservative number relative to the emissivity goals of the instrument. The detector noise is for a two readout (beginning and end) read mode and is assumed to be independent of the integration time. Equation 1 describes the zodiacal background in photons per second per cm per (an per steradian. 

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