Light sources, filters and detectors

Photochemistry of Organic Compounds From Concepts to Practice Petr Klan and Jakob Wirz 2009 P. Klan and J. Wirz. ISBN 978-1-405-19088-6 

The working temperature of tungsten-filament incandescent lamps lies between 2200 and 3000 K. Therefore, they emit light mostly in the visible and infrared parts of the spectrum. Such a source of radiation may be useful in photoreactions of coloured chromophores, for example in the photodissociation of bromine or chlorine molecules to initiate photohalogenation reactions (Section 6.6.1). 

Adapted by permission from Newport Corp. Oriel product line 

Some lasers that are frequently used in the photochemical laboratory and their properties are listed in Table 8.4. 

Optical filters can modify the spectral output from spectrally broad light sources and essentially monochromatic light can be obtained from light sources with several sharp but widely separated emission lines, such as the low-pressure mercury arcs. Commercially available glass Liters and readily prepared filter solutions are inexpensive alternatives to 

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In recent years, the evolution of the technological components required for IR sensor systems has been denoted by a significant miniaturisation of light sources, optics and detectors. Essentially, an IR sensor consists of (i) a polychromatic or monochromatic radiation source, (ii) a sensor head and (iii) a spectral analyser with a detector. As sensors where all optical elements can be included in the sensor head are the exception rather than the rule, also various optics, waveguides and filters may form essential parts of IR-optical chemical sensors. Another important building block, in particular when aiming at sensors capable of detecting trace levels, are modifications of the sensor element itself. 

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... 

Depending on the light source, the UV detector can operate from 190 nm and well into the visible area. The UV detector can be of various kinds, for example, filter photometric detectors, spectrophotometric detectors, and diode array detectors. 

Fluorescence Measurements, Figure 3 The schematic concept of a confocal fluorescence microscope configuration. LS=light source, 0D = optical detector, ExF = excitation filter, EmF=emission filter, DM = dichroic mirror, L = lens, FM = position of fluorochrome molecules, ESP = Light-source pinhole, and DP = detector pinhole. Note that the emission light coming from out-of-fbcus fluorochrome molecules is blocked away by the detector pinhole... 

Procedure for determining quantum yield of a photochemical reactor The set-up of the apparatus has been discussed above. The choice of light source, filter, reactor cell and detector depends upon the wavelength range used for the photochemical reaction. 

Photometric Moisture Analysis TTis analyzer reqiiires a light source, a filter wheel rotated by a synchronous motor, a sample cell, a detector to measure the light transmitted, and associated electronics. Water has two absorption bands in the near infrared region at 1400 and 1900 nm. This analyzer can measure moisture in liquid or gaseous samples at levels from 5 ppm up to 100 percent, depending on other chemical species in the sample. Response time is less than 1 s, and samples can be run up to 300°C and 400 psig. 

Optical methods are especially useful for the selective detection of CO and C02 concentrations. In low-priced sensors, a simple miniature light bulb is used as IR-source. The radiation emitted enters an absorption chamber, through which the flue gas is pumped. An added interference filter lets only the absorption spectra of the target gas pass. The IR detector determines the reduction of the light intensity, which is then transformed into an electrical signal. The correlation between the source intensity and the received intensity is given in the Lambert-Beer equation. 

C02-IR detection systems for example contain a light source emitting infrared light ideally at wavelengths around 4.2 pm (often a simple light bulb), an absorption path (the so called cuvette), a spectral filter and a detector (thermopile). The filter is normally integrated into the detector housing. 

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