Light emission, source

The amount of light emitted by a source is measured by its luminance or by its luminous intensity, which are defined in Figure 18.2. Intrinsic light emission relates to the amount of light emitted per unit area (luminance). Table 18.1 lists approximate luminances for some common light sources. 

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

Particulate emissions have their greatest impact on terrestrial ecosystems in the vicinity of emissions sources. Ecological alterations may be the result of particulate emissions that include toxic elements. Furthermore, the presence of fine particulates may cause light scattering, known as atmospheric haze, reducing visibility and adversely affecting transport safety, property values, and aesthetics. 

The source of light emission. Each individual animal in a colony of Pyrosoma has two groups of luminous cells at the entrance to the branchial sac (Herring, 1978a). The luminous cells contain tubular inclusions whose identity has been a matter of dispute for nearly a century. Buchner (1914) originally suggested that the inclusions were symbiotic luminous bacteria. However, efforts to cultivate luminous bacteria from the luminous cells of Pyrosoma consistently failed. In... 

Watanabe, H., Nagoshi, T., and Inaba, H. (1993). Luminescence of a bacterial luciferase intermediate by reaction with H2O2 the evolutionary origin of luciferase and source of endogenous light emission. Biochim. Biophys. Acta 1141 297-302. 

Fig. 9.2. Light from the emission source is broken into line components with a prism. A grating would provide the same result.

A number of studies have documented that concentrations of some of the directly emitted species found in outdoor atmospheres can be quite high indoors if there are emission sources present such as combustion heaters, gas stoves, or tobacco smoke. In addition, there is evidence for chemistry analogous to that occurring outdoors taking place in indoor air environments, with modifications for different light intensities and wavelength distributions, shorter residence times, and different relative concentrations of reactants. In Chapter 15, we briefly summarize what is known about the chemical composition and chemistry of indoor atmospheres. 

Surface measurements of NO-, have been made using folded light paths and a light source such as a Xe arc lamp (Platt, 1994 Plane and Smith, 1995). Concentrations as high as 350 ppt have been observed in polluted urban areas (Platt et al., 1980b, 1981 Platt and Janssen, 1995), although many times even in polluted areas, the concentrations are below 20 ppt (Bier-mann et al., 1988). This likely reflects the balance between sources and sinks. For example, since N03 reacts rapidly with NO, significant concentrations of N03 will not be observed close to NO emission sources. 

In flame emission spectroscopy, light emission is caused by a thermal effect and not by a photon, as it is in atomic fluorescence. Flame emission, which is used solely for quantification, is distinguished from atomic emission, used for qualitative and quantitative analyses. This latter, more general term is reserved for a spectral method of analysis that uses high temperature thermal sources and a higher performance optical arrangement. 

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