Light intensity, common units

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. 

The total intensity of sunlight outside the earth s atmosphere is characterized by the solar constant, defined as the total amount of light received per unit area normal to the direction of propagation of the light the mean value is 1368 W m-2, although variations from this mean are common (Lean, 1991). 

All infrared spectrometers generate data that are contained in the infrared spectrum (see Fig. 10.1). The spectrum represents the ratio of transmitted intensities with and without sample at each wavelength. This intensity ratio is called transmittance (7 ) can be replaced by percent transmission (%7 ) or by absorbance A = log(l/T). If the experiment is conducted using reflected or diffuse light, pseudo-absorbance units are used (cf. 10.10.2). Finally, it is common to report wavelengths in terms of wave number v (cm-1 or kaysers) knowing that ... 

Figure 7.32 Kinetics of luminescence of pyrene following laser flash excitation. L, laser pulse profile M, monomer emission, E, excimer emission rise and decay. Horizontal axis, time in ns vertical axis, light intensity in arbitrary units. The three kinetic curves are normalized to a common maximum...

The common units of light intensity or power density of light emitted or received are as shown above. Care should be taken in comparing luminances. For example. Table 18.1 reveals that a tungsten filament lamp has about a tenth of the luminance of the sun, but the area of the sun s emitting surface is massively greater than that of a filament lamp, and therefore the luminous intensity of the sun is massively greater than the luminous intensity of the filament lamp. 

The quantification algorithm most commonly used in dc GD-OES depth profiling is based on the concept of emission yield [4.184], Ri] , according to the observation that the emitted light per sputtered mass unit (i. e. emission yield) is an almost matrix-independent constant for each element, if the source is operated under constant excitation conditions. In this approach the observed line intensity, /ijt, is described by the concentration, Ci, of element, i, in the sample, j, and by the sputtering rate g, ... 

Deviation refractometers are the most commonly used. This version of the DRI measures the deflection in the location of a light beam on the surface of a photodiode by the difference in refractive index between the polymer solution and pure solvent. The Fresnel-type refractometers operate on the principle that the intensity of light reflected from a glass-liquid interface is dependent on the incident angle and the RI difference between the two phases. The deviation and Fresnel detectors typically have cell volumes of 5 to 10 pi, detection limits of about 5 x 10-6 refractive index units (RIU), and a range of 10 7 to 10 3 RIU.156 The deflection-type DRI is relatively insensitive to the buildup of contaminants on the sample cell and is therefore of special utility in laboratories that process large numbers of samples, such as industrial laboratories. 

PUVA is the acronym originally introduced to describe the combined administration of psoralen and subsequent exposure to high-intensity ultraviolet radiation from an artificial source (UVA). In the United States, orally administered 8-methoxypsor-alen (8-MOP) is the psoralen most frequently used in combination with UVA light therapy and will be the focus of this discussion. However, the term PUVA also refers to therapy with other oral and topical psoralens, most commonly 4,5, 8-trimethylpsoralen (TMP), a synthetic psoralen, and 5-methoxypsoralen (5-MOP), not available in the United States. 

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