Light spectral distribution

Fiqure 5. Absorption (a), fluorescence excitation (b) and fluorescence (c) spectra of compound 12 in EPIP at 77 K. Recording (b) and excitation (c) wavelengths are shown in brackets. Correction of exciting light spectral distribution for spectra c has been done up to 600 nm. 

C is the concentration of limiting reactant in mol/L, c is the chemiluminescence quantum yield in ein/mol, and P is a photopic factor that is determined by the sensitivity of the human eye to the spectral distribution of the light. Because the human eye is most responsive to yellow light, where the photopic factor for a yellow fluorescer such as fluorescein can be as high as 0.85, blue or red formulations have inherently lower light capacities. 

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

The rate of photolytic transformations in aquatic systems also depends on the intensity and spectral distribution of light in the medium (24). Light intensity decreases exponentially with depth. This fact, known as the Beer-Lambert law, can be stated mathematically as d(Eo)/dZ = -K(Eo), where Eo = photon scalar irradiance (photons/cm2/sec), Z = depth (m), and K = diffuse attenuation coefficient for irradiance (/m). The product of light intensity, chemical absorptivity, and reaction quantum yield, when integrated across the solar spectrum, yields a pseudo-first-order photochemical transformation rate constant. 

Specific solar radiation conditions are defined by the air mass (AM) value. The spectral distribution and total flux of radiation outside the Earth s atmosphere, similar to the radiation of a black body of 5,900 K, has been defined as AM-0. The AM-1 and AM-1.5 are defined as the path length of the solar light relative to a vertical position of the Sun above the terrestrial absorber, which is at the equator when the incidence of sunlight is vertical (90°) and 41.8°, respectively. The AM-1.5 conditions are achieved when the solar flux is 982 Wm2. However, for convenience purpose the flux of the standardized AM-1.5 spectrum has been corrected to 1,000 Wm2. 

Emission spectra have been recorded for electron injection into Au and Ag spherical electrodes and hole injection into Au(lll) planar electrodes. These processes were brought about in solutions of acetonitrile containing tetrabutylammonium hexafluoro-phosphate (TBAHP), using the trans-stilbene radical anion as the electron injector and the thianthrene radical cation as hole injector. The spectrum for the hole injection process into planar Au(lll) electrodes has been resolved into the P S-polarised components of the emitted light. A comparison of the spectral distribution of emitted light for the above electron injection process, occurring at both Au and Ag... 

With this new cell geometry for planar electrodes the threshold energy is no longer dependent on the degree of electronic compensation for the IR drop and always coincides closely with the excitation energy. Therefore, these spectra are more likely to represent the true joint optical density of states for the system than those reported previously /1-4/. Consequently, this data does merit more rigorous interpretation with respect to the spectral distribution of the emitted light and the polarisation dependence of the emission. 

But not all materials emit the same amount of light when heated to the same temperature there is a spectral distribution of electromagnetic waves. For example, a piece of glass and a piece of iron when heated in the same furnace look different the glass is nearly colourless yet feels hotter to the skin because it emits more infrared light conversely, the iron glows because it emits visible as well as infrared light. 

Warnings are often given that acceleration factors for relating artificial light sources with service are meaningless, because of both the variation in solar irradiation and the variation in spectral distribution. Regardless of this, acceleration factors are estimated, and indeed have to be if any extrapolation from accelerated tests is to be made. 

The relationship of degradation to light intensity is certainly nearer to linear than is the case with temperature. This implied time scale would add considerably to the effort needed to collect data at a series of acceleration levels. Also, only very recent weathering apparatus would be capable of operating over a range of irradiance levels. The result, in practice, is that it is extremely uncommon for more than one irradiance level to be used and degradation is assumed to be proportional to irradiance. By this is meant irradiance of the same spectral distribution. Different distributions, particularly in the UV regions, will inevitably produce different results. 

Some important examples of real radiation sources with spectral distributions close to the Planck distribntion are the sun (which shows a spectrum consistent with T = 6000 K) and the bright tnngsten wire of a light bulb T = 2800 K). 

The spectral distribution of light emitted from a fluorescent strip light contains more UV component than does a normal tungsten bulb, and thus it will emit more photons of a sufficiently high energy to effect the degradation reaction shown in equation (9.6). 

With the availability of lasers, Brillouin scattering can now be used more confidently to study electron-phonon interactions and to probe the energy, damping and relative weight of the various hydro-dynamic collective modes in anharmonic insulating crystals.The connection between the intensity and spectral distribution of scattered light and the nuclear displacement-displacement correlation function has been extensively discussed by Griffin 236). 

Application of this technique to measurements of the spectral distribution of tight scattered from a pure SF fluid at its critical point was present by Ford and Benedek The scattering is produced by entropy fluctuations which decay very slowly in the critical region. Therefore the spectrum of the scattered light is extremely narrow (10 - lO cps) and can only be observed by this light beating technique 240a)... 

The velocity, density and temperature of a streaming gas can be determined by measuring the magnitude, frequency and spectral distribution of Rayleigh-scattered light from two simultaneously pulsed ruby lasers with parallel beams and slightly different frequencies 246)... 

The spectral distribution of light scattered from a plasma depends on its wavelength Xq, the electron Debye length and the scattering... 

The light absorption of rhodopsin is in the visible range, with a maximum at about 500 nm. The absorption properties of the visual pigment are thus optimally adjusted to the spectral distribution of sunlight. 

Outside the atmosphere, the solar flux approximates blackbody emission at 5770 K. However, light absorption or scattering by atmospheric constituents modifies the spectral distribution. The attenuation due to the presence of various naturally occurring atmospheric constituents is shown by the hatched areas in Fig. 3.12. 

FIGURE 16.5 Typical spectral distribution from a sunlamp (reprinted with permission of North American Philips Lighting Corporation). 

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