Solar radiation spectral distribution

The spectral distribution of this radiation is given in Table 4.3, from which we can easily see that radiation with wavelengths below 150nm represents only a tiny fraction of the total. The energy distribution of the solar radiation corresponds to that from a black body with a temperature of around 5,000 K. 

Table 4.3 Spectral distribution of the optical solar radiation in the Earth s atmosphere. Data taken from the Smithsonian Physical Tables (1959)...

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. 

The sun s total radiation output is approximately equivalent to that of a blackbody at 10,350°R (5750 K). However, its maximum intensity occurs at a wavelength that corresponds to a temperature of 11,070°R (6150 K) as given hy Wien s displacement law. A figure plotting solar irradiance versus spectral distribution of solar energy is given in Fig. 9. See also Solar Energy. 

Leckner. B. (1978) The spectral distribution of solar radiation at the Earth s surface - elements of a model. Solar Energy.. 20, 143. 

Solar radiation consists of photons of different energies E. Of particular interest is the spectral distribution n(E), which describes how the photons are distributed over the different energy values. The quantity n(E) indicates the number of photons of specific energy E per unit surface area per unit energy per unit time. From this distribution we define the total photon flux as... 

Both these effects require knowledge of the spectral and spacial radiation distributions of the radiation flux on a surface. The determination of both these distributions at a given location is a difficult instrumentation problem. In this paper, the effect of scattering processes in the atmosphere on the available energy of solar radiation on a surface is examined. Both the spectral and spacial effects of Rayleigh scattering are demonstrated. 

The measurement of the spectral distribution of solar radiation outside the atmosphere and the subsequent association of this spectral distribution with the spectral distribution of radiation in a blackbody cavity has, I believe, biased the attempts to characterize the actual radiation in the atmosphere to an undue extent. Figure 1 indicates typical spectral distributions of radiation in the atmosphere as compared to that of solar radiation outside the atmosphere. Outside the atmosphere m 0 and if the flux is directly through m 1. If slanted at and angle from the zenith angle 90, then m is approximately 1/cos 60. 

Figure 1. Spectral distribution of direct solar radiation through a clear atmosphere of different air masses.

Figure 2. Spectral distribution of scattered solar radiation compared to extraterrestrial and direct solar radiation.

In dealing with problems of solar radiation, as opposed to blackbody radiation, the effect of the solid angle in which the radiation is confined has been examined (2-4) by considering the volume density of photons to be reduced. Landsberg(6) considers dilute radiation in the sense that the spectral distribution is retained but the radiation density is reduced. This leads to defining the temperature of a spectral component as... 

Fig. 8-63 Spectral distribution of solar radiation as functions of atmospheric conditions and angle of incidence according to Ref. 15.

The sun is not a "perfect" radiator, nor does it have uniform composition. The sun is composed of about 92% hydrogen, 7.8% helium. The remaining 0.2% of the sun is made up of about 60 other elements, mainly metals such as iron, magnesium, and chromium. Carbon, silicon, and most other elements are present as well.1 The inte raction of the atoms and ions of these elements with the radiation created by the annihilation of matter deep within the sun modifies and adds structure to the solar spectral distribution of energy. Astrophysicists such as Kurucz have used quantum calculations and the relative abundance of elements in the sun to compute the theo retical spectral distribution from first principles.5 Figure 1 shows a plot of the Kurucz computed spectral distribution at very high resolution (0.005 nanometer at UV) as well as an inset showing much lower resolution (0.5 nanometer in UV to 5 nm in IR) plot. 

Figure 4-5. Wavelength distributions of the sun s photons incident on the earth s atmosphere and its surface. The curve for the solar irradiation on the atmosphere is an idealized one based on Planck s radiation distribution formula (Eq. 4.3a). The spectral distribution and the amount of solar irradiation reaching the earth s surface depend on clouds, other atmospheric conditions, altitude, and the sun s angle in the sky. The pattern indicatedby the lower curve is appropriate at sea level on a clear day with the sun overhead.

Besides the absorption of the various components of solar irradiation, additional infrared (IR), or thermal, radiation is also absorbed by a leaf (see Eq. 7.2 and Fig. 7-1). Any object with a temperature above 0 K ( absolute zero ) emits such thermal radiation, including a leaf s surroundings as well as the sky (see Fig. 6-11). The peak in the spectral distribution of thermal radiation can be described by Wien s displacement law, which states that the wavelength for maximum emission of energy, A,max, times the surface temperature of the emitting body, T, equals 2.90 x 106 nm K (Eq. 4.4b). Because the temperature of the surroundings is generally near 290 K, A,max for radiation from them is close to... 

D65, daylight, with a CCT of 6500K is defined by the CIE. The International Standards Organization Standard ISO 10977 (1993) refers to this fact. D65 is also known as D6500 or Standard Illuminant D by the CIE, represents daylight over the spectral range 300 to 830 nm was first adopted in 1966. This standard is not a particular lamp but an internationally agreed to spectral power distribution for solar radiation, issued by the CIE as "Technical Report, Solar Irradiance," first edition... 

Commission Internationale de L Eclairage (CIE). International Commission on Illumination. Recommendation for the integrated irradiance and spectral distribution of simulated solar radiation for testing purposes. CIE, No. 20 (TC 2.2). Paris, France Bureau Central de la CIE, 1972. 

Spectral distribution of solar radiation just outside the alinosphere, at the surface of the carlli on a typical day, and comparison with blackbody radiation at 5780 K. 

In solar energy applications, the spectral distribution of incident solar radiation is very different than the spectral distribution of emitted radiation by the... 

Solar Radiation. Of all the factors which collectively determine the amount and spectral distribution of the radiation entering a surface layer of the atmosphere, the best established appear to be the spectral irradiance outside the atmosphere and the attenuation by molecular scattering. The absorption coefficients of ozone are well established, but no easy method exists for determining the amount of ozone in a vertical profile of the atmosphere at a given time. The measurement of the particulate content of the atmosphere and its correlation with atmospheric transmission is a field in which much remains to be accomplished. Surprisingly few data exist on the spectral distribution of sky radiation and its variation with solar elevation and atmospheric conditions. The effect of clouds is of secondary importance, as intense smog generally occurs under a clear sky. 

The hemispherical total absorptivity is not only a property of the absorbing surface. Rather, it depends on the spectral distribution of the incident radiation energy. This is shown by the different values of a for the mainly short-wave solar radiation, in which the absorption properties at small wavelengths are decisive, and for the incident radiation from an earthly source, for which the long-wave portion of the absorption spectrum a (X,T) is of importance. 

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