Reference Spectral Distributions

From the discussion in the preceding Sections, even without addressing the influence of clouds, it is clear that the terrestrial solar spectrum is highly variable. So, how can 

Several national and international consensus standards organizations, such as the American Society for Testing and Materials (ASTM) and the International Standards Organization (ISO) have adopted a reference standard extraterrestrial spectral distribution (ASTM E490-00a), and terrestrial reference spectral distributions for direct beam and total hemispherical (on a 37° tilted south facing surface) spectra at a prescribed air mass of 1.5 (ASTM G173-03).2 10 11 

The extraterrestrial reference spectral distribution was assembled from a number of recent diverse space-based (satellite) measurement sources. The ETR reference spectrum is normalized to a total integrated irradiance of 1366.1 Wm 2. 

The terrestrial reference spectra required a more extensive set of criteria to be met. Specifically, a set of reasonable conditions that could occur rather commonly in nature should be used. The conditions for the terrestrial reference spectra were chosen to meet the following criteria 12 

These criteria resulted in the choice of a relatively simple, but accurate spectral model of Gueymard (SMARTS Simple Model for Atmospheric Transmission of Sunshine) to compute the reference standard terrestrial spectra.7 

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Colorimetry [1.17] - [1.19]. The principles of colorimetry are based on the fact that all color stimuli can be simulated by additively mixing only three selected color stimuli (trichromatic principle). A color stimulus can, however, also be produced by mixing the spectral colors. Thus, it has a spectral distribution, which in the case of nonluminous, perceived colors is called the spectral reflectance q (2). After defining three reference stimuli, the trichromatic principle allows a three-dimensional color space to be built up in which the color coordinates (tristimulus values) can be interpreted as components of a vector (CIE system for standards, see Table 1, Colorimetry CIE = Commission Internationale de l Eclairage). For uncolored illumination the three CIE tristimulus values depend on the spectral reflectance as follows ... 

Source compensation Pulse-to-pulse intensity variations and intensity fluctuations in the spectrometric excitation source are often the dominant noise source affecting the performance of the detection system. However, since OIDs are parallel multichannel detectors, these intensityvariations do equally and simultaneously affect the entire spectral distribution as a whole. Thus, with the aid of a single-channel reference detector, monitoring a portion of the source s light flux, it is possible to accurately normalize for spectrum-to-spectrum variations and practically eliminate these and any other source flicker noise related effects. 

Planck constant — To describe the spectral distribution of energy of black body radiation -> Planck made the ad hoc assumption that the radiant energy could exist only in discrete quanta which were proportional to the frequency E = hu with h = 6.62 6 0 6 93(11) x 10 - 34 Js. Before 2003 the accepted value was 6.6260755(40) x 10-34 Js = 4.1356692(12) x 10-15 eV s. The quantity h later was referred to as Planck s constant. 

The efficiency in this section is defined at 25°C under 1000 W/m2 of sunlight intensity with the standard global air mass 1.5 spectral distribution. Thus, 15 percent module efficiency refers to peak watt efficiency (Wp) and implies that 15 percent of the incident sunlight energy is converted to electricity. 

Most often, AOD is referred to a specific wavelength, usually 500 nm, or occasionally 550 nm. An AOD of 0.01 at 500 nm represents very clean, pristine, clean atmosphere. Values of AOD at 500 nm of 0.1 to 0.2 are quite typical of average conditions. Values of 0.4 or greater represent a heavy aerosol load and very hazy skies. Figure 8 shows the clear sky direct beam spectral distribution at Air Mass 1.5 for a range of AOD from 0.05 to 0.40... 

It is well known that the spectral distribution and irradiance of the solar radiation at the Earth s surface depend on the location and is subjected to seasonal and diurnal variations. Therefore, a reference spectrum is needed as a basis for comparison with the spectral energy distribution of artificial light sources. Data from CIE No. 15 1971 (colorimetry official recommendations of the International Commission on Illumination) that recommend a standard illuminant D65 with a scheduled color temperature of approximately 6500 K have been used as a basis over the years. 

In contrast to absorption spectroscopy the use of a reference is rather difficult. It requires a sample for which absorption and fluorescence spectra both exhibit the same spectral distribution as the probe. This is quite impossible. For this reason in some instrumentation one tries to eliminate fluctuations in the intensity of the light source. The excitation beam is split and a small amount of the intensity of excitation is focused on a photodiode. Nevertheless neither the changes in the optics of the instrumentation nor the variance of the transmission curve with wavelength can be overcome. However there is a way to calibrate the spectral distribution of a fluorimeter in-... 

Standard for determining PV efficiencies in the US [44], A reference cell is a solid-state PV device that has had its calibrated under the AM 1.5 G spectrum. Short-circuit current is the parameter that is most sensitive to the spectral distribution of the light source [44]. It is important that the band gap of the reference cell be as close as possible to the material being tested, because the calibration cell can only correct for spectral mismatch if it absorbs the same portion of the spectrum as the test electrode. Otherwise, a reference cell can be fitted with short-pass filters, to mimic wider band gap devices, and calibrated. Calibration of reference cells can be performed by any of several recognized institutions (NREL, Fraunhofer, AIST, and others). Integration of the spectral response over the AM 1.5 G spectrum is another method that can be used to obtain jsc and yield a reference cell that does not require outside testing. 

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