The energy flow within the sun gives rise to a surface temperature of roughly 5800K. Hence, the spectrum of the solar radiation is like that of a 5800K blackbody with a fine structure because of absorption in the cool peripheral solar gas (Fraunhofer lines).
The solar constant is defined as the irradiance of the sun on the outer atmosphere of the earth when the distance between the earth and sun is 1 AU (the mean earth-sun distance of 149,597,890km). The total integrated irradiance over the entire spectrum is the solar constant.
Extraterrestrial and Terrestrial Spectra
The extraterrestrial spectrum is the spectrum of the solar radiation on the outer atmosphere of the earth as illustrated in Figure 1, showing a range of 200-2500nm that includes 96.3% of the total irradiance with the large part of the remaining 3.7% at longer wavelengths. The spectrum of the solar radiation at the surface of the earth has many different components. The rays coming directly from the sun are called direct radiation, while the rays scattered from the sky and the surroundings are called diffuse radiation. The total ground radiation is termed as the global radiation.
Figure 1. Spectrum of the solar radiation outside the earth’s atmosphere compared to the spectrum of a 5800K blackbody.
For global irradiance, the direction of the target surface needs to be defined. The target surface has to face the incoming beam for direction radiation. The ground spectrum relies on the distance that must be traversed by the solar radiation through the atmosphere. At any location, the distance that must be traversed by the radiation to reach ground level varies as the day progresses. Hence, the ground solar radiation level changes as the day progresses and reaches to zero at night. Similarly, there are changes in the spectrum of the radiation through each day due to the changing absorption and scattering length.
With the sun overhead, direct radiation traverses directly through the atmosphere to reach the ground, with all the air mass overhead. This radiation is called Air Mass 1 Direct (AM1D) radiation and a level reference site is used for standardization purposes. Similarly, the global radiation with the sun overhead is known as Air Mass 1 Global (AM1G) radiation. Since the extraterrestrial spectrum traverses through no air mass, it is termed as the Air Mass 0 (AM 0) spectrum. The atmospheric path for any zenith angle is delineated corresponding to the overhead air mass (Figure 2). The actual path length can relative to air masses of below 1 (high altitude sites) to much higher air masses just before sunset. Oriel Solar Simulators employ filters to reproduce spectra relative to air masses of 0, 1, 1.5 and 2.
Figure 2. The path length, in units of Air Mass, changes with the zenith angle.
Oriel Xenon Based Solar Simulators
Oriel Solar Simulators offer the spectral match much closer to the solar spectra available from any source. Although the match is not exact, it is more than adequate for several applications. The optics of Oriel Simulators is shown in Figure 3. The xenon arc lamp at the heart of the instrument produces a 5800K blackbody-like spectrum with occasional line structure. The small high radiance arc facilitates efficient beam collimation.
Figure 3. Light path of an Oriel Solar Simulator.
Oriel Solar Simulators include low F/# collection, collimation, and optical beam homogenization and filtering, thus yielding a continuous output with solar-like spectrum in a homogenous collimated beam. The direct terrestrial beam is simulated by the beam collimation, thus enabling characterization of radiation induced phenomena.
The Role of Filters
The intense line output of the xenon lamp makes the lamp differ from the solar spectrum in the range of 800-1100nm. The mismatch can be minimized using an AM 0 filter. Nevertheless, the use of any reasonably economical filter is not able to eliminate the line structure without careful modification of the remaining spectrum. The relevance of the remaining infrared mismatch relies on the application. Oriel Instruments’ AM 1, AM 1.5 and AM 2 filters can also alter the visible and ultraviolet regions of the spectrum for a better match to the standard solar spectra.
UV Solar Simulators
Oriel's Enhanced UV Solar Sources are like the full spectrum Solar Simulators, but with modified spectral distribution to decrease visible and IR regions of the spectrum. These solar simulators are ideal for areas demanding an intense UV source free from the difficulties associated with visible and infrared heating. This is crucial for biological work on live subjects. A 1000W Solar Simulator featuring exposure control devices and air mass filters is depicted in Figure 4.
Figure 4. A 1000 W Solar Simulator with Air Mass Filters and Exposure Control Instruments.
The UV-A and UV-B regions are gaining attention owing to the fact that natural sunlight includes UV-A and UV-B radiation. The three configurations of UV Solar Simulators are as follows:
- UV-A, UV-B and UV-C - This simulator generates UV from 210 to 400nm.
- UV-A and UV-B – This system has a UV-C blocking filter, generating UV from 280 to 400nm.
- UV-A – This simulator features UV-B and C blocking filters, producing UV from 320 to 400nm.
About Oriel Instruments
Oriel Instruments, a Newport Corporation brand, was founded in 1969 and quickly gained a reputation as an innovative supplier of products for the making and measuring of light. Today, the Oriel brand represents leading instruments, such as light sources covering a broad range from UV to IR, pulsed or continuous, and low to high power.
Oriel also offers monochromators and spectrographs as well as flexible FT-IR spectrometers, which make it easy for users across many industries to build instruments for specific applications. Oriel is also a leader in the area of Photovoltaics with its offering of solar simulators that allow you to simulate hours of solar radiation in minutes. Oriel continues to bring innovative products and solutions to Newport customers around the world.
This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.
For more information on this source, please visit Oriel Instruments.