Solar Simulators: Interpreting Spectral Irradiance Graphs
When designing our solar simulator, we faced some interesting questions. These included: how exactly should you define the solar spectrum? Or, more fundamentally, how can we best represent it? In a related article, we discuss how solar simulator irradiance relates to spectral mismatch. This introduced another question: do we try to achieve a closer spectral match of the AM1.5G spectrum? Or, should we aim to achieve an irradiance of 1000 W/m2 or 1 Sun over our given spectral range? Past the broader guidelines of “try to replicate the solar spectrum as accurately as possible,” we found that the distinctions between some of these nuanced points are not 100% clear.
The aim of this article is to contextualise the spectral irradiance graphs you see throughout our website and elsewhere. Hopefully, this provides a bit of insight into how this information can be presented, and why we have chosen to present it the way we have.
AM1.5G Spectrum
The above graph shows the full AM1.5G spectrum between ~350 nm – 2500 nm. This entire spectrum integrates to 1000 W/m2 and this is where we get the standard power irradiance value of 1 Sun. However, approximately 700 W/m2 lies between 350 nm – 1000 nm.
The bulk of solar simulator illumination is often concentrated within the visible light region, as this is considered to be the most “useful portion” of the spectrum. This is largely because longer wavelength photons won’t be absorbed by solar cell materials if the photon energy is lower than the band gap. Therefore, when solar simulators are discussed, you will often see the AM1.5G spectrum presented as it is in the graph below.
This graph shows the spectrum in terms of its most “useful” part, i.e. between 300 nm and 1000 nm. This is useful as it discounts the less relevant portions of the spectrum. However, it is important to remember the integrated irradiance over these wavelengths is roughly 700 W/m2, and that a significant amount of the incident power is not shown in the above graph.
Comparing Solar Simulators to the AM1.5 Spectrum
When looking at a spectral irradiance graph, you should always ask yourself what irradiance the data was measured at. It is common practice for companies to scale their spectral irradiance graphs (usually 0.1 – 0.5 Suns) in order to more conveniently compare the solar simulator spectral shape to the AM1.5G spectrum.
Although this can give you useful information about the spectral shape of your solar simulator, we think that it is important to be clear when a spectrum isn’t measured at 1 Sun. Remember, the total spectral irradiance of a solar simulator may not be obvious on first viewing.
To more clearly illustrate this point, we have plotted the spectrum of the Ossila Solar Simulator in a few different ways.
Illustrating 1 Sun Irradiance
If we plot the spectrum of our calibrated Ossila Solar Spectrum at 1 Sun, the peaks appear much larger than the AM1.5G spectrum.
However, if we zoom out on these spectra, we can see that the integrated area underneath this Ossila spectra is the same as the total integrated area of the AM1.5 spectra, i.e. 1 Sun. The Ossila Solar Simulator comes calibrated at this spectral distribution. This means that when you turn it on, the emitted light has a 1 Sun spectral irradiance with the above spectral distribution. However, you can vary the light output to adapt this spectrum to fit the curve better if you see fit.
Illustrating Spectral Shape Compared to AM1.5G Spectrum
The graph shown below illustrates the spectral distribution of the Ossila Solar Simulator at 70% intensity (0.7 Sun). You can see that this spectrum more closely aligns with the AM 1.5G spectrum. These reduced curves are useful to illustrate the spectral match of a solar simulator. However, there appears to be no real guidance on how logical this spectral scaling needs to be. Looking at other solar simulator companies, we have found this scaling factor to be anywhere between ~0.15 – 0.55 Suns - we presume this depends on which better fits the AM 1.5 spectral shape. We have chosen this value of 0.7 Suns as this should be most representative of the portion of the AM1.5G spectrum which lies between these wavelengths.
The shape of the spectrum in the graph above has not been altered at all. Some distributers clarify that they do not change the spectral shape of their spectrum to align with the AM1.5G spectrum. For arc lamps, it is taken for granted that you can’t change the shape of the output spectrum. However, with the emergence of LED solar simulators, you can adapt the shape of your emitted spectrum if desired.
For example, in the graph below, the individual outputs of the Ossila Solar Simulator have been optimised to fit the AM1.5G spectra by eye, rather than according to calibration standards. It is important to note that in most cases, varying the output spectra significantly means that the LEDs in this format will not necessarily meet calibration standards.
Understanding spectral irradiance data is very important if you are considering buying a solar simulator. We state clearly the total irradiance for the irradiance graphs shown on our website. However, the spectra shown by other manufacturer's is often not taken at 1 Sun Irradiance. These graphs should be considered only to demonstrate spectral match.
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