Since the sun powers all of the life processes on Earth, being able to harness its energy and light source to invent, test and apply technology for mankind’s improvement is crucial.
The sun is the brightest object in the sky and something we are all familiar with. Life on our planet would not be possible without the sun. It provides the energy for the plants we eat and those that decayed into the fuel we burn today to harvest energy.
This article outlines a concept called ‘solar simulation’ – a technology that mimics the sun for use in research labs, industry applications and commercial use.
The main objective of solar simulation technology is to produce illumination approximating natural sunlight in order to provide a controllable indoor test facility under laboratory conditions.
The instrument used to simulate sunlight in a laboratory setting is called a solar simulator (sometimes called a sun simulator) . A solar simulator has a light source that is designed to offer similar intensity and spectral composition to that of natural sunlight.
Sunlight is composed of all the colors that humans can see. We can separate white light or sunlight into different colors when we pass it through a glass prism (red, orange, yellow, green, blue, indigo, violet).
Similar to the element in your stove or oven, the sun emits light because of its temperature. We can closely approximate the sun’s surface temperature to be about 5800 Kelvin (K), or 5500 Celsius (C).
Sunlight comes from the energy emitted in the form of electromagnetic radiation given off of the hot surface of the sun. So the sun’s radiation spectrum matches a 5800 K blackbody.
The amount of sunlight falling on the earth’s atmosphere changes over a year by about 6.6% due to the variation in the earth/sun distance, and solar activity variations cause sunlight to change up to 1%. Additionally, all the radiation that reaches the ground passes through the atmosphere, which modifies the spectrum by absorption and scattering. As we can see, the specific physical and spectroscopic properties of sunlight are different in different parts of the world, year, day, and even altitude. In addition, due to the earth’s curved surface, the sun’s radiation that reaches the earth’s surface does not strike all areas of the planet at the same angle. For example, when the sun is nearly overhead it hits directly near the equator but more obliquely near the poles.
Before anyone can start reproducing sunlight, they have to have a way of measuring and talking about it. There are two main methods of counting and quantifying light, known as radiometry and photometry.
Radiometry is the science of measuring light of any wavelength, that is, in any portion of the electromagnetic spectrum. Photometry, on the other hand, is only concerned with the measurement of visible light, with a specific view (i.e weighting) toward how strongly or weakly the human eye responds to these wavelengths.
Even though our eyes only perceive the visible portion of the electromagnetic spectrum, other wavelengths produced by the sun (such as UV or IR) also play an important role in many light-sensitive processes. Light-sensitive chemical reactions are also known as photochemical processes, and there are many that humans and all life rely on. For example, depending on the type of chlorophyll, plants wavelengths ranging from the UV to the IR to produce useful energy. Additionally, materials (organic or inorganic) can be designed to absorb light in a broad spectrum of wavelengths even outside of visible light.
Because the Sun emits visible light as well as many other useful and necessary wavelengths, it makes most sense to use radiometry to measure sunlight.
Radiometry measures light of any wavelength, so comprises the most broadly-applicable measurement units when talking about the entire electromagnetic spectrum.
Radiant flux is the light energy per unit time which is emitted, transmitted reflected or received by an object. It has units of Watts (W, or Joules of energy per second, J/s). It is also sometimes called radiant power or optical power. Because this unit of measurement doesn’t depend on wavelength, it can be used to measure any kind of electromagnetic radiation.
Spectral flux is like the radiant flux, but specific to a wavelength interval. If we want to know how much power is received, transmitted or emitted per wavelength of light, then we talk about spectral flux. This quantity is useful because it tells you how widely spread out your radiant power is over the electromagnetic spectrum. It has units of Watts per nanometer (W/nm). If the radiation is being described in terms of frequency (instead of wavelength), the spectral flux can have units of Watts per Hertz (W/Hz).
As an example, let’s say we have two 1 W light sources. One of them has a spectral flux of 2 W/nm, whereas the other has a spectral flux of 1 W/nm. The first source’s light is twice as concentrated in wavelength space.
Usually, though, light sources vary in spectral flux depending on the wavelength or color of light, so you’ll see spectral flux plotted as a curve (a function of wavelength).
Irradiance is the radiant flux shining on or received by a specific surface area. In other words, it’s the received power of light per unit area. It has units of Watts per square meter (W/m2) or other variants like mW/cm2 (because most detectors have dimensional areas in the range of cm2 rather than m2).
This is one of most commonly used radiometric units, simply because it’s easy to measure, report and share, for most measurements are made by a detector with a finite area. It’s hard to measure the full power (radiant flux) emitted by a source, but it’s much easier to put a detector of a given area in the path of light and make one measurement.
Sometimes irradiance is also referred to as intensity or optical intensity, but this naming should be avoided because it’s too easy to confuse with radiant intensity.
Radiance is defined as irradiance per unit of solid angle. If you’re not familiar with solid angle, you can think of it as a two-dimensional angle. A rough way to distinguish this from irradiance is that irradiance describes the power striking a specific surface from all angles, whereas the radiance describes the power striking a specific surface from a specific angle. It is not nearly as commonly used as irradiance.
One of the uses of radiance is that, because it takes distance into account through the calculation of solid angle, radiance itself doesn’t depend on distance. So you can go to other planets in the solar system and the Sun will have a different irradiance, but still have the same radiance.
Radiant Intensity of a light source is its radiant flux per unit of solid angle. We won’t go into too much detail here because it’s not needed for fully understanding solar simulators.
Briefly, though, this quantity captures how the light emission is changing as a function of the source’s angle.
As mentioned earlier, photometric units of light measurement are concerned with how light is perceived by the human eye, i.e. visible light. It is less useful when talking about measuring sunlight, but because it’s used so often in commercial lighting, it’s worth a brief discussion.
Like radiant flux, luminous flux is the emitted optical energy per unit time. However, luminous flux is weighted by the sensitivity of the human eye, which varies as a function of wavelength. The human eye’s response is usually separated into two categories, photopic and scotopic vision. Photopic vision is the eye’s response under well-lit conditions (mediated by cone cells), whereas scotopic vision is the eye’s response under low-light conditions (mediated by rod cells). Luminous flux specifically accounts for the photopic response of the human eye. The curves that describe the human eye response are usually called luminosity functions.
Luminous flux is measured in a unit called lumens, written as lm. Commercial indoor lights are usually expressed in lumens because regular illumination needs are only concerned with the visible part of the spectrum.
The luminous intensity of a source is the luminous flux per unit solid angle (remember, solid angles are a bit like two-dimensional angles). This quantity accounts for directional variances in light.
The units of luminous intensity are lumens per steradian (lm/sr) also known as candelas (cd).
Illuminance is the photometric analogy to irradiance. In other words, it is the visible light power per unit area. It has units of lumens per square meter (lm/m2), also known as lux (lx).
Luminance is the photometric analogy to radiance. In other words, it is the visible light power per unit area per unit solid angle. It has units of lumens per square meter per steradian (lm/m2/sr).
When talking about the amount of sunlight falling on the Earth, irradiance is the quantity used, which as discussed above is the light power per unit area. The solar irradiance received by Earth’s atmosphere changes over a year by about 6.6% due to slight variations in the Earth/Sun distance. The sun’s own activity changes result in emission variations of up to 1%.
Before light even enters our atmosphere, then, there’s variation in how much Earth receives.
Practically, though, we’re interested in the light that actually reaches the ground, which means light that has to pass through our atmosphere. The atmosphere modifies the spectrum by absorbing and scattering light.
Additionally, due to the Earth’s curved surface, the sun’s radiation reaching the surface does not strike all areas of the planet at the same angle. Depending on the angle of the Sun, light will have to travel through more or less atmosphere, and the absorption and scatter will change. For example, when the sun is highest at noon in a timezone, it is directly overhead near the equator but more oblique near the poles. The position of the Sun throughout the day will also change depending on the season because of the Earth’s axial tilt.
Even though our eyes only perceive the visible portion of the electromagnetic spectrum, other wavelengths produced by the sun (such as UV or IR) also play an important role in many photochemical processes. For example, depending on the type of chlorophyll, plants can absorb wavelengths that range from the UV to the IR to produce useful energy. Additionally, materials (organic or inorganic) can be designed to absorb light in a broad spectrum of wavelengths even outside of visible light.
As we can see, the answers to what is sunlight, and how much sunlight we get, are not that straightforward.
To be able to consistently test photochemical processes in a lab setting, we need to have a standard definition of what is defined as sunlight and a reliable and controllable source of light. Artificial light, provided by solar simulators, allows you to mimic the sunlight spectra that meets international standards which are needed to have consistent and reproducible experiments.
We’ve given ourselves some tools to talk about and quantify light in a meaningful way. To fully answer the question of how much sunlight the Earth gets, we need to understand more about all the different colors of light emitted by the sun.
Solar spectrum is defined as the electromagnetic spectral distribution emitted by the sun or received by a collector or instrument on Earth.
The sun radiates solar energy or sunlight by electromagnetic waves over a range of wavelengths known as the Solar Spectrum.
The Sun emits radiation from X-rays to radio waves, but the surface of the earth receives mainly wavelengths between 350 nm and 4000 nm. The region visible to humans is restricted to 400 nm to 700 nm, approximately 43% of the total energy.
The solar spectrum is generated by the sun’s surface, which as we discussed can be modeled by a black body of 5800 K. As we also mentioned, the spectrum we get at the Earth’s surface is modified because of atmospheric absorption and scattering.
Complex atmospheric processes may considerably modify the solar spectrum that reaches the earth’s surface. For example, gas-phase H2O and CO2 are strong absorbers of solar infrared radiation.
Molecules have many different ways of absorbing light. When they absorb light, that energy is converted into another form. That energy can be converted into a molecule’s vibrational, rotational or electronic energies, and the exact wavelengths of light it will respond to depend on the molecule in question.
Water vapor, for example, undergoes rotational absorption in the microwave and infrared, and undergoes vibrational absorption in the near and mid-infrared wavelengths. It experiences electronic absorption (where an electron is boosted to a higher energy state) in ultraviolet wavelengths.
Carbon dioxide, oxygen and other molecules each have their own unique absorption spectrum that impacts the spectrum of sunlight we see. Some examples of absorption spectra are given below.
In the visible range, precipitation, clouds, and sandstorms reduce solar radiation. Because most ultraviolet radiation is absorbed from the solar spectrum and does not reach the earth’s surface, the peak of the solar radiation that reaches the earth’s surface is in the visible part of the spectrum. Of the total radiation, about 3/4 ultimately reaches the earth. The energy distribution of the solar spectrum is approximately 5% UV, 43% visible and 52% infra-red.
In summary, the sunlight we see varies because of solar activity, Earth-sun distance, precipitation, solar angle, clouds, sandstorms and more. You can probably appreciate how it’s difficult to give a simple answer to “how much sunlight do we get” when it depends on so many variables!
If we want any hope of recreating sunlight, we have to have a basis to start from, though. To be able to consistently test photochemical processes or solar cells in a lab setting, for example, we need to have a reliable, reproducible and controllable source of light.
Solar simulators aim to mimic sunlight by artificial means. In order for them to do this reliably and consistently, international standards needed to be defined.
This was done in a set of workshops in 1975 and 1977 sponsored by ERDA and NASA where they published a report of standard terrestrial solar cell measurement procedures, including detailed descriptions of standard solar simulators. During these workshops, scientists averaged the observable sun intensity, as well as the spectrum, and reached an agreement on a baseline.
In this report, the standard solar irradiance was chosen to be 1000 watts per square meter (W/m2) and an air mass of AM1.5 Global was chosen as the spectral composition to represent sunlight on earth.
Don’t worry if you don’t yet know what an “air mass” is—we’ll get to that shortly.
Both of these standards (for intensity and spectral composition) were incorporated into ASTM standards (such as ASTM E927, ASTM G173-03, and ASTM E490) as well as many other international standards (such as IEC 60904-9 and JIC C8912). These standards validate and qualify solar simulators, and define a common ground around sunlight.
The answer to the questions of “What is sunlight” and “How much sun do we get” is that standard sunlight has an irradiance of 1000 W/m2 and a spectral distribution matching AM1.5G.
The standard AM1.5G spectrum can be seen below:
When you first read the term “air mass” you might have wondered what relevance the mass of air has with sunlight and the solar spectrum. You’re not alone! It’s a term that can definitely cause some confusion, but is very important for understanding the way different solar spectra are defined. It’s doubly confusing because there are similar definitions in the disciplines of meteorology and solar energy.
In meteorology, air mass is defined as a volume of air with a specified temperature and vapor content. Two similar air masses will therefore have gases and particles exhibiting similar chemical and spectral behaviour. This is not the definition that’s most useful in the discipline of solar energy.
In the solar energy field, air mass is better referred to as “air mass coefficient” and defines how much atmosphere there is between you and the Sun. As we discussed earlier, the atmosphere absorbs and scatters light, so how much of it is vital to knowing and quantifying the spectrum of light. The air mass coefficient tells you the relative distance (or path length) that light has to travel through the atmosphere before it gets to you.
While these two definitions share some similarities, it is important to recognize that they are not interchangeable. Therefore, when you’re in solar energy circles, the term “air mass” really refers to “air mass coefficient.” We’ll go through some specific examples below.
Earlier, we described how the sun’s emission spectrum can be closely approximated by a 5800 K black body. This emitted radiation travels 150 million kilometers and arrives at Earth’s orbit. Because it has traveled through the vacuum of space, there were no particles to absorb or scatter the light.
What is the air mass coefficient of such a spectrum? To answer that, we first must answer: how much atmosphere has the light traveled through?
The answer, in this case, is none—zero. There’s been no atmospheric transmission whatsoever.
This spectrum is what we call Air Mass 0 (AM0), indicating the sunlight has not interacted with any of the Earth’s atmosphere.
Next, we’ll discuss examples of spectra that have passed through the Earth’s atmosphere.
When we measure the solar spectrum on Earth, it is different from the 5800 K black body radiation of AM0 due to scattering (of blue light, for example) and absorption (of red light, for example) by molecules in the Earth’s atmosphere. Overall, the spectrum is going to be attenuated or diminished after passing through air. The more atmosphere sunlight passes through, the greater the light’s attenuation.
We’ve already discussed how air mass (or air mass coefficient) is the path length of a direct sunbeam through the atmosphere. Now we can introduce some definitions and specifics of how to calculate it.
The air mass coefficient defines “1” as the path length light travels through when the Sun is directly overhead at sea level. The spectrum of light after travelling through this path length of atmosphere is what we call Air Mass 1 or AM1. In this case, the sun’s direct radiation passes vertically through the atmosphere in the shortest possible path.
This point directly above a particular location is known as the zenith. expressed as a ratio relative to the sun at the zenith (a zenith is an imaginary point directly above a particular location) above a sea-level location. In this case, the sun’s direct radiation passes vertically through the atmosphere in the shortest possible path. This is known as Air Mass 1.
AM1 is the baseline from which the rest of the spectra can be defined. Air mass is expressed as a ratio relative to the sun at the zenith above a sea-level location.
If we have twice the path length as that of seaside sunlight zenith, then that spectrum is referred to as AM2. If we have four times the path length, that’s AM4, and so on.
These spectra refer to fractions of the standard path length when the sun is at its zenith shining on the sea. If you climb up a mountain until you are at half the atmosphere’s height, the light when the sun is directly overhead will be travelling through half the path length, and you’ll be exposed to the AM0.5 spectrum. If you fly a plane until you’re at 90% of the height of the atmosphere’s height, with only 10% remaining, then the plane will be exposed to AM0.1 (i.e. 10% of the path length of AM1).
Because we have topography and don’t spend all our time at sea level, we need spectral definitions for when the sun is directly overhead, and we’re above the sea. These are all the spectra from AM0.1 to AM0.9.
These spectra correspond to sunlight that has traveled through path lengths much longer than direct overhead sunlight at sea level (1.1 times, 1.5 times, 2 times, 5 times, and 40 times to be specific).
Your next question might be: when does light travel through that path length? For that, we’ll need to do some calculations, but the short answer is that as the sun changes its angle on the horizon, the AM spectrum changes.
Because the air mass coefficient is calculated as the ratio of the sun’s actual path length to that at the sun’s zenith, we can use some basic geometry to start seeing when AM1.1, AM1.5, etc. might actually apply.
The fundamental thing that’s changing when the air mass coefficient changes is the zenith angle; that is, the angle between the sun’s current position and a line directly overhead. The higher the zenith angle, the lower the sun is in the sky, and the more atmosphere sunlight has to travel through.
To know when a specific AM spectrum applies then is really a question of figuring out what the sun’s zenith angle for that AM spectrum really is.
Because most major population centers of the world (Europe, China, Japan, the United States, northern India, southern Africa and Australia) lie in mid-latitudes, an AM number that represents mid-latitudes is the most commonly used to characterize the performance of solar cells. AM1.5 atmosphere thickness represents a zenith angle of z=48.2°.
During the summer months, the AM number for mid-latitudes is less than 1.5, and higher figures apply during the morning and evening. Therefore, AM1.5 is a useful representation of the atmosphere thickness as a yearly average for mid-latitudes. This air mass of 1.5 was selected as the standard spectra in the 1970s for standardization purposes based on a solar radiance analysis in the United States.
The “G” in AM1.5G stands for “global,” and is the first of the suffixes that add more information and specificity to the spectra.
There are a few suffixes added to the AM-specified spectra, and they take into account different effects to maximize the specificity of a spectrum. This extra level of detail makes it easier for researchers around the world to compare their results to one another by ensuring that all aspects of sunlight behaviour are considered and called out.
The “D” in AM1.5D stands for directed, and corresponds to sunlight that directly shines on a point on Earth (i.e. with no reflections or scattering).
AM1.5G corresponds to a global spectrum that includes diffuse and scattered light. It also specifies that it’s considering light that will be received by a 37 degree tilted surface. The reason for the 37 degrees is that the standard was originally defined for solar cells (photovoltaic cells) which are commonly mounted at an angle.
Standard spectra include AM0, AM1.5G, AM1.5D. These are defined by ASTM E490, ASTM G173-03 and other standards bodies in an effort to provide standard test conditions so that experiments and results can be compared, and get a reasonable approximation for real-world performance. AM0 and AM1.5G are by far the most commonly-used test spectra.
The spectrum generated by sunlight at AM1 (at 0° from the zenith) to AM1.1 (at 25° from the zenith) is a useful range for estimating the performance of solar cells in equatorial and tropical regions.
Other AM values are used to approximate sunlight at regions other than mid-latitudes or at higher elevations. AM2 and 3 (z=60° and z=70° respectively) for example, are useful to determine the solar performance of some devices (e.g. solar cells) at higher latitudes such as those in northern Europe. An AM value of 40 is typically regarded as being the air mass value of the horizontal direction (z=90°) at the equator.
A solar simulator is a device whose light source offers similar intensity and spectral composition to natural sunlight. Solar simulators (also called “sunlight simulators”) are scientific equipment used to replicate sunlight in controlled laboratory environments. They are essential for research and testing of products and processes that either use or are affected by sunlight – like solar cells, solar fuels, sunscreens, plastics, coatings, and other photosensitive materials.
The major components of solar simulators are a light source and power supply, optics and filters used to modify the output beam, and the controls needed to operate the simulator.
There are two approaches to produce “artificial sunlight”:
Because there are differences in the way artificial light is generated and the way sunlight is generated, it is necessary to take additional steps to match both sunlight’s intensity and spectral composition.
There are three criteria used to evaluate the match between natural sunlight and a solar simulator’s output:
The main component of the solar simulator is the light-source. The most widely used light source is a short-arc and long-arc Xenon (Xe) lamp but other light sources are also used such as: metal halide (MH) arc lamps, quartz tungsten halogen (QTH) lamps and more recently light emitting diodes (LEDs).
The Xenon arc lamp has dominated the market for the past 40+ years because the Xenon bulb has a relatively continuous spectrum from 300 nm to 2000 nm.
However, when compared to LEDs, lamp-based systems generally have lower temporal stability, higher operating cost and a shorter lifetime.
LEDs can also be used as a light source and offer several advantages over other systems such as:
Recent advancements in LED technology has allowed this technology to push to a full sun spectral coverage from 300 nm to 1850 nm.
Read more about the different types of solar simulator light sources below.
Note: for Solar simulators, it is convenient to describe the irradiance of the simulator in terms of “suns.” One “sun” is equivalent to irradiance of one solar constant which is equal to 1000 W/m2 (1 sun=1000 W/m2).
These are typically dictated by the light source used. For example, arc lamp power supplies are typically highly complex devices that have to manage a high voltage ignition stage in order to establish the arc. For QTH lamps, a comparatively simpler DC source is required with a compatible power output. For LEDs, only a relatively simple power supply is required.
Optics & filters are often used, but aren’t actually necessary for all light sources. When using solar simulators, you are using an artificial light source to generate the spectra needed. However, there may be differences between artificial light sources and natural sunlight, both in intensity and spectral composition.
Filters and optics are used and can be modified to cause the output light to closely resemble the desired spectra.
The optical layout of a solar simulator varies considerably depending on many variables: light sources used, the area of illumination, the spectral output.
Usually, major concerns for the optical system of a solar simulation system, beyond the fact that it should be able to attain its required classification, is that it should be simple to use and maintain.
A collimator lens can be added to solar simulators to generate a collimated beam in which the electromagnetic radiation has parallel rays and as a result the light spreads minimally as it travels.
When a collimating lens is not used, the beam angle will be larger. Additionally, an integrator lens is also typically used in solar simulators to achieve beam uniformity.
Systems that do not use the aid of any lenses to homogenize the light typically require a minimum of two mirrors to change the beam direction, homogenize the light beam and collimate the light, which results in loss of light.
A lambertian back reflector can also be added to solar simulators to randomize the direction of the reflected light and provide a more uniform radiance.
The quality of a solar simulator is graded on the spatial uniformity across a defined illumination area, the temporal stability through the experiment, and the spectral match to the sun from a clearly defined reference spectrum.
Solar simulators can be divided into two general categories: continuous or steady-state and pulsed.
The continuous type is a form of light source in which illumination is continuous in time. This type of device is most commonly used for low intensity testing of one or up to several suns (1000 W/m2).
The second type is the pulsed system. This simulator has flashes/pulses with typical durations of milliseconds and very high intensities of up to several thousand suns are possible.
Indoor solar simulation started at the beginning of the 1960s with a series of research programs that were sponsored by NASA.
These programs were aimed at developing a ground test facility that can simulate the space environment for earth satellites and other spacecraft testing. The chamber for this testing was named “Space Environment Test Chamber” and in this chamber, a solar simulator was used to simulate the space solar radiation.
After testing and comparing, a mercury Xenon lamp was chosen as the best light source. Jet Propulsion Laboratory (JPL) NASA created a series of several large solar simulators to meet the increasing test requirement for space technology.
In 1986 the European Space Agency (ESA) created a solar simulator to meet the European space plan. The ESA solar simulator was larger and had a simpler optical set up than that of JPL. Later Russia, South Korea, and Japan constructed their own large-scale solar simulators.
These space solar simulators paved the way to much of the fundamental research and testing for solar simulation today.
Solar simulators are needed to have a common basis for comparing solar devices and for the design of large arrays. This created an industry need for testing solar cells and other devices under control conditions and thus a need for accessible solar simulators.
Several companies such as Hoffman Electronics Corp. (using a combination of Xenon arc lamps and tungsten bulbs), Optical Coating Laboratory (modifying light sources via optical filters) along with Spectrolab X25 were some of the pioneers of standard solar simulator design and manufacturing.
Then, in the 1970s due to the development of the photovoltaic industry, having a standard measuring method became a necessity. Early in the development of photovoltaic (PV) cells, the performance was tested with light sources that needed to be continually calibrated by using expensive large-scale space simulator chambers. A standard was important in order to determine the performance of samples from a single source, to compare samples of different designs, to study changes in device performance as a function of time, and to provide system design data to engineers and marketing. Because of this, the first solar cell standard procedure was set in 1975 and updated in 1977.
In 1978 the Subcommittee on Photovoltaic Electric Power Systems of ASTM Committee E-44 started developing standard methods for measuring the electrical performance of photovoltaic devices. In 1985 a series of revised ASTM standards were finally available.
Solar simulators are fundamental for photovoltaic measurements being done in both research and industry, and since the illuminated current vs. voltage (I-V) is sensitive to the spectrum, intensity, and temperature, looking for new light sources and developing higher accuracy optical systems based on the leading standards became a priority.
Additionally, standard PV solar simulators had issues with lowering the average power and temperature fluctuations (caused by prolonged exposure to the light). As solar cell manufacturers started to ramp up manufacturing, large area simulators that were capable of testing modules were developed. To minimize power use and excessive heat generation, the illumination times were reduced, decreasing the measurement time. As a result of trying to solve this issue, the pulsed solar simulators were designed and developed. In parallel, additional solar simulator development continued, including the development of multisource simulators for improved accuracy, high-intensity simulators for multijunction and concentrator systems, and LED systems. With the development of high power LED technology in the 1990s, solar simulators were developed to use this new light source, which offered advantages such as tunable spectra, high accuracy, long operating life, and output control of the light source with a 30- 50 nm resolution. LEDs consume less energy, pack much smaller than conventional lamp-based housing and LEDs can be controlled within microseconds or operated stably at one light output intensity continuously for a long time.
Whether using an artificial illumination source for photovoltaic characterization, photochemistry experiments, or environmental testing, correct maintenance and use of your light source will improve your experimental results. When searching for a solar simulator you will continuously come across the term “Class AAA” rating – but what does it mean? If you’re not quite sure what that is, and how it applies to your research, we have the answer.
The standards are set by governing bodies such as ASTM, IEC, and JIS and are used to determine the quality and accuracy of a solar simulator device’s illumination. The specific standards that govern solar simulation are JIS-C8912, IEC 60904-9, and ASTM-E927-10.
Some classes you may see range from Class C to Class AAA, depending on the solar simulator – which may lead one to think that a Class AAA simulator is better than a Class A simulator.
In fact, “Class AAA” is a shorthand for three different parameters of the standard, meaning Class A Spectral Match, Class A Spatial Uniformity, and Class A Temporal Stability.
While the ASTM, IEC, and JIS standards vary slightly, the overlapping minimum requirements for each Class are tabulated below:
Based on the value of spectral match obtained from the equation, the solar simulator may be classified as class A, B or C for spectral match.
The Class A spectral match requires a factor of 0.75 – 1.25 between the artificial light source and the standard spectra in each wavelength region.
If your research focuses on improving solar cell efficiency for energy production, you can improve the link between your results and real-world application by utilizing the spectral match provided by a Class AAA solar simulator.
Spatial uniformity describes the distribution and consistency of irradiance over an area. The parameter is reported as “spatial non-uniformity” and is calculated from the difference of the maximum and minimum irradiance values in an area.
Spatial uniformity is crucial to ensuring an even distribution of light across an entire experimental area. The Class A should have a spatial non-uniformity of less than 2%. Solar simulators may have a total illumination area that is much larger than the Class A uniformity area, so it’s important that the high-uniformity area can cover your samples.
In order to quantify spatial non-uniformity of a solar simulator the irradiance of the simulator beam over the test area is mapped. The test area is divided into a grid of measurement positions and the following equation that is found on the standards is applied to calculate the non-uniformity in percentage terms.
The number of measurement positions depends on the size of the test area and the standard being used.
Temporal Stability is the consistency of light output over a period of time.
Traditional solar simulators (e.g., Xenon, metal halide, or tungsten bulb-based lamps) are known for changing spectra and drifting intensity over time, whereas more advanced LED technology has substantially better stability and longer lifetime.
Another thing to take into account is the fact that different light sources have different temporal stability at different time scales (minute vs sub-second time scales). For example:
Whether performing experiments that require several months of continuous light or when testing multiple devices consecutively, you would require the lowest temporal instability in order to gather accurate data on the performance of your devices.
Two parameters are used for measuring temporal stability; short term instability (STI) and long term instability (LTI).
STI relates to the data sampling time of a data set (irradiance, current, voltage) during a current-voltage (I-V) measurement. This value of temporal instability may be different between data sets on the I-V curve. STI is determined by the worst case in this situation. For batch testing of modules or solar cells that during I-V measurement have no irradiance monitoring, the STI is related to the time period (minutes to hours) between irradiance determination.
LTI is related to the time period of interest. For I-V measurements this refers to the time for measuring the entire I-V curve. For irradiation exposure tests it is related to the time period (hours to months) of exposure. The following equation is used when determining temporal instability of irradiance (in percent):
When it comes to choosing the right solar simulator for your research, the Class AAA rating is necessary, but depending on your experiments, it may not be sufficient. Some solar simulators offer performances that surpass Class AAA rating in spectral mismatch (smaller range than 0.75-1.25), spectral non-uniformity (less than 2) and temporal instability (less than 0.5 for short temp and less that 2 for long term). It’s important to consider how the limitations in the classifications may impact your results, especially if your research is sensitive to changes in time or responds to different spectral ranges
Solar simulators can be broadly classified into two general categories: pulsed or flashed and continuous or steady state
A steady-state solar simulator has a light source that is continuous over time. Most of the specifications from the standards directly apply to this type of solar simulator. Steady-state tends to be used for smaller areas and is most commonly used in low-intensity testing. They are usually able to generate between 1 sun (1sun=1000 W/m2) up to several suns. Steady-state solar simulators can have several different types of lamps to be able to extend the spectrum in the far IR.
Pulsed and flashed solar simulators, unlike the steady state, do not have continuous light source over time. This type of solar simulator was invented to prevent heat build up in the tested device generated by lamp light source. Not having continuous illumination can be achieved by either having a light source that is turned on and off (flash solar simulators) or by using a shutter to block the light.
The first method involves a flash of illumination that lasts several milliseconds. Using this method each flash may be able to generate very high intensities of up to several suns. The main issue that sometimes arises from this type of simulator is that it is technically challenging to obtain reproducible intensities and spectra from one flash to the other.
Because the light source is not continuously on, temporal stability does not apply directly to this simulator but we can measure reproducibility by comparing one flash to another.
Alternatively, instead of having the light source turned on and off, a shutter can be used to quickly block or unblock the light from a continuous source. Typically, these pulses range from 100 milliseconds (ms) and up to 800 ms for special Xe Long Pulse Systems.
Solar simulators can also differ in spectra and irradiance distribution. Solar simulators can use many different types of lamps as a light source or a combination of lamps. The type of lamp determines the spectral power distribution and can be modified using optical filters. The optics determine efficiency and irradiance geometry.
Solar simulators can be classified as A, B, or C based on the spatial uniformity across a defined illumination area, the temporal stability through the experiment, and the spectral match to the sun from a clearly defined reference spectrum.
Below is a description of the most common types of light sources used in a solar simulators: xenon arc lamp, metal halide arc lamp, quartz tungsten halogen lamp and LEDs.
Xenon arc lamps are the most widely used light source for solar simulators. The main reason for this is that it provides a stable spectral that matches reasonably well to sunlight even unfiltered.
Xenon arc lamps have a spectrum almost identical to that of a sun at 5800 K. It does generate strong emission lines in the infrared from 800 nm -1000 nm but these can be eliminated using optical filters.
An additional advantage that the xenon arc lamp has is that when there are variations in power, this does not result in any significant shift in the spectral balance. Also, high-pressure short arc lamps are able to produce high intensity light beams. However, there are several disadvantages with a xenon arc lamp when used in solar simulators. Below is a summary of pros and cons:
The metal halide arc lamp is an arc source utilizing a mercury vapor arc with metal halide additives and it produces illumination that matches the spectra typically for temperatures in the range of 5000 K to 6000 K.
It was introduced as the light source for solar simulators when the compact source iodide was developed. Below is a summary of pros and cons:
Commonly referred to as QTH, Quartz Tungsten Halogen is an incandescent lamp that consists of a tungsten filament in a sealed transparent container that has a mixture of an inert gas and a halogen.
They are more commonly used in multi-source solar simulators because the tungsten filament can only reach temperatures of less than 3400 K which is lower than the 5800 K of the surface of the sun. As a result, they radiate weaker in the shorter wavelengths (blue and UV portion) but stronger in the infrared portion.
Below is a summary of pros and cons:
LEDs are solid-state devices that don’t require the maintenance or have the hazards related to a pressurized lamp.
When used in solar simulators, they are able to provide a more dynamic functionality solar simulator that meets the experimental needs for solar energy.
In the past, the main issue was the light intensity insufficiency when fully LED solar generators were designed but the issue was later solved when researchers were able to develop high power LED technology. In 2012 researchers from the University of Illinois at Urbana-Champaign (UIUC) presented a fully LED solar simulator design, which covers the AM1.5G solar spectrum and achieves Class C uniformity over an area of 100 mm × 50 mm.
Currently, LED based solar simulators are able to achieve any AM spectra to precision well in excess of that required by the standards.
LED as a light source has been shown to have several advantages when compared to traditional light sources such as lower cost, they are more compact, and typically consumes less power. Below is a summary of pros and cons:
For the spectra generated by the xenon and the metal halide lamp, we get a very wide bandwidth ranging from 200 nm to well past 2500 nm. Additionally, the xenon and the metal halide lamp spectra show large spikes which are inconsistent with the target spectrum (these spikes usually have no significant impact on solar cell characterization).
On the other hand, for the LED system we see a narrow emission bands, tuned to match the 400 nm to 1100 nm range specified in the ASTM standard and no spikes are observed.
Variable spectrum, multi-channel, programmable spectra, dynamic spectra – All terms found while searching for a solar simulator, but what exactly do they mean?
Most solar simulators on the market today ship with what is called a “fixed” spectrum, typically with a AM1.5G match. This means that the intensity of the light can be dimmed, however the spectral profile cannot be changed.
With a variable spectrum module, both the spectrum and intensity the light emits can be adjusted and changed, making a much more versatile instrument!
For many applications the AM1.5G spectra doesn’t make sense. In these cases, it is essential to have the ability to characterize at a different spectrum.
If the research being conducted is not done for mid-latitudes applications, a solar spectrum representative of other air masses may be needed. For example, in the development of photovoltaic materials for use in orbital satellites, you may need a spectrum that represents that of the sun in outer space. Therefore, an AM0 spectrum should be used in order to perform an accurate test.
Or when developing photocatalysis systems, they should be just as functional in the northern hemisphere during the winter months, as at the equator in the summer months. So you may need to run tests using AM1 for the equator, AM 1.5G for the mid-latitude and AM2 or AM3 for high latitudes such as those in northern Europe.
With programmable spectra, this is possible. Traditionally, a lab would require individual instruments or different filters in order to replicate AM1.5G, AM0, AM1, AM2 etc. Now with variable spectrum technology, a single piece of research equipment can reliably reproduce virtually any naturally occurring spectra (and a large range outside of natural conditions as well). This decreases overall expenses, all while improving research.
A solar simulator, simply put, is a source of photons. Therefore if you need photons to conduct your research then you will need a stable light source in order to have reliable and reproducible data.
Photons are the elementary particle of light and exhibit properties of both waves and of particles.
Photons have different energies depending on wavelength; the shorter the wavelength the higher the energy. When they interact with matter, depending on the energy of the photon, a chemical reaction may take place if the photon is absorbed by a molecule or atom – this is known as photochemistry.
In nature, photochemistry is responsible for some of the reactions that are fundamental for life such as; photosynthesis, formation of vitamin D and our ability to see.
Because photochemical reactions happen through a different pathway than those that are driven by temperature (they are usually able to proceed through high energy intermediates which overcome high activation energies in a short period of time), they are typically faster.
On the flip side, because light is so efficient at driving some chemical reactions, it can also be destructive – causing photodegradation of materials like plastics, organic molecules, DNA, etc.
Solar simulators are commonly used for testing solar cells, sunscreens, plastics, sunlight sensitive devices and other materials in a controlled laboratory setting. A solar simulator is useful in experiments that require a stable and reliable light source.
You can use a solar simulator if you need to study any type of phenomenon that is driven by light (photons), for example in:
To study the conversion of light into energy by testing solar panel performance in different regions, new potential materials, etc.
The acceleration of a chemical reaction by using a material which absorbs light as the catalyst.
To test the performance of sunscreens overtime, determine if they are broad spectrum or determine the SPF (Sun Protection Factor).
* Or to use solar simulators in the testing of any of the thousands of other light driven reactions. *
Solar cells (also known as a photovoltaic system) convert the energy from light (usually solar energy) to electric energy by a process known as the photovoltaic effect. They can be made from a single crystal (e.g. perovskite solar cells), crystalline (e.g. silicon) or amorphous material (e.g. amorphous silicon). Solar cells work by absorbing sunlight which generates either electron-hole pairs or excitons. Then, charge carriers of opposite types are separated and extracted into an external circuit generating electricity and thus renewable energy.
When working with solar cells, the best measure for its performance is to calculate its efficiency.
Solar panels are a collection of solar cells. Most commercially available solar panels have at least a 10% efficiency but significantly higher efficiencies are currently attainable as published by the NREL best research-cell efficiencies. In order to accurately calculate the efficiency of solar cells, you need to use a Class AAA solar simulator.
Additionally you need to perform standardized testing that allows the comparison of devices manufactured at different companies and laboratories with different technologies to be compared.
Measuring solar cells requires a stable light source that closely matches the conditions of sunlight. You need to match both the intensity and the spectrum to the AM1.5G standard. Your first idea might be to simply use the sun itself. But as previously stated in the section there are variations in atmospheric conditions and the solar spectrum changes throughout the day and year which requires correction to compare results accurately.
The solution is to use a sun simulator that provides
These requirements are essential in obtaining an accuracy of better than 2%.
Photoelectrochemistry is a field of science that studies the interaction of light with electrochemical systems. One of the main fields of photoelectrochemistry is semiconductors which are commonly used in photovoltaic devices. One of the uses of semiconductors is create artificial photosynthesis (photogeneration cells).
Solar simulators are very useful in many fields of photochemistry. For example, they are essential when studying sunscreens. Experiments need to be conducted to determine the Sun Protection Factor (SPF) of a given sunscreen. One technique is based on calculations according to Cosmetics Europe (COLIPA) protocols, which measures the transmittance of a given sunscreen sample across the UV spectrum weighted against the erythemal effect and solar intensity at each wavelength. This is done by first taking the pre-irradiated UV-vis absorbance of the sample and then using the equation below to calculate the SPF value.
where EE is the erythemal effect spectrum, I is the solar intensity spectrum, T is the sunscreen transmittance, Abs is the sunscreen absorbance, and CF = 10 so that a standard 8% homosalate sunscreen would calculate to an SPF of 4. The normalized values of EE*I, which are constants measured in 5 nm increments, define a sunscreen which transmits all light as having an SPF of 1.0, while one that absorbs all light as having an SPF of infinity.
Solar simulators are increasingly useful in testing visible light photocatalysis.
Photocatalysis describes chemical transformations that require light as an energy input to proceed.
To test visible light activated photocatalysis, you need to have a stable light source of visible light in order to get reliable and reproducible data and to be able to accurately assess and compare the performance of the photocatalytic reaction, study the reaction pathway, accurately calculate yield among other variables.
A solar simulator is a perfect instrument to be able to provide light source for photocatalytical studies.
Solar simulators are widely used in environmental testing. For example, in the study of environmental aquatic systems, a wide variety of photochemical reactions with active chromophores occur in natural waters when illuminated by wavelengths in the solar spectrum.
In order to study aquatic systems (eg. photochemical degradation of organic chemicals in aquatic media) or to test the photochemical degradation of different pollutants, a stable light source that mimics natural sunlight is needed.
Another use of solar simulators in environmental science is for testing photocatalysts.
A photocatalyst increases the rate of a chemical reaction without itself undergoing any permanent chemical change by using a light.
Most of the substances used as photocatalysts are semiconductors. Semiconductor photocatalysts tend to be heterogeneous catalysts (a type of catalysis in which the catalyst occupies a different phase from the reactants and products making them very useful in the treatment of organic contaminants in water and air.
The most common material used for this purpose is TiO2 although other types can also be used.
To test the performance of photocatalysts that are meant for environmental purposes, you need to use a light source that mimics sunlight which a solar simulator provides.
Furthermore, it is advantageous to have a solar simulator with variable spectrum capability so that you can test photocatalyst performance in environments that represent different places in the world.
Light is a fundamental component of life on our planet. Photobiology is a large discipline that includes studies of both the beneficial and harmful effects of light. It covers topics from the atomic level to that of ecological communities.
Biologists need to be able to simulate natural light to study how sunlight interacts with living organisms.
For example, studying photosynthesis is extremely important because of our quality of life (better food sources, CO2 conversion to Oxygen) and our very existence, depends on it.
Photosynthesis research can show us how to produce new crop strains that will make much better use of the sunlight they absorb, develop new herbicides by disrupting photosynthesis, etc.
Many photosynthesis studies require a controlled laboratory environment, including a solar simulator. Solar simulators are also used in other areas of photobiology such as to study vision, biological effects of ultraviolet radiation, circadian rhythms, and bioluminescence.
Additionally, another important use of solar simulators is to conduct photostability studies.
Stability testing is a key aspect while formulating any pharmaceutical product. The photostability studies are conducted with the main objective of ensuring that light exposure does not lead to dangerous changes in the dosage of the active ingredient.
In 1996, the Food and Drug Administration (FDA) issued the ICH Harmonized Tripartite Guideline on Stability Testing of New Drug Substances and Products for industry and notes that light testing should be an integral part of stress testing.
The stress testing requires a light source capable of producing an output similar to the D65/ID65 emission standard (i.e. artificial daylight that is designed to produce an output similar to the D65/ID65 emission standard).
Solar simulators are commonly used in many types of phototherapy. Phototherapy is used to treat medical conditions using light. One of the major uses of phototherapy is in the treatment of skin disorders such as eczema, psoriasis, vitiligo, itchy skin and T-cell lymphoma.
Phototherapy typically involves using light to reduce cell growth and skin inflammation. It can also be used to study sleep disorders, and in cancer treatments such as Photodynamic Therapy (PDT).
PDT is a treatment that uses special drugs, called photosensitizing agents, along with light to kill cancer cells. The drugs only work after they have been activated or “turned on” by light. Solar simulators are useful in studying and characterizing potential drugs that can be used as a photosensitizer for photodynamic therapy.
Additionally, solar simulators can be used with the intent of using UV exposure to induce cancers in mice and other biological test subjects, to provide an experimental foundation for testing other things such as skin lotions, chemotherapeutic drugs, and more.
Solar simulators were initially created to help simulate the space environment for earth satellites and another spacecraft testing, and they are still being used for this purpose.
The environment in space is quite different. Because there is no atmosphere, the solar spectrum is different (AM0). A satellite orbiting the earth undergoes rapid day-night cycles and is in direct sunlight for 50% or more of its operational lifetime. The intensity for AM0 is 1366 W/m2, much brighter than we see on Earth. Because of the extreme cost of launching something to orbit, there is a need for ground-based sun simulators that can mimic the optical environment of orbit to adequately perform testing before launching something into space.
Multijunction solar cells use a combination of semiconductor materials to capture and convert a large range of photon energies.
While multijunction solar cells are extremely expensive, they have much higher performance in terms of W/kg. When balanced against the launch costs, the additional expense can be justified, provided that they can be optimized in advance. Consequently, solar simulators that are optimized for AM0 are required to test space solar cell performance.
In addition, solar simulators that provide the AM0 spectrum are used to improve the understanding of chemical evolution in organic-rich astrophysical environments (comets, meteorites, Titan, interstellar medium), and where organic matter is being looked for (Martian surface and subsurface).
Solar ultraviolet photons are a major source of energy to initiate chemical reactions in the solar system, and many experimental programs on Earth are devoted to studies of the evolution of organic molecules through such chemical reactions.
How you choose a solar simulator will depend on what you are going to use it for. First, you need to decide if you want a steady state (continuously on) solar simulator or a pulsed solar simulator.
Another factor that you need to take into account is the spectral match. Depending on which experiment and how exact you want your data to be, you may also need a solar simulator that very closely matches the sun spectrum.
Class A (±25%) has the closest match to the sun’s spectrum, followed by Class B (±40%) and then followed by Class C (+100/-60%).
Typically the closer it matches the spectra the more expensive the solar simulator would be. If no tolerance for spectrum match is needed then a lamp may be more appropriate.
Intensity of the light beam is another factor that needs to be taken into account. The international standards define the intensity of the light for a solar simulator with an AM1.5G filter at 1,000W/m2 – which is called 1 sun. Unless your experimental needs require testing under more than one sun, the excess wattage will result in more than one sun in small targets. The best way to select the power of a solar simulator is to determine the largest area you will be testing and then determine how many suns you want those areas to have (typically since the point is to mimic the sun, the beam should mimic 1 sun).
Non-uniformity refers to the uniformity of the light beam on the illuminated area.
The international standards specify the non-uniformity for each class; Class A (≤2% for area of illumination equal to or less than 30cm x 30cm, or a diameter of 30cm and ≤3% for area of illumination is greater than 30cm x 30cm, or a diameter of 30cm), Class B (≤5%) and Class C (≤10%).
If your experiment requires a very uniform beam over the illuminated area you need to choose a solar simulator that meets Class A requirement. However, if this is not an important factor in your experimental setup (for example a stirring solution) then a Class B or C or even a lamp may be sufficient.
Temporal stability measures how stable the irradiation beam is over time. International standards define a Class A Temporal Stability (≤2%), Class B (≤5%) and Class C (≤10%).
Solar radiation is very stable, so if you need to closely mimic the sun then you need to choose a Class A simulator. However, if changes on the irradiation beam will not affect your experiment (for example a Qualitative measurement) a lower Class may be used.
A shutter is a very convenient component of a solar simulator. If a shutter is not present in a solar simulator, the alternative is to turn the lamp on and off. However, depending on your light source turning the system on and off may reduce its lifetime. So an aspect to consider is whether the shutter is automated or manual. This is a very important consideration depending on the type of research that will be conducted. For example, when testing new photocatalyst performance where throughput is not an issue, manual control may be sufficient. On the other hand, if you will be testing many samples or want to have more flexibility, an automated shutter may be the right choice.
Finally, another component that you may need to have in a solar simulator is the light intensity feedback. This will give you the ability to monitor the light intensity output. This is especially important when there are light power supply fluctuations. Additionally, this gives you the ability to normalize your IV measurements to account for power fluctuations.
The main objective of solar simulation is to produce illumination approximating natural sunlight in order to provide a controllable indoor test facility under laboratory conditions.
With an increasing desire to use renewable energy and a growing concern on how to protect our skin and belongings from sun; research around sunlight has become increasingly important. So if you are studying photovoltaic solar panels to harness the sun’s energy or you are developing better sunscreens to protect us from the sun or you are doing any other type of research that involves sunlight – acquiring advanced solar simulation technology for precision measurement is a must.
Not if you want reliable reproducible measurements. A lamp (of any type), will give irradiation that, without filters, will not reproduce the sun spectrum. Additionally, if you do not have a power supply that is stable you will have variations of irradiance intensity over time. If you need to conduct experiments that require a light source that closely mimics the sun, you need to use a solar simulator.
It is a measure of the amount of light produced within specific wavelength bands compared to the standard spectra.
It depends on the lighting technology. Under the Class AAA designation, ultraviolet light between 300 nm and 400 nm is not specified. For Xenon bulbs there will be a small amount of UV, where as for LED solar simulators the UV can be omitted entirely.
Is the air mass spectrum standard at mid-latitudes.
No, there is no need to have any special environment unless your specific research requires it (for example a glove box).
Yes, you can use a solar simulator to conduct any type of research that needs a light source that mimics the sun. Additionally, with dynamic spectra technology, you can match the spectrum to your desired environment.
Whether you need a pulsed or a steady state simulator, solar simulators should be chosen according to your needs (see the How to Choose section)
The short answer is that the sun has an illuminance of about 100k lux (lumens per square meter) on a perpendicular surface at sea level.
As we discussed earlier, though, the unit of lumens is a photometric one, which only considers wavelengths of light visible by the human eye. So this measure is really missing out on a lot of the radiation the sun is actually emitting. As far as the solar industry is concerned, radiometric units are preferred (for example, radiant flux in W/m2 instead of luminosity in lumens).
For more details on the difference between radiometric and photometric units, and what these units mean, please see above section.
The short answer is that “spectra” is the plural form of “spectrum.”
The word spectrum originates from the latin word for “image” or “apparition.” Because of its latin origin, the plural of spectrum is “spectra.”
However, as all languages morph over time, and since spectrum as a word has been adopted into the English language, the use of “spectrums” as a plural form has gradually become more common and widely accepted.