Perfect Artificial Sunlight - Guidance for Broad Applications of Solar Simulators
What is the solar spectrum?
Sunlight is a portion of the electromagnetic radiation given off by the Sun. The spectrum of the Sun’s solar radiation is close to that of a black body with a temperature of about 5,800 K. 99.9% of the energy is concentrated in the infrared, visible and ultraviolet regions.
The solar spectrum is divided into five regions in order of wavelength (Reference: Naylor, Mark; Kevin C. Farmer. Sun damage and prevention; http://telemedicine.org/sundam/sundam2.4.1.html):
- UVC (Ultraviolet C): which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to the human eye). Due to absorption by the atmosphere very little reaches Earth’s surface. This spectrum of radiation has germicidal properties, as used in germicidal lamps.
- UVB (Ultraviolet B): UVB range spans 280 to 315 nm. It is also greatly absorbed by the Earth’s atmosphere, and along with UVC causes the photochemical reaction leading to the production of the ozone layer. It directly damages DNA and causes sunburn. In addition to this short-term effect, it enhances skin ageing and significantly promotes the development of skin cancer but is also required for vitamin D synthesis in the skin of mammals.
- UVA (Ultraviolet A): UVA spans 315 to 400 nm. This band has less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths and tanning beds) and PUVA therapy for psoriasis.
- Visible: Visible light spans 400 to 760 nm. This range is visible to the naked eye. It is also the strongest output range of the Sun’s total irradiance spectrum.
- Infrared: Infrared spans 700 nm to 1 mm. It comprises an important part of the electromagnetic radiation that reaches Earth. Scientists divide the infrared range into three types based on wavelength:
Infrared – A：760 nm to 1,400 nm
Infrared – B：1,400 nm to 3,000 nm
Infrared – C：3,000 nm to 1 mm
The solar spectrum varies along with the factors of time, atmospheric thickness, and cloud thickness. Scattering from the sky and reflecting from the surrounding will also produce a lot of diffuse light.
Figure 1. Solar irradiance spectrum above atmosphere (yellow) and at earth surface (red) (from Wikipedia: https://en.wikipedia.org/wiki/Sunlight).
What is Air Mass? How does Air Mass affect sunlight?
Air Mass (AM)：Air mass is defined as the degree of influence of the atmosphere on the amount of sunlight received by the Earth’s surface. Different Air Mass also represents different solar spectrum. The elevation angle of the sun’s light changes with time. When the sun is at different elevation angles, the light will pass through the atmosphere of different thicknesses, which is Air Mass.
Formula of Air Mass (AM): AM＝ 1/cosθ， θ is the angle between the sun and the ground.
Figure 2. Different air mass represents different solar spectrums.
AM0: The spectrum outside the atmosphere, approximated by the 5,800 K black body, is referred to as “AM0”, meaning “zero atmospheres”. Solar cells used for space power applications, like those on communications satellites, are generally characterized using AM0.
AM1: The spectrum after travelling through the atmosphere to sea level with the sun directly overhead is referred to, by definition, as “AM1.” This means “one atmosphere.”
AM1.5: 1.5 atmosphere thickness, corresponds to a solar zenith angle of θ=48.2°. The illuminance for daylight under A.M.1.5 is given as 109,870 lux (corresponding with the A.M. 1.5 spectrum to 1000.4 W/m2).
AM 1.5D: The direct radiation of the test plane when sunlight passes through 1.5 times thickness of the atmosphere at an angle of θ=48.2°.
AM 1.5G: The global radiation of the test plane when sunlight passes through 1.5 times thickness of the atmosphere at an angle of θ=48.2°. The sum of the direct and diffuse radiation.
The difference between AM 1.5G and AM1.5D: Solar radiation refers to the electromagnetic waves and particle streams emitted by the sun into space. The solar radiation energy received by earth is only one-twentieth billionth of the total solar radiation. Common solar radiation nouns are:
- Direct: When sunlight passes through the atmosphere, part of it reaches the ground without changing its radiation direction.
- Diffuse: Another portion of solar radiation that changes direction after being reflected and scattered by the atmosphere.
- Global: The sum of the direct and diffuse solar radiation reaching the ground after the weakening of the atmosphere.
The relationship between Direct/Diffuse/Global is as the formula：Global= Direct * cos(θ)+Diffuse
Since solar cells are mainly affected by light, the power generation of solar cells are significantly affected in different regions and at different times. In order to consistently evaluate the power generation of solar cells, the international standard of test conditions are defined: AM 1.5, 1000W/m2, 25 °C (AM=1/cosθ).
Solar Constant: The solar constant is a flux density measuring mean solar electromagnetic radiation (total solar irradiance) per unit area. It is measured on a surface perpendicular to the rays.
It is measured by satellite as being 1366 watts per square meter (AM 0 =1366 W/m2)
The cross-sectional area of the earth is 127,400,000 square kilometers, so the power received by the entire earth is 1.74×1017 watts.
Because there are often sunspots and other solar activities on the surface of the sun, the solar constant is not a physical constant; the variation range is about 1% in a year.
International standards for evaluating solar simulators and related parameters for calculating formula
AM Spectral Irradiance: When sunlight passes through the atmosphere, it will be affected by air particles and generate a lot of diffuse light. When the incident angle is different, the distance of sunlight passing through the atmosphere is different accordingly, and the degree of scattering is also different. ASTM (American Society for Testing and Materials) and IEC (International Electrotechnical Commission) have formulated the standard values of AM spectral irradiance as follows:
|AM||International Standard||Irradiance (W/m2)|
|AM 0||ASTM E490||1366.1|
|AM 1.5 G||ASTM G173||1000.4|
|AM 1.5 D||ASTM G173||900.1|
|AM 1.5 G||IEC 60904-3||1000|
IEC 60904-3 defines AM1.5G spectral irradiance data. It is commonly used for the measurement of ground–mounted PV modules or solar cells. There are two types of irradiance wavelength range: Restricted and Extended. Shown as below (Table 1 & Table 2):
Table 1 – Global reference solar spectral irradiance distribution given in IEC 60904-3 contribution of wavelength intervals to total irradiance in the restricted wavelength range 400 nm to 1100 nm:
|Percentage of total irradiance in the wavelength 400 nm to 1100 nm|
|Cumulative integrated irradiance
|1||400 to 500||18.4||18.4|
|2||500 to 600||19.9||38.3|
|3||600 to 700||18.4||56.7|
|4||700 to 800||14.9||71.6|
|5||800 to 900||12.5||84.1|
|6||900 to 1100||15.9||100.0|
Table 2 – Global reference solar spectral irradiance distribution given in IEC 60904-3 contribution of wavelength intervals to total irradiance in the extended wavelength range 300 nm to 1200 nm:
|Percentage of total irradiance in the wavelength 400 nm to 1100 nm|
|Cumulative integrated irradiance
|1||300 to 470||16.61||16.61|
|2||470 to 561||16.74||33.35|
|3||561 to 657||16.67||50.02|
|4||657 to 772||16.63||66.65|
|5||772 to 919||16.66||83.31|
|6||919 to 1200||16.69||100.00|
In addition to irradiance, IEC (International Electrotechnical Commission) has published IEC 60904-9, which defines the regulations for evaluating the solar simulators. Important parameters and descriptions are as follows:
- Solar simulator: equipment employing a light source with a spectral distribution similar to the natural sunlight used to evaluate characteristics of PV devices.
- Spatial non-uniformity of irradiance: Spatial non-uniformity of irradiance over the designed test area. Use an irradiance detector (ex: photodiode) to measure the irradiance at different positions on the specified irradiation surface (the measuring value is photocurrent), and then calculate it through the following formula:
Note: During the measurement, the step width of each movement of the detector should not be greater than one-fifth of the minimum dimension of the designed test area.
- Spectral match: spectral match of a solar simulator defined by the deviation from AM 1.5 reference spectral irradiance as laid down in IEC 60904-3. There are two types of the wavelength range: 400~1100 nm (Table 1) & 300~1200 nm (Table 2).
- Temporal instability: this is a stability index. To ensure the accuracy of solar cell efficiency measurement, the output beam of the solar simulator is required to maintain a stable illuminance. The calculation formula is as follows:
According to the different IV measurement systems, the instability can be divided into two categories:
- Short-term Instability (STI): When there are three separate data input lines that simultaneously store vales of irradiance, current and voltage, the temporal instability is Class A (if spectral classification is performed in the restricted wavelength range 400 nm to 1100 nm) or Class A+ (if spectral classification is performed in the extended wavelength range 300 nm to 1200 nm) for STI. *Note: The delay in simultaneous triggering of the three multiple channels is typically less than 10 ns.
- Long-term Instability (LTI): For a three channel I-V measurement (irradiance, current, voltage) with a pulsed or stead-state solar simulator, the value is the time for acquiring the I-V characteristic.
According to IEC 60904-9:2020, the solar simulators are classified below:
|Classification||Spectral match to all intervals specified in Table 1 or Table 2||Spatial non-uniformity of irradiance|
|Temporal Instability: Short term instability of irradiance, STI|
|Temporal Instability: Long term instability of irradiance, LTI
|A+ (*)||0.875 to 1.125||1||0.25||1|
|A||0.75 to 1.25||2||0.5||2|
|B||0.6 to 1.4||5||2||5|
|C||0.4 to 2.0||10||10||10|
Note*：Class A+ is defined for the wavelength range of 300~1200 nm only.
Fields of application of solar simulators
Solar simulators can be applied in a wide range of fields, including energy science, biotechnology, environmental engineering, etc.:
|Energy Science||Performance testing of solar cells or PV modules
New energy development (e.g.: water splitting)
|Biotechnology||Medical/Cosmetic research and application|
|Material Science||New material development and testing
|Architecture||Weathering Life Testing of Building Materials
Color study of coatings/paint
|Agriculture||Experiments and Tests of Agriculture, Forestry, Fisheries and Animal Husbandry|
|Environmental Engineering||Research of the interaction between humans and the environment|
How to choose a solar simulator?
|AM 0||Space application, Satellite||AM 0 Solar Simulator: SS-ZXR|
|AM 1.5||Photovoltaic Device Research||AM1.5G Solar Simulator: SS-X|
|Novel Photovoltaic Devices or Tandem Photovoltaic Device Research||AM1.5G Solar Simulator (Spectrum Adjustable): SS-PST100R|
|New energy development (e.g.: water splitting)|
|Xe Light Source: ALS-300-G2|
|Environmentally sensitive material development (Glovebox integration)||Customized Light Source: SS-XRC, SS-FZ5|
|Indoor||Research on Indoor Photovoltaic Devices||Ambient Light Simulator: ILS-30|