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 ra

diation 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 (sun spectrum, sunlight spectrum, spectrum of the sun) is divided into five regions in order of wavelength. (Reference: Naylor, Mark; Kevin C. Farmer. Sun damage and prevention):

1. UVC (Ultraviolet C): This spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at a higher frequency than violet light (and, hence, is 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.
2. UVB (Ultraviolet B): The 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.
3. UVA (Ultraviolet A): UVA spans 315 to 400 nm. This band is less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths and tanning beds) and PUVA therapy for psoriasis.
4. 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.
5. 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 factors of time, atmospheric thickness, and cloud thickness. Scattering from the sky and reflecting from the surroundings also produce a lot of diffuse light.

Figure 1. Solar irradiance spectrum above the atmosphere (yellow) and at the Earth’s surface (red)(from Wikipedia: https://en.wikipedia.org/wiki/Sunlight).

What is Air Mass in solar spectrum?

Air Mass (AM): Air mass is defined as the extent to which the atmosphere influences the amount of sunlight received by the Earth’s surface. Different Air Masses also represent different solar spectra. The elevation angle of the sun’s light changes over time. When the sun is at different elevation angles, the light will pass through varying thicknesses of the atmosphere, which is referred to as Air Mass.

How does Air Mass affect sunlight? AM0, AM1 and AM1.5 spectra

Figure 2. Different air mass represents different solar spectrums.

Formula of Air Mass (AM): AM = 1/cosθ, where θ is the angle between the sun and the ground.

• 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 communication 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).

Figure 3. AM0 and AM1.5G Reference Spectra

Table 1: ASTM Percentage of Total Irradiance for three standard spectra

What is AM1.5D and AM1.5G spectra?

• AM 1.5D:

The direct radiation of the test plane occurs when sunlight passes through 1.5 times the thickness of the atmosphere at an angle of θ=48.2°.

• AM 1.5G:

The global radiation of the test plane occurs when sunlight passes through 1.5 times the thickness of the atmosphere at an angle of θ=48.2°. This is the sum of the direct and diffuse radiation.

What is the difference between AM1.5G and AM1.5D spectra?

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 terms are:

• Direct: When sunlight passes through the atmosphere, part of it reaches the ground without changing its direction of radiation.
• Diffuse: Another portion of solar radiation 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 atmosphere weakens it.

The relationship between Direct, Diffuse, and Global is as follows: Global = Direct * cos(θ) + Diffuse.

What is the irradiance intensity of AM1.5G and AM1.5D?

The irradiance intensity of AM1.5G and AM1.5D spectra is calculated with SMARTS v 2.9.2, with inputs chosen per international standard IEC 60904-3-Ed2. In PV and solar cell IV testing applications, AM1.5G is the standard solar spectrum for terrestrial PV testing. The irradiance intensity is 1000 W/m² under one sun condition. For concentrated PV devices, only the direct part of solar radiation can be collected and sensed by the solar cells. Therefore, the AM1.5D spectrum is usually used as the testing standard spectrum. The irradiance intensity of AM1.5D under one-sun condition is 900 W/m².

What is one sun intensity and Solar constant?

The solar constant is a flux density measurement of mean solar electromagnetic radiation (total solar irradiance) per unit area. It is measured on a surface perpendicular to the rays.

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.

It is measured by satellite as being 1366 watts per square meter (AM 0 =1366 W/m²). This intensity is also called AM0 one sun intensity, which represents the sun intensity under the AM0 spectrum.

What is solar spectrum and its range?

There are several international standards for evaluating solar simulation and its range.

When sunlight passes through the atmosphere, it is affected by air particles, generating a lot of diffuse light. The distance sunlight travels through the atmosphere varies with the incident angle, leading to different degrees of scattering. This is referred to as Air Mass (AM). The ASTM (American Society for Testing and Materials) and IEC (International Electrotechnical Commission) have formulated standard values for AM spectral irradiance, specifying the solar spectrum and its intensity range:

YOU CAN DOWNLOAD THE AM0, AM1.5G, AND AM1.5D STANDARD SPECTRA HERE:

Resources & Download

AM0 Spectrum

AM1.5D Spectrum

AM1.5G Spectrum

IEC 60904-3 defines AM1.5G spectral irradiance data, commonly used for the measurement of ground-mounted PV modules or solar cells. There are two types of irradiance wavelength ranges: Restricted and Extended, as shown below (Table 1 & Table 2):

Table 2 – The global reference solar spectral irradiance distribution given in IEC 60904-9 shows the contribution of wavelength intervals to total irradiance in the restricted wavelength range of 400 nm to 1100 nm:

Table 3 – The global reference solar spectral irradiance distribution, as given in IEC 60904-9, shows the contribution of wavelength intervals to the total irradiance in the extended wavelength range of 300 nm to 1200 nm.

You can also refer to: https://en.wikipedia.org/wiki/Solar_simulator

What is solar simulator?

According to IEC 60904-9, a solar simulator (solar simulation, sun simulator, sunlight simulator) is described as equipment that uses a light source with a spectral distribution similar to natural sunlight to evaluate the characteristics of PV devices. Therefore, the solar simulator is an artificial light source that can generate or simulate sunlight with a similar spectral distribution and light intensity.

Various types of solar simulators are commonly used to determine the current-voltage (I-V) characteristics of PV devices. Typically, these work as single-lamp systems with photovoltaic devices placed in designated test areas or as multi-lamp systems based on the superposition of light cones. Examples include:

1. Pulsed single-lamp or multi-lamp solar simulators: operate in a dark room. The distance between the light source and the photovoltaic device is usually several meters. Baffles are used to suppress internal reflections from walls.
2. The pulsed solar simulator: operates in an enclosure, and the distance between the light source and the photovoltaic device is usually less than one meter. Diffusers and reflectors are used to achieve a specified spatial uniformity of irradiance.
3. Steady-state single-lamp or multi-lamp solar simulators: operate in a dark room. The distance between the light source and the photovoltaic device is typically several meters.
4. LED-based multi-lamp solar simulators: operate when the distance between the light source and the PV device is typically less than 1 meter.

Pulsed solar simulators can be further subdivided into long-pulse systems that acquire the total I-V characteristic or part of the I-V characteristic during a flash, and systems that acquire one I-V data point per flash. Several lamp types are available in the multi-lamp solar simulator. These instruments are spectrally tunable and can superimpose different spectral irradiances emitted by different types of lamps. If available, in addition to ratings, reported test data should be referenced to assess the suitability of the solar simulator for a particular use or test purpose.

Multi-lamp systems can be further subdivided into systems in which each lamp illuminates the entire test area, and systems in which a single lamp illuminates only a portion of the test area.

In addition to light sources, lamp power supplies, and optics, I-V data acquisition, electronic loads, and operating software may also be part of the solar simulator. Requirements for related measurement techniques are contained in other parts of the IEC 60904 series.

What is the purpose of solar simulator?

What are solar simulators used for?

Solar simulators can be applied in a wide range of fields, including energy science, biotechnology, environmental engineering, etc. The following table shows the field and the applications of solar simulators.

How to evaluate the performance of a solar simulator?

According to IEC 90904-9, it defines the regulations for evaluating solar simulators. The important parameters and descriptions are as follows:

• Spatial non-uniformity of irradiance
• Spectral match of irradiance
• Intensity instability of irradiance

Spatial non-uniformity of irradiance: This refers to the spatial non-uniformity of irradiance over the designed test area. An irradiance detector (e.g., photodiode) is used to measure the irradiance at different positions on the specified irradiation surface (the measuring value is photocurrent), and then it is calculated using 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: The spectral match of a solar simulator is 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, instability can be divided into two categories:

Short-term Instability (STI): When there are three separate data input lines that simultaneously store values 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 steady-state solar simulator, the value is the time for acquiring the I-V characteristic.

What is the grade of a solar simulator?

A solar simulator may be one of four classes (A+, A, B, or C) for each of the three categories – spectral match, spatial non-uniformity, and temporal instability. Each simulator is rated with three letters in order of spectral match, non-uniformity of irradiance in the test plane, and temporal instability (for example: CBA, meaning a class C spectral match, a class B spatial non-uniformity, and a class A temporal instability).

The solar simulator classification should be periodically checked in order to prove that classification is maintained. For example, spectral irradiance may change with the operation time of the used lamp, or uniformity of irradiance may be influenced by the reflection conditions in the test chamber.

According to IEC 60904-9:2020, the solar simulators are classified below:

Note*：Class A+ is defined for the wavelength range of 300~1200 nm only.

What is AAA solar simulator?

An example of solar simulator classification for I-V measurement is shown in Table. The classification for non-uniformity of irradiance depends on size of interest.

As we can see, the classification result is A＋AA+, which represents that the spectral match is class A+, the non-uniformity of irradiance within 10 cm x 10cm is class A, and the temporal instability of irradiance is class A+.

When one mentions a AAA class solar simulator, it means that the solar simulator has a class A spectral match, class A non-uniformity of irradiance, and class A temporal instability of irradiance.

How to build a solar simulator?

Solar simulators are used to simulate solar irradiance and spectrum. A solar simulator typically consists of three main components:

(1) The light source and associated power supply

(2) Any optics and filters needed to modify the output beam to meet classification requirements

(3) The necessary controls to operate the simulator

Design of Solar Simulator

If you are interested in the design and components of Xe lamp solar simulator, you can check this article SS-X

How to measure the spectral match of a solar simulator?

The spectroradiometer should be appropriate for the spectral-match measurement task. It is necessary to ensure that the sensitivity of the sensor is suitable in the wavelength range of interest.

For the pulsed-type solar simulator, many cautions need to be paid attention to. The integration time of the spectroradiometer should be suitable for the pulse length of the simulator. The spectrum of the simulator might change during the light pulse. In the case of spectral shifts, differences in spectral responsivities between the irradiance monitor and the DUT will introduce spectral mismatch error. The integration time should be less than half of the pulse length.

According to the instructions of IEC 60904-9, the following features and parameters may determine the quality of spectral irradiance measurement:

• Wavelength resolution: The wavelength resolution of the spectroradiometer should be equal to or better than 5 nm in the visible range (300 nm to 900 nm) and 10 nm in the near-infrared range (900 nm to 1200 nm).
• NOTE: The wavelength resolution is a measure of the ability of the spectroradiometer to separate two spectral lines that are close together.
• Non-linearity of sensor element(s): Spectroradiometers are typically calibrated with a tungsten calibration lamp at a low irradiance level. However, the spectral intensity of solar simulators may differ considerably from calibration conditions.
• Stray light or second-order wavelength effects.
• Angular response of input optics: This parameter has a significant impact in the presence of diffuse light.

There are many available measurement techniques. However, the CCD spectroradiometer technique is the most common due to its compact size and excellent spectral performance. Spectral irradiance measurement with CCD spectroradiometers typically requires the use of two instruments to cover the relevant wavelength range (for example, with both a Si and InGaAs detector respectively). The Si CCD usually covers 300-1100nm, and InGaAs CCD can cover 900-1700nm. By combining these two instruments, and proper cosine-correction optics, the spectral distribution of a solar simulator can be measured appropriately.

The above irradiance spectrum is acquired by a Si and InGaAs combined spectroradiometer, which covers the 300-1200nm wavelength range. The spectrum is divided into six sections, and the spectral ratios are calculated according to IEC 60904-9:2020. The classification of each section is labeled. The final results show that the worst spectral match level of this solar simulator is about A class.

How to measure the Non-uniformity of irradiance of a solar simulator?

It is recommended to use encapsulated crystalline silicon cells (WPVS type) as irradiance detectors for determining the non-uniformity of irradiance in the test area of the simulator by measuring their short-circuit current. The time response of the irradiance detector should conform to the characteristics of the simulator being measured. The irradiance detector should have a spectral responsivity appropriate for the solar simulator and for the spectral responsivity of the PV device to be measured.

For the PV cell solar simulator, a 2 cm x 2 cm detector size should be defined. Care should be taken that multiple reflections between the output lens of the light source and the irradiance detector do not cause measurement errors. Change the positions of the irradiance detector and record the photocurrent signal of the detector. After finishing the 8 x 8 =64 matrix, calculate the spatial non-uniformity using the non-uniformity equation mentioned above. The class of the simulator for non-uniformity can be determined.

The 8×8 matrix is measured using a 2 cm x 2 cm encapsulated WPVS Si reference cell. The max and min intensities are labeled. The spatial non-uniformity is 1.39%, which is class A.