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How to Check Solar Simulator Calibration

How to Check Solar Simulator Calibration

It is important to ensure that your solar simulator is outputting a consistent spectral output. Different solar simulators will have different bulb lifetimes, and the spectrum delivered by a solar simulator may change over time - both in the short and long term. Therefore, you should check the calibration of your solar simulator periodically. This way you will be sure that your solar simulator continues to emit light that accurately replicates the solar spectrum.

Power vs Spectral Match

The accuracy of a solar simulator can be defined in terms of its spectral match or its irradiance. The average irradiance of the solar spectrum is 1000 W/m2 (100 mW/cm2 or 1 Sun), and most solar simulators try to meet at least this standard. The irradiance of a solar simulator can be easily measured using a standard silicon reference cell calibrated to 1 Sun.

However, there can be changes in the spectral distribution of your solar simulator over time, which you could miss using only irradiance measurements. Unfortunately, if you want to accurately measure spectral drift, you will need to check the output of your solar simulator with a highly calibrated spectrophotometer. You can perform an approximate reading using a spectrometer - but this will be a qualitative measurement rather than for direct calibration. For this reason, it is important to bear in mind the usable lifetime of your lamp. If you are using a lamp with a short lifetime, such as an arc lamp, make sure its performance is monitored and that the bulb is changed when necessary.

Initial Calibrations

The goal of any solar simulator is to accurately replicate the solar spectrum, so it is vital that the light produced by your solar simulator is thoroughly and accurately characterised before use. All solar simulators are classified according to well-defined calibration standards, such as the IEC 60904-9:2020 International Standard.

This initial calibration is the job of solar simulator manufacturers. The Ossila Solar Simulator lamp is calibrated for an 8.5 cm working distance using a broadband spectroradiometer. A spectroradiometer can measure the intensity, power and irradiance of light at different wavelengths and is calibrated so that it can be very accurate in these measurements. This measurement can give you the spectral power distribution (SPD curve) of a light source.

Every one of the Ossila Solar Simulator lamps is thoroughly calibrated before distribution to ensure that it meets IEC 60904-9:2020 calibration standards. We ensure that it meets AAA grade classification over an area of 15 mm area diameter and an ABA classification over a 25 mm area. You can therefore be assured that when you buy an Ossila Solar Simulator Lamp, it will come fully calibrated and ready to use.

Checking Spectral Output

The optimum distance between measurement surface and the solar simulator

While the manufacturer performs the initial calibration, you may wish to check the measurement of your solar simulator once you have it set-up in your laboratory. If you are using a solar simulator with a variable height stand, such as with the Ossila Manual Solar Cell Testing Kit, you will need to ensure your solar simulator is set up correctly to make sure there is optimum distance between the light source and the test system. The Ossila Solar Simulator needs to be 8.5 cm away from the surface of the device being tested. Any slight shift in this distance will affect the intensity of light on your sample and may influence your measured device metrics. 

Always keep in mind that the output of your solar simulator may change over time. Depending on what type of light source you are using, this change will happen at different rates. This fluctuation might happen over a short time scale (over the course of one of several measurements). An example of this short-term fluctuation is if the lamp exhibits spectral flicker. Alternatively, this change may happen over a longer period of time (over a period of weeks or months). For example, as arc lamps age, they exhibit an effect known as spectral drift.

LED light sources tend to have much longer lifetimes that their arc lamp alternatives, so this shouldn't be as much of an issue with the Ossila Solar Simulator. However, as mentioned before when using the variable height stand, you may wish to check the relative position hasn't changed over time - even millimetres of movement can make a difference.

Whatever light source you are using, it is good practice to check the calibration of your solar simulator regularly. The most common method for checking solar simulator calibration is using a calibrated reference solar cell, which outputs a standard value at 1 Sun. You can also use an external device such as a photodiode or light sensor to track the relative performance of your solar simulator over time. However, photodiodes/light sensors do not give you an objective measure of irradiance or spectral shape so you cannot use these measurements to compare the performance of different solar simulators.

Reference Solar Cell

The most common way to check the output spectrum of your solar simulator is using a reference cell. This is usually a stable device of known photovoltaic performance. This reference device should ideally be the same type of PV device as the device you are testing. However, the most common choice of reference cell is Silicon PV, due to its dependable performance. The reference cell output is often calibrated to read 1.00 or another standard measurement under a 1000 W/m2 irradiance at 25 °C. This cell is encapsulated with a protective layer to prevent damage to the device and ensure long cell lifetime.

This is the recognised way of checking the calibration of a solar simulator, so it is probably worth investing in a reference solar cell for your PV testing lab.

Reference solar cell to check calibration
Reference solar cells can be used to monitor the calibration of your solar simulator

Using a Standard Reference Cell to Check your Solar Simulator

  1. Turn on your solar simulator and allow the light source to warm up. LED Solar Simulators can achieve 1 Sun irradiance almost immediately, but it is good practice to leave it for a few minutes to ensure stable output. For arc lamps systems, you may need to wait 30 minutes until consistent output is guaranteed.

  2. Place the reference cell where your device will be. Try to be as accurate as possible in this, considering both its lateral placement and distance from the light source.

  3. Check the output of your reference cell. If it reads the defined standard output for 1 Sun, you can begin testing your devices.

  4. If the reference cell reads out a different value, then you can adjust the height or power supply of your light source until the reference cell reads the standard value.

NOTE You should not have to vary the height or power output too much to achieve 1 Sun irradiance.

If you are having to vary this largely, this could mean the bulb is reaching the end of its lifetime and needs to be changed. For an arc lamp, this will usually be after 1000 hours use. For LED light sources, this might be >10,000 hours.

Photodiode and Other Light Sensors

Other light sensing electronics can be used to monitor the performance of your solar simulator. For example, you can use photodiodes to track relative performance.

Photodiodes are small photojunction devices that contain a diode. Under reverse bias, photodiodes will exhibit a reverse-leakage of electric current. This reverse leakage is increased massively once the device is illuminated, which leads to increased current flow through the device. The current produced is proportional to the intensity of light onto the device surface. By plotting current output against light intensity for a particular photodiode, you can define the relationship between these properties for that one specific device. The actual current output of a photodiode varies depending on the type of photodiode you are using. There can even be some discrepancy between photodiodes of the same type. Therefore, you cannot use these devices to objectively measure irradiance, if you haven't seen how that particular photodiode performs under a calibrated 1000 W/m2 light source at a set distance.

You can however use photodiodes to track the relative performance of your solar simulator. To do this you will need to measure current output at 1 Sun light intensity - when your solar simulator is first set up in your lab. You can then repeat this measurement at any time to check if the irradiance of your solar simulator has changed. For example, if the current output is decreasing, you know this means the spectral irradiance must also be decreasing. If you are doing this, it is important that each measurement is taken at the same place (consider height and lateral placement) with the same external conditions.

Other light sensors such as known as photosensors and photodetectors, also respond light intensity fluctuations the device surface. Another example of a light sensor is a light dependent resistor. LDRs contain a semiconductor is more conductive when its illuminated.

Solar Simulator

Solar Simulator

Contributing Authors

Written by

Dr. Mary O'Kane

Application Scientist

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