How Can I Use a Solar Simulator to Qualify Optical Detectors?

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Solar simulators, lights that reproduce sunlight, can be used for a wide variety of optical sensor applications, from cameras to sun sensors to solar cells.

The exact method(s) to use for the qualification of photodetectors vary depending on the application, but we’ll try to cover some aspects to consider for the most common ones, including cameras, hyperspectral sensors, sun sensors, solar cells, photodiodes and LiDAR.

First of all, no matter the application, it’s important to be aware of seven key parameters of a solar simulator.

There are the three that are covered by the solar simulator standards (that determine whether it’s Class AAA, BBC, or other), which are spectral match, spatial uniformity and temporal stability.

  • Spectral match indicates how closely the spectrum matches sunlight.
  • Spatial uniformity describes how much the intensity changes over the area of interest.
  • Temporal stability describes how much the intensity varies in time.

The next two parameters are spectral coverage and spectral deviation which refer to how much of a target spectrum is actually covered by a solar simulation, and the total error at all wavelengths, respectively. For more on these parameters, please see our comprehensive IEC standards article.

The last two parameters that need to be considered are uniform volume and angular distribution, which we’ll discuss more below. This article is organized according to the parameters to consider, and examples of assessment and impact on different applications will be discussed along the way.

Spectral Considerations

Spectral Range

For photodetectors, only select portions of the spectrum that actually matter. If you are testing a visible wavelength camera, then you may only need light from 400 nm to 700 nm, so a Class A spectral match solar simulator from 400 nm – 1100 nm or beyond is unnecessary. If you are testing a hyperspectral camera, however, you may need to have a solar spectrum that extends into the infrared, and exactly how far will be specific to your device. Therefore, for your application, you should scrutinize spectral match only in the areas of your device’s responsivity. A silicon solar cell’s responsivity range can be very different from a multi-junction, so get a light source that matches what you’re trying to test. Closely related to the spectral match are spectral coverage and deviation. As mentioned above, you only really need to worry about spectral coverage in the responsivity regions of your photodetector, and a similar principle applies for spectral deviation. If your photodetector is insensitive to changes in irradiance over a given spectral bin, then you may be able to relax your spectral deviation requirements, for example. If, on the other hand, your photodetector has a large number of narrow spectral bins that dictate its response, then you’ll likely want to ensure that you not only have good spectral coverage of these bins, but that the deviation is low for all of them as well.

Spectral Tunability

The other spectral aspect worth considering is whether or not you’d like to tune the spectrum to assess specific your device’s sensitivity to different wavelength bands. Are you trying to quantify the impact of green-light glare on your camera’s image quality? The interference of afternoon sunlight on a remote sensor, or the determination of reflectance confusion on LiDAR throughout the day? Having a solar simulator that can tune specific spectral regions (something that can be done with a good LED solar simulator) may greatly facilitate the tests you need. Tuning individual channels can, for example, allow a camera developer to assess cross-channel interference by comparing RGB channel responsivity of full spectrum light against the responsivity of individually-active wavelength channels.

Spatial Uniformity Considerations

Spatial uniformity may determine the error bars on your device’s qualification if, for example, you’re trying to calibrate a camera’s pixel-to-pixel variation across an illuminated area. Depending on the area you’re trying to qualify, you may need to know more specific information about the resolution of the spatial uniformity measurements for a given solar simulator. Clarifying with the solar simulator manufacturer is important, because a Class A uniformity solar simulator with measurements every 2.5 cm over a wide plane is different from one where the measurements were only made every 10 cm. Knowing the degree to which you need to assess your device’s spatial sensitivity, and making sure your solar simulator’s output matches those requirements or at least meets them part way, is important in optical detector qualification under artificial sunlight.

Uniform Volume Considerations

This aspect of uniformity is not often mentioned, but is vital when trying to test non-planar devices. Because a solar simulator rarely produces perfectly collimated light, the intensity and spatial uniformity of the irradiance field will change as a function of working distance. In other words, the brightness will likely decrease and the field won’t have the same uniform area the farther you move away from the light source. We refer to this uniform area and the distance over which it is maintained as “uniform volume” because it is a 3-dimensional space in which tests can be carried out with confidence. In practice, this means ensuring that your device falls within the uniform volume, or is coplanar with the solar simulator manufacturer’s recommended working plane. If your device under test (DUT) is planar, then this is usually trivial. If, however, your DUT is non-planar, then you’ll need to know the dimensions of a solar simulator’s uniform volume. Ensuring your tolerance for intensity and uniformity changes are aligned with a solar simulator’s performance changes with distance is key to achieving desired results.

Temporal Stability Considerations

Temporal stability is usually the easiest parameter for a solar simulator to achieve, but some optical detector validation might have more exacting requirements. If you’re wanting to automate testing, or test intermittent behaviour by shuttering or turning the lights on and off, you want to make sure the shutter’s opening, light warmup, and/or automated turn-on is of a shorter duration than the behaviour you’re trying to assess. For example, if you want to qualify solar cells on a production line, knowing how quickly the solar simulator can turn on accurately is essential to determining the fastest allowable throughput. If you’re trying to simulate sun blockages or rapid attenuation changes on a satellite’s sun tracker, you’ll again want to know that the solar simulator’s ability to change is representative of the phenomena you’re trying to investigate.

Considerations of the Angular Distribution of Light

Sunlight is mostly collimated, i.e. its rays are parallel to one another. Solar simulators, on the other hand, are rarely perfectly collimated. For many optical devices, this difference is unimportant. For example, a solar cell’s current and voltage performance won’t really be affected by the angular content of the incident light — it will still convert photons to electron-hole pairs and undergo the same processes.

In other applications, however, the angular distribution of light will matter a great deal. In sun sensors or star trackers which need to determine the sun’s direction to within a few degrees of precision, a solar simulator may require light that’s as close to collimated as possible, with a divergence no larger than the desired precision of the sun sensor. For UAV remote sensing applications trying to assess the impact of the sun’s angle on gathered images, the incident angle of light may also be important, and require more stringent collimation.

The angular distribution of light is a bit tricky to understand, so we have an entire article describing angle of emission. In the end, it’s most important to ensure that the solar simulator’s angular content meets the needs of what you’re trying to investigate for your photodetector.

Summary

With all these considerations in mind, a solar simulator is nevertheless an indispensable tool for researchers around the world to repeatedly produce light in laboratory conditions. 

Solar simulators can add significant value for validation of optical sensors that are impacted by interference from or that are dependent on sunlight. Before starting, it’s important to:

  • Match the solar simulator’s spectral output (including spectral match, spectral coverage and spectral deviation) to the spectral sensitivity of your device(s)
  • Ensure the uniformity of illumination will give you the error bars you’re seeking in spatial qualification
  • Ensure the intensity drop-off and uniformity changes as a function of distance (i.e. the “uniform volume”) will be able to accommodate your target device(s)
  • Ensure the time constants of  the solar simulator’s operation, including shutters, warm-up and any automation, will allow you to test the time-varying phenomena of interest. 
  • Match the solar simulator’s angular distribution of light to the angular sensitivity of your device or application

References – Links to other relevant G2V articles for more information

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