This article refers primarily to solar irradiance, the light energy emanating from the sun as measured from the Earth after passing through the atmosphere.
In general, objects either absorb, reflect, or transmit incoming light (irradiance). The percentage of light that an object reflects is called reflectance. Reflectance is an inherent property of an object; it does not depend on lighting conditions. For example, most healthy plants reflect more green light than red or blue light (this is why plants appear green). Stressed plants tend to reflect more red and yellow light, which is why we typically see leaves change their color in autumn. Below is an example of a typical plant reflectance curve:
What is a calibrated reflectance panel?
Calibrated reflectance panels consist of a surface with known, carefully-measured reflectance values in a range of wavelengths. Below is an example reflectance curve of a calibrated reflectance panel:
How are calibrated reflectance panels used?
A properly-taken image of a calibrated reflectance panel can be used to determine the solar irradiance on the ground at the time of the capture. Because the panel has a known reflectance curve, the raw pixel values of the panel area can be converted to reflectance values. This information can then be used to calibrate other images taken under the same lighting conditions. Photos of the calibrated panel must be taken carefully to avoid shadowing the panel.
What is a an irradiance/sunshine sensor?
Light sensors are designed to directly and continuously measure solar irradiance. Rather than measure reflected light, light sensors point up at the sky to measure the incoming light energy directly. Several factors can affect these measurements, including the current weather conditions (especially cloud cover), as well as the sensor’s orientation relative to the sun as it flies.
How are light sensors used?
Since they continuously measure irradiance, light sensors are most effective at correcting for global changes in lighting conditions, such as when the sky is completely overcast and irradiance fluctuates during a flight. They cannot reliably correct for partly cloudy days where popcorn clouds may shadow part of the earth but do not shadow the sensor (or vice versa).
The light sensor should be mounted on a mast with a clear view of the sky, and completely unobstructed, so that no part of the aircraft can cast a shadow on the sensor itself.
In addition to using a light sensor, we strongly recommend using a calibrated reflectance panel every time you collect data.
Why do we recommend both calibrated reflectance panels and light sensors for data collection?
The goal is to produce high quality, reflectance-compensated output. Capturing multiple data points helps to confirm the quality of the collected information and serves to increase confidence in the overall result. While light sensors capture changing lighting conditions throughout a flight, they do so from a higher altitude, whereas calibrated panels provide the most reliable measurement of irradiance on the ground.
Ideally, the photogrammetry tools used for processing this data should be able to combine and use multiple methods for radiometric correction.
When flying in the recommended weather conditions, irradiance should not change much during a flight. Our best practices guide covers this topic and more, including how to avoid shadows, which have a significant impact on the data you collect.