How to caculate absolute reflectance

[Kikysee]

In MicaSense Image Processing Tutorial 1, If a gradient or a high standard deviation (>3% absolute reflectance) is noticed across the panel area it is possible that the panel was captured under inconsistent lighting conditions (e.g. next to a wall or vehicle) or it was captured too close to the edge of the image where the optical calibration is the least accurate.
But how to calculate absolute reflectance?
Can someone help me?
Thank you very much.