Light may be generated or emitted from an object through five processes that take place on the molecular scale (see the movies from NASA that illustrate these processes).
Plants, and most objects within our landscapes, do not emit light within the wavelengths we can detect with our hand-held sensors or our eyes.
There are several sources of the light we see. These sources should be measured to compare the proportions and amounts of light of different wavelengths, such as the ultraviolet, visible, and near infrared light.
The diagram shows how to position a light emitting diode (LED) and voltmeter to measure the light emitted from the sun.
Notice that the top of the LED is pointed directly toward the light source. Remember there is an angle of sensitivity for each LED, so always check the alignment of the LED so its top is pointing at the light you are measuring.
Using LEDs to Measure Transmission of Light Through Materials
Using LEDs to Measure Reflection of Light from Surfaces