The Intensity parameter sets the intensity of a light source, i.e turning it on and off. Please keep in mind that layers receiving this light as a single light source will not be visible anymore.
Intensity 0% (value 0) off
Intensity 100% (value 255) on
You can control the Intensity parameter externally, simply assign a value between 0-255.
For external control e.g. via the Widget Designer the Parameter List is of interest.