All lights have a color, shadow, and intensity property. The shadow properties determine whether shadows caused by this light will have soft mapped, soft sampled, or sharp edges. The intensity of a light is its brightness. In AutoCAD, you can choose the light intensity to be either generic, international (SI), or American.
The generic choice results in a non-photometirc workflow, where the lights have an intensity factor, color, and user-selectable attenuation type. The value is 0.
The international unit system results in a photometric workflow with physically correct light intensity. This is the deafult lighting unit with a value of 1.
The American choice is the same as the international unit, with the only difference being that the illuminance values are provided in foot-candle instead of lux. This has a value of 2.
Use AcDbDatabase::setLightingUnits() to control the lighting units.