Corona supports Candela (cd), Lumen (lm) and Lux (lx) lighting units. If it's not specified (for example in standard lights, self-illumination, or Corona Light material intensity), the unit used is Watts per steradian per square meter - W/(sr.m^2).
This means that when there is no tone mapping applied to the image (not even gamma correction), the W/(sr.m^2) value of 1 results in pixel RGB value 1.