Intensity
Units - allows choosing the light units. Using correct units is essential when you work with the VRayPhysicalCamera. The light will automatically take the scene units scale into consideration to produce the correct result for the scale you are working with. The possible values are:
Default (image) - the color and multiplier directly determine the visible color of the light without any conversion. The light surface will appear with the given color in the final image when seen directly by the camera (assuming there is no color mapping involved).
Lumious power (lm) - total emitted visible light power measured in lumens. When this setting is used, the intensity of the light will not depend on its size. A typical 100W electric bulb emits about 1500 lumens of light.
Luminance (lm/m^2/sr) - visible light surface power measured in lumens per square meter per steradian. When this setting is used, the intensity of the light depends on its size.
Radiant power (W) - total emitted visible light power measured in watts. When using this setting, the intensity of the light does not depend on its size. Keep in mind that this is not the same as the electric power consumed by a light bulb for example. A typical 100W light bulb only emits between 2 and 3 watts as visible light.
Radiance (W/m²/sr) - visible light surface power measured in watts per square meter per steradian. When this setting is used, the intensity of the light depends on its size.