Previous versions of Vray calculated light intensity in a different way compared to how it calculates today. I will explain the old way first and then move on to how the latest version calculates light intensity as this may be beneficial if you are going back through older scenes.
Vray used to use watts as a measurement for light intensity, which is a measurement of power and not a true reflection on the amount of visible light. You could easily fall into the habit of adding Vray plane lights into a scene and cranking up the multiplier until you are happy with the general illumination but not really knowing how much light is actually being emitted.
Vray assumed that 1 generic unit is equal to 1 metre. The default (image) light multiplier defined the light intensity in watts per square meter. So 1 unit (watt) would cover 1 metre squared (Watt/m^2).
This means if a Vray plane light had a half length and width of 5 metres, the actual light size is 10 metres x 10 metres. The total emitted light was 100 watts. In order to get an accurate value of visible light, you would need to factor in the efficiency of the light source. For a 100 W tungsten incandescent bulb, it actually only emits between 2 and 3 watts of visible light. Therefore, it was never really a true reflection of light intensity.
In addition to this, because Vray worked in metres, if you had your system units as millimetres or inches, the intensity would be incorrect. You had to apply calculations to the multiplier if you were working in anything but metres.
Introducing a more accurate method for measuring light intensity
Luminous power is now a part of the latest version of Vray. This intensity value is taken from the overall luminous efficacy.
To get from overall luminous efficacy to luminous power (lumen), you multiply it by the bulb wattage. So a 100 W tungsten incandescent bulb that has a 17.5 overall luminous efficacy is 1750 lumens.
If lumious power (lm) is selected, the size of the actual light source will have no effect on intensity but the multiplier will do. If you select Luminance (lm/m^2/sr), the intensity of the light depends on its size.
1750 lumens may seem too dark in some scenes. It is advised that you use the Vray physical camera so that you can control the exposure. Do not adjust the light multiplier as you know that this is physically correct, the over luminance of your scene will depend on your gamma, camera exposure and material settings. When using luminous power (lm), it is important to make sure that the light is at real world scale. Although it does not affect intensity, it does affect the accuracy and position of shadows. A 100 W tungsten incandescent bulb is about 50mm in diameter.
Although not recommended, Vray still allows you to use watts as a unit of measurement. Select radiant power (W) as the unit of measurement and enter your desired wattage. Again there is also an option to have the light intensity dependant on its size by selecting radiance (W/m²/sr).
For lumens and watts, it does not matter what system units you use. The output will be correct, as long as the scene has been modelled in real world scale.
Finally there is the default (image) multiplier. This has no reference to a real world measurement and is purely dependent on the multiplier and the light size that together control the intensity/brightness.
Lumens are considered to be the best method for measuring accurate light intensity, but it does require realistic surroundings. If your material and camera settings are not physically correct, then default (image) would be the better choice.