Operation | Lux meter
Shortcut: I 
This option activates and deactivates the Lux meter. When the Lux meter is active, you can take light measurements of the light falling onto an object.
The mouse cursor will change for 
when activated
The taking of the measurements start when you first click in a window. The program will then gather and store information on the scene as it is at that moment. It will use this information to calculate the illumination, and this information will not change until you leave the 'Lux meter' by selecting it again.
This also means that you should not change anything in the scene while you are using the lux meter.
When you click somewhere in a window, the lux meter will start calculating (and will do so as long as you keep the mouse button down) the light falling onto the position you are pointing at.
The information will be displayed in the 'Statusbar' at the bottom of the application. It will look something like:
Illuminance 6827.39 lx. (= 6217.39 direct + 600 ambient) Position (m.)( -0.524, 0.000,-2.577 )
This means 6827.39 lux falls on the position -0.524, 0.000,-2.577 (of which 6217.39 lux directly from light sources and 600 from ambient light).