It was quite interesting to use a Lux-meter sitting at the bottom of a tank without water and then start filling the tank up.

I was very obvious the water made the tank act like a opto fibre =)

Regarding the difference between Lux- and PAR-readers the measure kindof the same range, only that Lux-meters are not that sensitive to blue and red light.

Lumens are emitted inside the PAR-range (400-800 nm though, so if you know the spectral distribution of your light you could actually with some mathematics convert the Lux-measure to a PAR-reading.