A lux meter wouldn't give you an accurate reading of light intensity, but it would give a good indication of the distribution of intensity around the light or in the water, or in the water vs in air. Just a thought: doesn't every bulb have a fixed relationship between lux and PAR, based on the spectrum produced by the bulb? That would be a number, like .26 PAR to Lux, and of course it would apply only to new bulbs. Is that right? If so, we should urge bulb manufacturers to provide that for every bulb.