Need high light? Not quite, here's an example

That photo brings up a question I have been wondering about. Imagine a 50 gallon tank with a top surface that is 2 feet by 2 feet. Put a 2 inch diameter, really bright light, with a parabolic reflector, about 2 inches above the water. Let's say that light is a really compact fluorescent tube of 50 watts. So, we have 1 watt per gallon of light, right? But, the area directly under that bulb has very much more light intensity, so in that area one could grow any known aquatic plant, with no light issues at all. Areas not directly under that light would be very much low light areas, where few plants would grow well. Am I right so far?

The tank you showed has a strip light, mounted close to the water, located over the back half of the tank. I think the area directly under that light is a high light area, with far more than the average 1.5 watts per gallon in effective light.

This has interested me because when I reduced the lighting on my 45 gallon tank from 110 watts to 72 watts, the high light demanding plants acted as if nothing had happened. But, my light, like that in the photo, is over the back half of my tank, so the area immediately under the light must be seeing much higher light intensity that the front half of the tank.

Unfortunately, I'm not sure where this is leading me.

 

If that is a 325 litres tank (ADA-H 1.2x0.45x0.6) then 0.4 watts/litre converts to over 200 mmol/m2 PAR. That is not low light by my standards.

watts per gallon and watts per litres does not scale well for larger and smaller tanks and cause a lot of confusion.

 

How long a photo period do you use with these lights?