WPG - PC vs. MH?

[ibnozn]

I recently upgraded the light over my 20G from a 65 watt PC 6700K to a 70 watt 10000K Metal Halide. I'm super happy with the light but I'm trying to get a sense of how much more light I have. It seems quite a bit brighter than the PC, more than just a 5 watt difference. The MH bulb output is supposedly 3100 lumens. Any idea what wattage fluorescent bulb this would be comparable to? Watts per gallon generally refers to fluorescent lighting, so I'd like to know roughly what I have.

 

[VaughnH]

What you have will depend almost entirely on how high above the water you hang that MH fixture. The light from it will drop off in intensity proportional to the inverse square of the distance from the bulb to the substrate (if you are concerned about growing ground covers.)

It is very hard to judge light intensity with your eyes. Our eyes are designed to function well under lighting conditions from moonlight to noon sunlight. So, they have great auto exposure controls. But, that kind of washes out their usefulness as light meters.

Fluorescent lights tend to be sitting right on top of the tank, so a watts per gallon rule can mean something with them. Once the light is raised, that rule is not useful.

 

[ibnozn]

It's about 6.5" above the water. The PC was 2.5".

 

[Gerryd]

Vaughn,

Can you put this in more layman's terms??

The light from it will drop off in intensity proportional to the inverse square of the distance from the bulb to the substrate

What is an inverse square?

Could you give me an example?

Sorry, a little dense here............and school was a LONG time ago Not that I paid much attention anyway lol