Ouestion: effect of distance of lighting above water level

[jrreese]

I have a question about the distance of lighting above the water level? is their a depletion of par percentage based on inches above water level?

[ReeferAl]

I'm not sure I understand your question. Are you asking whether the light output will fall off more over time depending on distance of the bulb from the water or whether the light levels will measure differently at different distances from the water?

Allen

[jrreese]

Will light levels measure differently based on the distance from the lighting to water

[ReeferAl]

The light levels will measure differently at different distances of the light meter's probe from the light. Assuming the distance of the probe from the light remains the same, the distance from the water won't have any effect on the readings.

Measurements of light levels within the tank will vary with different distances of the lighting from the water if the probe remains at the same location. In other words, if the distance of the light from the probe decreases then the light measurements will increase. This does not reflect any change in the bulb itself, only in the masured light level. The further you are from a light source the dimmer it will appear.

Allen

[jrreese]

Thanks so much Allen, You said in your article that one might decress the distance of the light source to the water to compensate for the lowering of intensity (par) with the age of the bulb. I was curious about what that distance should be, and it's effect on light source effectiveness. I have built a new hood with an incressed distance of the light source from the water and the tank seems to the eye to be a lot darker than before. So I was wondering about the effect it might be having on the corals, if any?

[ReeferAl]

The light levels will roughly follow the "inverse square law". That law of physics says that the light intensity of a point source will decrease with the inverse of the square of the distance. What that means is that if you double the distance the light level will decrease to 1/4.
It isn't quite as bad as that for our lighting since the reflectors disperse the light so it isn't a point source. This means that the levels we measure won't fall off as quickly with distance. This also means though that it is impossible to predict how much it will decrease with distance.
Without knowing the design of your hood I can't really tell you how you could lower the lights. I would say though that even 2 to 3 inches can make a significant difference in the higher areas of the aquarium. The bottom of the tank will be affected much less by small changes in distance.
Allen

[jrreese]

Thanks again for you reply, I think I will lower the light a little bit at a time to see the effect. Thanks again! John