Danny,
thanks for the input. I had read that article previously, but couldnt remember where it was from. I think an important thing to note is that the author states
it is not a fair comparison. First of all, having the lights suspended so high above the tank is not the ideal way to get light into the tank. Most tanks seem to have the lights between 5.5” to 8” off the water. Secondly, the water was fairly turbid, mixed with quite a bit of micro-bubbles from the skimmer. The tank was also bare-bottomed, meaning that these are the deepest readings I have taken, with the sensor a mere 1” off the bottom of the tank.
These bulbs were mounted 9.75 and 15.5 inches respectively off the waters surface. Have you heard of the inverse square law?
It is used in photography and is referenced for lighting subjects...but it applies here too. From petersons photographic library..." The inverse square law is the basis for our understanding of the relationship between distance and illumination level....The law states that the intensity of light decreases as the sqaure of the distance between light source and subjucet increases."
To clarify this the book makes a couple of comparisons..."Let's assume that you have a light source placed one foot from a subject, and you double that distance by moving your subject back two feet from the subject. You're not just cutting the light in half, as you might suspect. You're reducing the light by four times (two squared = four). If you now increase the the subject-to-light-source distance to three feet, you've decreased the illumination level to 1/9th its original intensity, (three squared = nine)."
So keep that in mind when placing lights above your tank and the height you go with. Most people will place their lights any where from 5-8 inches off the surface of their water. If you realize this and then go back and look at the picture of the sunlight supply tank, (mogul/SE ended on right @ 9.75 inches and HQI/DE on left @15.5 inches of height) you'll see that the results are the same when the DE is almost twice as far off the surface of the water. So if we apply the inverse square law here, by placing the DE pendant at the same height as the mogul/SE pendant, the DE would produce almost 4 times the light that the SE/mogul would.
Thats pretty significant to me....
For 4 times the light, I'll find a way to cool the water.
Another thing I noticed about this test was that the author didnt take measurements of the philipines underwater to get a reference to our tanks....just some thoughts,
Nick
