Holy LED light Hood Batman. Solaris.

[hahnmeister]

I think one of the reasons for the perception many people have of the LED's may be based in the fact that they provide less light usually.

Halides have gotten brighter and more efficient. A bulb like an Iwasaki 175watt 15,000K or a pheonix 14,000K which is brighter than some 10,000Ks would have been unheard of years back. That, and at one time, flat retrofit reflectors and spider reflectors were 'cuttung edge'... and now lumenarc style ones are more mainstream.

And then there are the ballasts... HQI and DE bulbs vs. probe start and M58 ballasts that burn out the bulbs faster.

Overall, 250 watt bulbs are now able to do what 400's once did, and 150's are able to do what 250's once did. VHO's and PC's have been replaced by T5s with 2-3x the output per watt.

Yet when we set up our tanks, we still tend to go with the old guidelines of what we might have used years ago.

The result is that many of our tanks are 'too bright'. And granted, corals might be used to such intensities in the wild sometimes, but there is also more flow, and food in nature... so its harder to reach a photosynthetic imbalance. Many of the corals we keep, as a conseqence, do better under slightly lower light than in the wild. If you keep a low nutrient system, with loads of light, but dont supply enough plankton (who does?), then its very likely your corals will turn pastel and fade.

I know I went through a 'whoah, what the heck happened' period when I replaced my pheonix 14,oooK with a duller 20,000K... the coral's polyps came out more and the corals all of a sudden grew faster.

So its quite possible that for some, going from a halide to LED is simply a benefit of having less light. Ripping out those 400 watt halides and putting in 250's might have done the same thing.

[Sanjay]

Originally posted by hahnmeister

Light rays that enter the water at more than 45 degrees will most likely not even enter the water,

Not true. Look at Fresnell equations for better explanation.

sanjay.

[hahnmeister]

Oops, thats right.... I needed a refresher. 45 is the critical angle for water back into air for total internal reflection. I must call my Physics 210 professor and apologize. No critical angle going from air to water... only the more dense medium to less dense. Still, it seems like most reflectors try to keep the light at an angle of 45 or less to reduce wasted light, since the greater the angle as the light passes the air/water boundary, the more will get reflected.

[pjf]

To achieve TIR, the light entering the water must strike the water at an angle less than 60 degrees from vertical. When the light strikes the water, it will be bent to less than 41 degrees from vertical (Snell’s Law). The rays striking the vertical glass wall will do so at greater than 49 degrees from perpendicular and be reflected back into the tank.

[hahnmeister]

You also need to be going from a higher density medium to a lower density, or total internal reflection isnt even possible.

[pjf]

It's true that some light is reflected off the surface of the water. Very little passes through the glass walls. You can test that by looking for glitter lines on the floor next to the sides of an aquarium (assuming that you don't have eggcrate louvers over your tank).

Try to see your lights through the glass & water. You may be able to see them by looking up through the side glass. What little you are able to see is caused by light striking the water at shallow angles (greater than 60 degrees from vertical).

[jnarowe]

hmmmm...I can tell you that enough light is reflected off my water to nearly blind me.

[roblack]

Hahn has made some good points about changes in lighting, and I wouldn't be surprised if growth changes with my corals under LEDs are actually related to dimmer lighting.

[jnarowe]

That would bum me out!

[pjf]

Dana Riddle (http://www.advancedaquarist.com/issu...04/feature.htm) found photo-inhibition at 260 PAR and photo-protection at 100 PAR with Montipora coral. I'm not surprised that reduced lighting can help. Perhaps we need to borrow some PAR meters before we shop for lighting.

This is actually an argument for two-tier lighting: metal halides during mid-day & actinics at dawn & dusk. The metal halides will light the lower depths. The actinics will allow the upper corals to photosynthesize when not inhibited by metal halides.