PAR depth

[brad]

I am trying to calculate the spectrum of sunlight in pure water at depth, to see how it matches reef tank lighting. I was hoping someone with more diving experience (especially deep diving) could tell me on a coral reef:
1) How far can you go down and still see anything, or what is the farthest you've heard of anyone diving if you cannot go below enough light for good visibility?
2) How far can you go down and still see red as red?
3) Do you know of a table or reference for any of this?

I am doing this more for personal curiosity than practical improvements to my lighting.

 

[philter4]

I dive deep, over 200 ft and you can still see well, the red spectrum is depressed, it looks either like maroon through brown depending on surface conditions, but just with practice you can tell if something is going to be bright red at the surface. As far as how far light penetrates, it is well below safe dive limits for humans, at deep dives everything has a blue tint, but you do not need an artificial light source to see. Sanjay Joshi has done many studies on PAR and has a web site, look up reef lighting and you should find all of the info that you need. In a recent talk he was asked about artificial light compared to natural light and his answer was something like (I don't remember the exact figures) at 18 inches in a tank is about 1/2 the natural light at 100 ft in the sunlight. This I believe because I have seen coral growing well at 100 ft on every dive I make in south FL.

 

[brad]

Thanks. Sanjay has been by far the greatest resource I have had for this information. I have never dived (dove?) in anything more exciting than Lake Erie or freshwater springs, and hearing first hand is worth more than any analytical model of spectrums and absorption. From listening to some bulb manufacturers, we'd assume 100 feet down there would be no light except blue.

 

[snorvich]

You lose color spectrum in the order of: Red, Orange, Yellow, Green, Blue, Violet as you go deeper. That is why many photographers use a torch at depth to see the coloration of the negative space.

 

[billsreef]

Red is pretty much gone after 30 feet. I've taking pictures of gray fish at 60 feet, only to find out after the film was developed that they were red Well developed coral reefs grow in 100 feet and deeper. While I've never been down to 400 feet, I'm told around this depth the reef changes over to sponge dominant coverage.

 

[tomg3reef]

brad,

I apologize in advance for answering your question of what time is it by telling you how to build a watch but .........

I too am curious about duplicating the amount of light and color (wave length) seen by the corals. I am told that most of the corals we try to keep ar found between 15 and 100 feet so I am concentrating my efforts on that depth and really want to zero in on the corals between 10and 20 meters (32.7 to 65.4 ft.)

I found a URL which had the attenuations for 10 bands of light between 400 nm (blue) to 700 nm (red-red) but I have misplaced it. In its place I have the chart from http://oceanworld.tamu.edu/resources/ocng_textbook/chapter06/images/Fig6-18.htm which gives the transmissivity of turbid tropical-sub tropical water (i.e. Cozumel reefs, Belize etc.)
This chart says that from 325 nm to 590 nm orange the transmissivity of this water is > than 90 % and from 425 nm (~actinic blue) to 550 nm (red-orange) the transmissivity is close to 96 %. PER METER OF DEPTH.

At 10 meters then the factor of transmissivity (whats left at that depth and color) is 0.96E10 or 0.96 raised to the tenth power 0.6645. This is applied to the surface intensity of the color band to get the intensity at 10 meters.

for the 96% spectrum the factor is 0.6645 at 10 meters
for the 96% spectrum this is 0.442 at 20 meters
for the 90% spectrum the factor is 0.348 at 10 meters
for the 90% spectrum the factor is 0.1215 at 20 meters

now I have to find the spectral intensities at the surface somewhere and I will know the light levels at various colors the corals are seeing.

 

[philter4]

I know this doesn't answer the question, but here is 2 photos, one with and one without a flash at around 100 feet
with flash

without

At that depth the spectrum is different, and you can see the blue tint, but you can still see the yellow spots and basic color pattern.
I know this doesn't answer the question of how much light is down there, but it gives you an idea of what you see.

 

[David P]

2 weeks ago I did a dive on an oil rig, @ 50 it was pitch black, I mean we used our lights to read our gauges. Then at 150' it was much brighter, with visability well in excess of 60'. Everything is pretty blue, but we use HID lighting to bring the pretty colors back. Then @ 70 while hanging at a deco stop the water turned to milk, and couldn't see much of anything.