Lighting - W.m/m2

[Robert Smith]

After some discussions about lighting I took Vinson's numbers and resulted in a defining moment of fish keeping. I've invented the "Smithon" and even defined it. Behold!

1) Using 2W/Gal at 700mm and 3.78L/Gal I get 370W/m2.

My working is:

2W/Gal = 2W/3.78L = 2W/3780cm3

For a 700mm deep tank, thats:

2W/3780cm3/700mm = 2W/3780cm3/70cm = 2W/54cm2 = 1W/27cm2 = 370W/m2

2) If we then assume that you can also scale the depth factor to get a universally scalable number in W.m/m2 (Watt metres per square meter):

370W/m2 at 700mm is 370W x 0.7m / m2 = 530 W.m/m2

I think the first constant (370W/m2) is safe to use because that is a linear factor, ie. more area equals more required light. The second constant (530W.m/m2) may not be because light penetration is not linear and different wavelengths of light penetrate water differently. A 70cm deep tank seems like a good compromise depth to me. Using this constant on a deeper tank runs the risk of pumping too much energy into the surface, perhaps causing evaporation and frying any surface dwelling fish.

As a reality check:

- A maximum of about 1000W/m2 reach the equator, but this typicaly averages about 800W, given clouds, dust etc.;

- In the USA the average ranges from about 125 to 375 W/m2 per year. This includes nights, clouds etc. So assuming 10 hours of darkeness and multiply that by 1.7 to get the daytime average I get about 200 to 640 W/m2;

So, I don't think 530W.m/m2 would cause any problems and could in fact be increased.

If you kept equatorial fish it seems you could possibly pump out 800W/m2 and not expect any downside for the surface dwelling bodies. So if I assume the deepest tank will be 1m, this gives a constant of 800W.m/m2, which is about 50% higher than the 2W/Gal number you had above.

So, I've officially created a new constant for fish keeping, the "Smithon" with units of Watt metres per square metre (W.m/m2).

Tank Type - Max Lighting (W.m/m2)

-----------------------------------------

Equatorial - 800

Tropical - 600

Temperate - 400

High/Low - 200

Do these numbers seem sensible to people with real-world experiance?

[Robert Smith]

Replying to yourself is not a good sign, but I realised I'd better put up my own tank numbers.

I have 50W of ultra-white tungsten halide (3500K) in a 450l x 350w x 450d tropical tank. This is:

50W x 0.45m / 0.16m2 = 140 W.m/m2

My lighting setup is a bit wierd and definitely under-powered, but according to my own calcs I should have about four times the lighting, or about 200W of lights.

[dimebag]

jeez man you lost me at the first line

[fishandchips]

It takes a few reads to fully understand it but there is some great info in this post.

[alanmin4304]

Don't wish to be too critical of you ingenous theory but it does not take into acount the efficiency of the lighting system. Watts is the power used rather than the actual light output. A not very efficient system will use a lot of power but most of that energy will be given off as heat and only a very small percentage as light. Your theory needs to express the relationship between light given off and water depth (not a straight line graph) and to also take into a/c the type of light for the purpose (e.g plants need red light but there is vitually none at one metre water depth. I know nothing about light requirements for marines but they will be different as will the light penetration of sea water). Good luck with your continuing work. Einstein only came up with some of the theories.

[Robert Smith]

You're right of course Alan, but the concept is still an improvement over the existing 2W/Gal rule (assuming that this actually a rule people use!)

I would assume that the 2W/Gal rule takes into account efficiency. But of what kind of light? Metal Halide or Fluro? Does anyone know?

My data based on sunlight does not take into account efficiency. It appears the numbers may be on the high side, because if you assume a fluoro is 10% efficient (ie. this is the amount of energy converted to light, and it ranges from about 6 to 15%) then you'd need 8000W of fluorescents to get 800W of incident light for an equatorial tank. 8000W seems excessive!

I should probably be usinf Lumens anyway.

[alanmin4304]

It is a difficult subject alright. Having grown plants in aquariums with artificial light, in a glasshouse with natural light and emersed with artificial light I can confirm that plants certainly respond differently in each situation so the type of light is difficult enough without trying to work out how much and for how long.

[AMAZONIAN]

Hi All

The "Colour" of the transmitted light has a VERY BIG IMPACT on the depth that it will penetrate into a given depth of water. :roll:

Some "Shades" of red are lost within 2 inches of the surface, the upshot is that to get that "Shade" to penetrate deeper you need more Wattage.

That means an increase in lumens and so in turn uses the extra wattage.

The calcs are interesting, but as an Electronics Engineer specialising in lighting and effects, I would want to be using the CONVENTIONAL charts to obtain the required result.