Watts vs Lumens

[bigjohnstud]

This question has been buggin me for sometime...

In planted tank terminalogy for lighting, we always talk about achieving ~3watts per gallon. Why isn't the uom of light used ie. lumens? Shouldn't we be talking about light intensity rather than power consumed by the bulb?

Hope someone can shed some light here...

[sherchoo]

Problem with tubes is they seldom provide the lumens data. It's been commonly marketed out as wattage. Also the lumens rating varies from tube to tube with a difference of only a few units. So to compare lumens, it's rather quite troublesome.

Also lumens do not tell what kind of tube is it. If someone goes to a shop saying 18W FL tube... the shop owner would immediately give the correct one. So much so, this has become a norm when classifying light tubes.

With regards to the 3W/g guide', I will always stress that this is a guide and should only applies to flourescent lighting.

[Allen_1971]

Well with lights there are a few measures... unfortunately all we typically get is wattage... and thats what we are usually stuck with. Thus you get these 3W/Gal guidelines.

Some tubes like philips gives the efficiency like 75lm/W for example... and sadly all tubes are not the same... comparing the philips fluotone T-8 tube it has a output of 76lm/W while the philips 865 T-8 has an efficiency of about 85lm/watt. This is over 10% higher output for the same wattage.

Other lighting information which we are typically missing are spectrum information... which is normally available on more expensive specialty tubes. All we get is the general color temp... and sometimes not even that.

Finally it would be even better if we could also get the luminance factor... which will tell us the cd/m^2. This would give us an idea of the light penetration of the tube. In general though the more concentrated the source... (e.g. MH) the better the penetration

[FC]

Typically:
FL 36
-about 3,200 lm for super bright
-less than 3,000 lm for normal ones
PL 36 about 2,700 lm

Colour Rendering (quality of the light):
-The higher the quality that come with the light, typically, the brightness went down at the same time. Due to the extra phospher coatings, I guess.
e.g.,
-Philips 865 comes with Ra of 85 and about 3,250lm
-Philips 965 comes with Ra of 95 and about 2900lm

[bigjohnstud]

Thanks Allen, Sherchoo and FC for shedding some lumens on this topic. I understand now that it is due to limited information on the products available in the market that led to measurement of light by watts rather than lumens. Cheers

[FC]

If you wish to calculate the light requirement in lumen that covers most plants, you may use this formula: 20,000 lumen per metre sq. This formula applies only to tank height of below 60 cm.

For example, the tank length and breadth are 1.2 by 0.6 m, then
The water surface area = 0.72 m2
Lumens required will be = 0.72 x 20,000 = 14,400 lumens.

Have fun.

[Allen_1971]

Once again take note that lm is also just a guide... one good example of this is arcadia's tubes... For example their original tropical tube is supposedly good for plants, and I know a couple of people who agree... however, their own webby specs that the output for a 36W lamp is a measly 1100lm... which is pretty dim. This is probably because the lamp is almost completely lacking ins the green spectrum...

Haha I think if we really want to be more accurate we have to look at the luminous density of the red and blue spectra... but frankly thats too much effort, and to difficult to do unless you have a light spectrometer.

[vinz]

Which is why W/G of Daylight is actually not too bad a guideline. Daylight tubes are close to white and therefore the three primary colours are well represented.