I have seen the article on minimum lighting threshold mentioned a few times (http://www.rexgrigg.com/mlt.html) and I would like to discuss it further. I may end up posting on the message board over there but thought it could make for some interesting discussion here....
This article is hard to take in a vacuum. I don’t know the intent of the author. By that I mean I don’t know if Rex is looking to achieve the maximum growth possible or merely long term success. That being said...
My biggest hang up with the article is that it makes the many small planted aquariums I have seen seem like they could have been a figment of my imagination. It is hard to refute the success of countless aquarists over the years (even if they did choose the amount of light for their tank based on a WPG rule that is ill-conceived or flawed or just plain wrong…).
The units of light intensity throughout the article are also confusing. Lux is lumens/sq. meter. A lumen is a unit of luminous flux, not to be confused with radiant flux. The difference being that luminous flux takes into account the eye’s sensitivity to varying wavelengths of light. Radiant flux is the measure of total power of the light. (thanks to wikipedia for the differentiation). The AH Supply site that Rex used for his lumens/watt figure actually warns against using lumens to make lighting decisions for aquaria.
I have been screwing around with light intensity, distance and some other stuff in excel using a list of standard aquarium sizes and from what I can see, the intensity based on distance not being used is a big oversight. The intensity of light on an object can be described as the Intensity at the source divided by the square of the distance from the source. The good 'ol 1/R^2 relationship...
If we use his statement that a 55g with 4WPG is a standard we would find that using the AH supply Lumen information the 55g has 18,400 lumens which works out to 45,705.09 lux or 4,246 Footcandles. Using the foot candles (as the height of the tank will be in feet) I calculated the intensity at the bottom of the tank as 1,386.5 footcandles.
Now lets take a look at a 10g with 4 WPG. That works out to 40 watts which means 836.36 lumens. Footcandles are equal to lumens/sq. feet, that works out to 1672.74. Because the distance from the source to the tank floor is less than a foot we will see the intesity actually increase (based on the definition of a foot candle, this makes sense). I calculated that to be 3763.658 footcandles. If this logic makes sense, 1.5 wpg (assuming same lumens/watt ratio which isn't a good thing to assume) would give the same amount of footcandles on the bottom of the tank.
I have rambled enough for now but I wanted to explain a couple of the reasons I don't completely buy the Rex Grigg article. It has been most thought provoking though. any thoughts?
This article is hard to take in a vacuum. I don’t know the intent of the author. By that I mean I don’t know if Rex is looking to achieve the maximum growth possible or merely long term success. That being said...
My biggest hang up with the article is that it makes the many small planted aquariums I have seen seem like they could have been a figment of my imagination. It is hard to refute the success of countless aquarists over the years (even if they did choose the amount of light for their tank based on a WPG rule that is ill-conceived or flawed or just plain wrong…).
The units of light intensity throughout the article are also confusing. Lux is lumens/sq. meter. A lumen is a unit of luminous flux, not to be confused with radiant flux. The difference being that luminous flux takes into account the eye’s sensitivity to varying wavelengths of light. Radiant flux is the measure of total power of the light. (thanks to wikipedia for the differentiation). The AH Supply site that Rex used for his lumens/watt figure actually warns against using lumens to make lighting decisions for aquaria.
I have been screwing around with light intensity, distance and some other stuff in excel using a list of standard aquarium sizes and from what I can see, the intensity based on distance not being used is a big oversight. The intensity of light on an object can be described as the Intensity at the source divided by the square of the distance from the source. The good 'ol 1/R^2 relationship...
If we use his statement that a 55g with 4WPG is a standard we would find that using the AH supply Lumen information the 55g has 18,400 lumens which works out to 45,705.09 lux or 4,246 Footcandles. Using the foot candles (as the height of the tank will be in feet) I calculated the intensity at the bottom of the tank as 1,386.5 footcandles.
Now lets take a look at a 10g with 4 WPG. That works out to 40 watts which means 836.36 lumens. Footcandles are equal to lumens/sq. feet, that works out to 1672.74. Because the distance from the source to the tank floor is less than a foot we will see the intesity actually increase (based on the definition of a foot candle, this makes sense). I calculated that to be 3763.658 footcandles. If this logic makes sense, 1.5 wpg (assuming same lumens/watt ratio which isn't a good thing to assume) would give the same amount of footcandles on the bottom of the tank.
I have rambled enough for now but I wanted to explain a couple of the reasons I don't completely buy the Rex Grigg article. It has been most thought provoking though. any thoughts?
