The PAR meter data from J.Downs, plus data I already had from others, is finally enough that I think it is now possible to predict the light intensity you will get for any size aquarium for any size T5HO fixtures.
To start with, light intensity drops off with the square of the distance you move from the source. This illustration shows why that is true:
Using that proportionality between light intensity and distance from the source, you can plot data from a variety of T5HO fixtures, and, from that determine how the intensity varies at various places on the substrate, and in the water column. For example:
This shows why the intensity near both front and rear walls of the tank is less than in the middle, and by about how much. Plus it shows why the intensity drops at the two ends of the tank, even though the bulb is the same length as the tank.
For years we looked for "watts per gallon" as a measure of the needed amount of light. That is a useless criteria, unless you consider only tanks of standard proportions, as well as with the same type of light fixture. If you take any light fixture and suspend it above a very large tank of water, that fixture will give about the same intensity directly under the bulb as when it is over a tank only 12 inches front to back.
From that it should be clear that it is only the height of the tank, the distance from the light to the substrate, that determines how intense the light is directly under the bulb. And, the intensity at the front and back of the tank is determined by the front to back dimension of the tank. The bulb length has no effect on intensity directly under the bulb, at least for 24 inch to 60 inch bulbs, other than helping to maintain the intensity near the ends of the tank.
So, it is possible to use the PAR data now available to determine with reasonable accuracy how much light you will get at the substrate level, for any height tank. This data is for T5HO lights of a quality comparable to Tek or Catalina fixtures. The data I saw showed no significant difference between those.
If one T5HO bulb, or row of bulbs, doesn't give you the intensity you want, you can use a second bulb, fairly close beside the first one. And, if you want better uniformity of the intensity over the front to back span of the tank, you can use two bulbs, separated by about 1/3 of the tanks depth.
Once you pick a light to get the intensity you want at the substrate, you are stuck with whatever intensity you get in other parts of the tank, except that you can raise the lights higher above the tank, which reduces the intensity at the substrate, and more importantly, reduces the extreme difference in intensity between the water surface and the substrate. So, the ideal light setup for most tanks we would use for planted tanks, would be 2 rows of T5HO bulbs, separated by about 1/3 of the tank depth, and raised enough to reduce the resultant intensity at the substrate to the level you want.
Cross posted on The Planted Tank Forum.
To start with, light intensity drops off with the square of the distance you move from the source. This illustration shows why that is true:
Using that proportionality between light intensity and distance from the source, you can plot data from a variety of T5HO fixtures, and, from that determine how the intensity varies at various places on the substrate, and in the water column. For example:
This shows why the intensity near both front and rear walls of the tank is less than in the middle, and by about how much. Plus it shows why the intensity drops at the two ends of the tank, even though the bulb is the same length as the tank.
For years we looked for "watts per gallon" as a measure of the needed amount of light. That is a useless criteria, unless you consider only tanks of standard proportions, as well as with the same type of light fixture. If you take any light fixture and suspend it above a very large tank of water, that fixture will give about the same intensity directly under the bulb as when it is over a tank only 12 inches front to back.
From that it should be clear that it is only the height of the tank, the distance from the light to the substrate, that determines how intense the light is directly under the bulb. And, the intensity at the front and back of the tank is determined by the front to back dimension of the tank. The bulb length has no effect on intensity directly under the bulb, at least for 24 inch to 60 inch bulbs, other than helping to maintain the intensity near the ends of the tank.
So, it is possible to use the PAR data now available to determine with reasonable accuracy how much light you will get at the substrate level, for any height tank. This data is for T5HO lights of a quality comparable to Tek or Catalina fixtures. The data I saw showed no significant difference between those.
If one T5HO bulb, or row of bulbs, doesn't give you the intensity you want, you can use a second bulb, fairly close beside the first one. And, if you want better uniformity of the intensity over the front to back span of the tank, you can use two bulbs, separated by about 1/3 of the tanks depth.
Once you pick a light to get the intensity you want at the substrate, you are stuck with whatever intensity you get in other parts of the tank, except that you can raise the lights higher above the tank, which reduces the intensity at the substrate, and more importantly, reduces the extreme difference in intensity between the water surface and the substrate. So, the ideal light setup for most tanks we would use for planted tanks, would be 2 rows of T5HO bulbs, separated by about 1/3 of the tank depth, and raised enough to reduce the resultant intensity at the substrate to the level you want.
Cross posted on The Planted Tank Forum.
