# **light position and plant growth**



## Anthony (Jan 11, 2005)

does the position of the light affect plant growth or is the watts per gallon the only thing to worry about. My tank is 18 inches from front to back and has 1 single lamp pc and one strip light totaling 80 watts


----------



## ringram (Jan 19, 2005)

The distance between the light source and the water does, indeed, play a major role in the plants' well-being. As you probably already know, the closer to the surface the light source is, the more intense the lighting. But in addition, light has what some people call a "footprint" - that is, a light of , say, 18" might have a footprint of 12", leaving some areas darker than others if the tank is longer than 12". The further you move the light away from the water, the wider the area covered due to water diffraction (I think?). Anyway, perhaps someone would care to elaborate, but what I have said is true, I just perhaps don't know the best way to explain it. I know there are others out there who would be able to add to this. Anyone?


----------



## Canoe2Can (Oct 31, 2004)

Diffraction has very little to do with what you are talking about here. Diffraction is just the way that light bends as it passes through different substances, and while this does happen in our tanks as the light passes through the glass top and the water, I don't think it would have a terribly large effect over the relatively small distances involved. 

What's going on here is the decrease in intensity over distance (can't think of the proper scientific term). Let's just say you have a particular light source that produces 1000 lumens per sq. foot at a distance of one foot. If you move it back to two feet, your lumens per sq. foot do not halve as some might expect, instead they quarter. Now you are spreading the same amount of light over 4 sq feet instead of one, so you end up with 250 lumens per sq. foot. Hence, in my 24 inch tall aquarium, the light is more intense at the halfway point as it is at the bottom. Raising the light a few inches off the surface to increase the spread of the light will inevitably lead to a drop off in intensity. Of course, if you need to, you need to, and 2 inches won't make a huge difference.

I also have a tank that is 18 inches wide. I have it lit with a 2 x 65 Watt Aqualight, and this seems to work okay. I keep the highest light plants in the middle right under the bulbs. I use lower light plants at the front and back of the tank.


----------



## glass-gardens.com (Apr 14, 2004)

I have two tanks set up within 4 to 5 feet of each other, I run blackouts routinely but not at the same time, when I run a blackout on one tank, the faster growing plants at least start to grow towards the tank still lit.

I generally try to achieve consistent lighting throughout the tanks and use taller plants to shade those needing less light. I find that the plants wanting higher light tend to grow towards the the area of highest intensity, at least stem plants and have used this to my advantage, encouraging them to grow in a certain direction.


----------



## Canoe2Can (Oct 31, 2004)

Now that I've thought about this a bit more, I'd better qualify a thing or two. For one thing, we generally use flourescent bulbs that produce light over a certain length instead of from a single point like I mentioned in my example. That means the decrease in light will be as you move further front and back of the light strip (the top edges of my tank at the front and back are practically dark). And having a good reflector will make a difference as well. 

And I do understand what ringram was saying. At a certain height, a light will have most of its effect over a certain width. Raising the light will increase the width that it lights well, but will also reduce the intensity over that width.


----------

