Dispersion, Angles & Spread of light units

Dispersion

Learning from lasers

In most lighting applications, there is significant light loss due to dispersion; the light from the fixture is spread over a larger area as the distance increases.

An example of a light source where there is very little dispersion loss is a laser pointer, or lasers in general. Because the light beams from a laser are collimated (parallel to each other), there is very little dispersion and light loss due to distance. This allows the laser dot to be brightly visible from long distances, even though the wattage of a laser (an everyday handheld pointer may have a power rating of only ~0.02 watts) can be tiny compared to the projected distance.

Apply this concept to planted tank lighting - good reflectors/lenses that produce light beams that shine directly down (parallel to tank sides) are less affected by scatter.

Learning from sunlight

Sunlight is nearly parallel when it hits the ground; there is no loss of light due to distance. If you measure light levels at a height of 3 feet, you will see little loss of light.

In conventional LED lightbars with "120 degree" lenses, there is significant light loss due to scattering. Substrate light levels drop quickly when these fixtures are raised above the waterline because a lot of light is cast outside the tank. Due to the dispersion pattern, the surface area near the fixture receives much more light than the substrate areas at the sides of the tank. Although common, these types of fixtures provide the worst type of distribution in a planted tank.

How to reduce spread

A narrow beam fixture (i.e., a 45 degree lens LED light bar) suspended high above the tank will provide a much more even distribution of light throughout the tank environment than a wide angle fixture sitting on top of the tank. If the lens quality is good, there will be little light loss due to dispersion even though the fixture is hung high.

The downside of these fixtures is that they require a setup that allows the fixture to be hung high. Also, other than custom manufacturers, there are fewer fixtures on the market that come standard with narrow lens angles.

Another approach is to use a distributed light array. Multiple T5 tubes with good reflectors and/or a wide LED array will give a much more even distribution of light in a tank. The downside is that these fixtures can take up a lot of space or be bulky.

When does it matter?

It matters when you are picky about plant growth.
Distributed distribution reduces self-shading of plants; leaves lower on the stem receive incident light from distributed fixtures. This means that plants are generally better lit.

In contrast, with point source lighting, the upper leaves receive a lot of light, but the leaves in the lower layers are deeply shaded. Aquascapers who are extremely picky about the quality of plant growth will prefer lighting systems with better dispersion.

If you have a vertical hardscape, this is also important. If you have a complex hardscape that takes up vertical space, point source lighting may be obstructed by the hardscape and plants adjacent to the hardscape will be in shade.

Distributed light sources can reach more areas. If you prefer complex hardscapes, fixtures with good spread are essential.