Light is the driving energy behind all plant growth in an indoor garden. It is the energy emitted from grow lights that allows plants to photosynthesize in the absence of the sun. This is why the best indoor gardeners make sure they have the most effective lighting systems in their gardens.

No matter how big or small a growroom is, a grower’s success is largely determined by the light energy. It is also a limited resource—regardless of whether a gardener is operating 1,000 watts or 10,000 watts, there is a limit to the amount of light energy for every growroom.

Growers who best use their available light energy will be more successful than growers operating the same-wattage rooms who are not making the most of their lighting. Another thing successful growers have in common is their understanding of how light energy works in an indoor garden environment.

Growers with a better understanding of the science behind light can make better use of horticultural lighting.

Physical Properties of Light Energy

Light sources emit energy in a 360-degree, circular pattern. This means the light spreads in all directions from the source unless redirected by a reflector or specialized lens. Some types of horticultural lighting, such as most LED fixtures, have specialized directional lenses already built into the fixture.

These lighting fixtures seem to emit light in a very directional pattern. Keep in mind that the light on these fixtures is actually being redirected and would otherwise emit light in the same 360-degree circular pattern as any other light source.

Light spreads as it travels and diminishes exponentially. In physics, this is known as the reverse square law. Most gardeners who have grown plants indoors have seen this first hand.

The fruit and flower sets on the top of the plant, closest to the light source, are always larger and more developed than those near the bottom of the plant that are farther away from the light source.

Although some of this has to do with the way the upper portion of the plant shades the lower portion of the plant, this commonly experienced phenomenon is mainly due to the reverse square law.

There is just less light energy available at the lower portion of the plant. The reverse square law is the main reason why many indoor growers have better results when growing short, robust plants instead of tall, lanky plants.

When growing outdoors, this is not an issue. The distance from the top of the plant to the bottom compared to the distance the light has already traveled from the sun to the Earth is insignificant.

In indoor horticulture, the inverse square law is relative and the distance from the top of the plant to the bottom is significant in comparison to the total distance the light is traveling. This is why it is so important for a grower to always have the light source as close as possible to the plant canopy without causing stress.

Maximizing Light Energy Efficiency with Reflection

Aside from minimizing the distance of the light source to the plant canopy, a grower can maximize light energy efficiency with reflection. Today, it has become practically commonplace for growers to use some sort of reflector with a high-intensity discharge (HID) lighting system.

Believe it or not, this was not always the case. In fact, just 15 years ago, it was common for many growers to hang bare bulbs in their growrooms. This, of course, wasted a lot of light energy as the light would travel in all directions from the light source and was not necessarily directed toward the plants.

These days, most indoor growers use reflectors to direct as much light as possible toward the plants. Light reflectors come in all shapes and sizes, but the primary function remains the same—to limit wasted light energy.

There are also light reflectors that are air- or water-cooled. These reflectors use air or water to remove a good portion of the heat created by HID bulbs as they emit light. The biggest advantage of these reflectors from a light energy standpoint is that they allow the grower to get the light source closer to the plant canopy without creating stress.

Some growers report yield increases of close to 10% by using an air- or water-cooled reflector. Many growers will also use reflective films or reflective paint on the walls of their growrooms to reflect light back toward the plants. Both light reflectors and reflective films can be made from a variety of reflective materials.

Flat white paint or flat white sheet metal are decent reflective surfaces. In fact, a flat white color has a reflective rating of around 85%. Mylar-type surfaces give a slightly higher reflective rating (just over 90%) and are commonly used by growers to create a reflective room.

Many reflectors use a pebbletone aluminum, which provides more than 90% reflectivity and effectively diffuses the light. The best reflector would actually be a mirror with a 100% reflectivity rating. However, mirrors are not the best choice for horticulturists because they do not diffuse the light in any way. In fact, if used in a light reflector, mirrors may be somewhat counterproductive because they could create additional hot spots.

Light Reflector Cross-patterns

When two or more light reflectors are placed in an indoor growroom and their light footprints (area of light that is projected from a reflector onto the plant canopy) overlap, this is known as a cross-pattern.

Setting up proper cross-patterns can go a long way in maximizing the efficiency of the room’s light energy. When the light from one reflector overlaps another, they combine energy. As previously discussed, light that travels farther from the light source becomes too weak to produce the desired growth but when that light is combined with another reflector’s light, the light energy can become powerful enough to produce high-quality growth. In other words, by properly spacing the reflectors, a grower can combine the light from two or more light fixtures to extend the area of usable light.

The way a grower sets up a cross-pattern will depend on the wattage of the light fixture, its distance from the plant canopy, and the size and type of reflector being used. A general rule of thumb is to observe a reflector’s footprint and then overlap the footprints of multiple reflectors, combining the light at the edges of the footprints.

With most reflectors, an overlap of a few feet is more than adequate to create a proper cross-pattern. Plants such as tomatoes or other fast-growing annuals that need higher amounts of light energy will require a more aggressive cross-pattern, which means the reflectors should be spaced closer together. Plants that require less light, such as lettuce or culinary herbs like sage, parsley and thyme, will perform just fine with lighting fixtures spaced farther apart.

Light Diffusion in Horticulture

Diffusion of light in horticultural applications refers to the way light energy is distributed over an area. The goal should be to create the most even light diffusion possible. This will create uniform growth within the garden. Light reflectors that use reflective materials designed to enhance light diffusion will produce more uniform growth and generate less hot spots.

Reflective materials that have built-in textures, like dimples (pebbletone aluminum), are designed to better distribute the light under the reflector. These materials cause the light to reflect in more than one direction, creating a more even distribution of light. Reflectors that produce better light diffusion are a great tool to help maximize lighting efficiency when using higher-wattage HID lights.

Some types of horticultural lighting, like tube fluorescents, are naturally better at light diffusion than HID fixtures. This is because tube fluorescents emit an even amount of light throughout the entire length of the bulb instead of all the light emitting from one singular focal point.

What results is an even light output directly under the fixture. This is one of the main reasons fluorescents are so popular for vegetative growth—they allow the grower to keep uniform growth throughout the entire garden without the need to rotate plants.

In Conclusion

Within our indoor gardens, we do everything we can to mimic nature and tweak variables to gain the highest yields possible. In nature, reflective materials are not necessary because of the great distance between the sun and the Earth.

In an indoor garden, however, reflecting light is one of the most important things a grower can do to get the most out of the available light energy. The use of a light reflector with an HID lighting system can increase the lighting’s efficiency by up to 50%. With energy rates continuing to rise, it is of the utmost importance to get the most out of every dollar spent on electricity.

A better understanding of light reflection, cross-patterns and diffusion could be the difference between a vibrant, successful garden and slow, spindly growth along with the frustration of wasted, costly electricity.

All of the most successful indoor horticulturists have one thing in common: they understand the effects of the inverse square law and the importance of using light energy to its maximum potential.