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Reflected Light Energy: Footprints and Cross-Patterns in Your Indoor Grow Room

Light footprints and cross patterns can enhance your grow. Light footprints and cross patterns can enhance your grow.


For years growers have tried to maximize their yields while minimizing costs and consumption in order to make their gardens as efficient as possible. There are many contributing factors that make up a highly productive indoor garden but none are as important to understand as light energy.

Diminishment Factor

One of the most important concepts for indoor horticulturists to grasp is the way light energy diminishes exponentially from its source. This fact hardly affects plants grown outdoors due to their relatively short height in relation to the distance from the light source (the sun). Plants grown indoors, however, are greatly affected by this phenomenon. In fact, the light energy can diminish 4-5 fold from the top of the plant to the bottom of the plant. This is why it is advantageous to grow shorter, bushier plants indoors.

The best way indoor horticulturists can negate the effects of this exponential diminishment of light energy is to place the light source as close to the plants as possible. This can be done more easily when a grower chooses an air- or water-cooled reflector, which reduces the immense heat produced by HID lighting.

The light energy is the single driving force that allows for all other plant functions to occur in an indoor environment.

Reflectors, in general, will make a huge difference in terms of the light energy that reaches the plants (assuming a horizontal garden set-up). When using reflectors, especially multiple reflectors in one room, it is imperative that the grower understands footprints and cross-patterns.


The footprint or light footprint is the area of light shaped by the reflector. This footprint is directly affected by the size and shape of the reflector and also the distance the reflector is placed above the plants.

Many growers assume that the largest reflector is always the best choice because it creates the largest footprint. This is not true, especially if the grower is using a smaller wattage unit, like a 400-watt. Remember that light diminishes from the light source, not from the point at which it was reflected. In other words, the distance the light has to travel within the reflector before it is directed downward also diminishes the light energy.

Generally speaking, smaller reflectors suit smaller wattage bulbs and larger reflectors are more suited for larger wattages. A good rule of thumb is to try to select a reflector that creates a footprint that is equivalent to 40-60 watts per square foot. For example, if a grower is using a 1000-watt bulb, he or she should try to choose a reflector that creates a footprint that covers approximately 15-25 square feet. This is where the distance from the reflector to the plant canopy comes into play. Most growers can safely place their reflectors 12-24 inches from the top of the plant canopy. This is the distance that should be taken into consideration when deciding which reflector’s footprint best suits your area.


A cross-pattern, in relation to indoor horticulture, is when two or more reflectors’ light footprints overlap each other. For growers to get the most efficient light set-up possible, cross-patterns must be taken into consideration.

When the light from multiple reflectors is overlapped, the light energy is combined. By strategically placing their reflectors, growers can create cross-patterns that will increase productivity and maximize efficiency. That being said, every grow room is different and so are, in some degree, the grower’s intentions. Some growers will place reflectors very close to one another to create higher watts per square foot in a given area. This allows the grower to create monster plants that yield very large fruits or flowers. Some growers will place reflectors much farther apart, trying to maintain a consistent watt per square foot ratio. This will allow for a very efficient garden that will produce an abundance of medium sized fruit or flowers.

The light energy is the single driving force that allows for all other plant functions to occur in an indoor environment. Whether purchasing new equipment or using current lighting fixtures, a deeper understanding of light energy will help any grower create a more productive grow room.

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Last modified on Thursday, 29 November 2012 08:47

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