LED Grow Lights: Footprints and Planning

The artificial light source is the main energy driving plant growth within an indoor garden. Although every indoor garden is different, they all have one thing in common: there is a limited amount of light energy.

Some larger indoor gardens may have tens of thousands of watts of artificial light, while smaller gardens may only have a couple hundred watts. Regardless of the amount of light energy that is found in the garden, a grower should do their best to efficiently use that light energy.

After all, the more efficiently a grower uses artificial light, the more productive the garden will be. When discussing efficient lighting systems and set-ups for an indoor garden, it only makes sense to discuss horticultural LEDs.

LEDs for Heightened Efficiency in the Grow Room

Since the inception of LEDs as a horticultural light source, growers have been tempted by promises of heightened efficiency and better return on investment. Although not all LED lighting systems are equal, they do, in general, offer some major benefits to indoor horticulturists.

LEDs are comprised of individual diodes (solid state devices that actually emit light). These individual diodes can produce specific light wavelengths. In other words, each diode can produce a specific color in the light spectrum. This opens up a world of customizable spectral outputs for horticultural lighting systems. The ability to customize LED lighting fixtures is probably the largest advantage for indoor horticulturists.

Our knowledge of plant physiology has shown us that plants have a heightened photosynthetic response to particular light wavelengths. LED lighting systems are capable of producing exclusively, or a higher amount of these particular wavelengths, which means the light energy is more useable for the plants.

This in itself increases the efficiency of LED lighting compared with other horticultural light sources. Another major efficiency factor associated with LEDs is temperature. In general, LED lighting systems produce less heat than HID or fluorescent lighting systems. For most indoor horticulturists, heat is unwanted and many counter-measures, such as air conditioners or fans, are implemented to remove the excess heat.

Since LEDs naturally produce less heat, they automatically require less cooling equipment and this reduces the overall operational costs of the garden.

Diminishing Light Energy and LEDs

Regardless of the type of lighting technology, the inverse square law still applies. In other words, even when using an LED fixture, the light energy will diminish exponentially from its source. Put another way, the farther away the plant canopy is from the light source, the less light energy that will be available.

On the other hand, if the plant canopy is placed too close to the light source, an over-saturation or burning effect can occur. Indoor horticulturists should do their best to place the plants as close to the light source as possible, without causing damage to the plants. Some horticulturists refer to this spot as the “sweet spot” for lighting.

The sweet spot is the spot where light energy is maximized without causing stress to the plants. Horticultural LEDs, like any other artificial light source, will have a sweet spot where the available light energy for the plants is the greatest. For most LED lighting systems, this sweet spot lies two to three feet under the light source.

Setting up LED Lighting Systems for Maximum Light Efficiency for Plants

After choosing an LED lighting system, a grower should take time to consider how to make the most efficient use of the lighting system. One drawback of horticultural LEDs is that there is such a vast difference between each system. However, when it comes to placement in the garden, the two biggest factors affecting the set-up of an LED lighting system are total wattage and lens type.

Wattage for LEDs

Although LEDs’ unique ability to produce higher amounts of PAR (photosynthetically active radiation) per watt of energy consumed makes comparing LEDs’ wattages to other lighting technologies’ wattages difficult, a grower can still use the wattage of an LED system to help determine the proper placement and spacing between LED lighting fixtures.

For example, a series of 400W LED fixtures would be positioned closer together than a series of 1,000W LED fixtures (assuming other factors that make up the LED system are similar). Generally speaking, the higher the wattage, the farther fixtures will be positioned from each other.

A higher wattage LED system also generally equates to a larger light foot print. A very general rule of thumb is to have 30-40 watts of LED per square foot of garden space. In other words, a 400W LED can cover a 10-15 square foot area. Nothing perturbs LED manufacturers more than making light coverage recommendations based on wattage.

However, the recommendations for light coverage differ so greatly from manufacturer to manufacturer that home hobbyists have no other choice but to use wattage as a base of measurement and make their own determinations by experimentation.

When comparing wattages from various LED lighting systems, make sure to look at the actual wattage consumption and not the amount of wattage the manufacturer claims the fixture will replace.

Secondary Optics

Horticultural LEDs are equipped with primary lenses on each diode. However, some manufacturers will add a secondary optic to help focus the light into either a wider or narrower beam.

Secondary optics never increase the amount of light being emitted, but can concentrate that light into a more direct focal point or diffuse the light into a wider coverage area. Most horticultural LED manufacturers use secondary optics as a way to focus the light into a more intense beam.

When the light is focused and the intensity is increased in one area, light diffusion is lost and the total coverage area is decreased. Although not always the case, manufacturers that utilize lower wattage diodes (1-3W) in their lighting systems tend to gravitate toward secondary lenses; whereas manufacturers that utilize 5W diodes or higher rarely use secondary optics.

Light Planning for LEDs

Successful grow lighting starts with a carefully considered lighting plan that is tailor-made to suit your specific project. A lighting plan calculates the best possible coordination of fixtures, reflectors, patterns and distances between lights and crop. This requires a thorough analysis of various factors, such as the desired light level, the light distribution, the most effective lighting height, the greenhouse structure, irrigation, screening, and heating.

Most reputable lighting manufacturers offer light planning as a value-added service, and it is recommended growers take advantage of a manufacturer’s expertise rather than wasting valuable time, effort, and productivity trying to figure out the complexities by themselves. As an example, P.L. Light Systems offers free light planning.

Understanding how a lighting plan works is key to getting the most efficient lighting set-up possible. By properly spacing LED lighting systems, a horticulturist can combine the light energy of two fixtures and expand the area of usable light. The overlapping light combines the energy emitted from each fixture.

By itself, the diminished light would be inadequate to produce proper growth, but when combined, there is enough light energy to sustain healthy growth rates. It is important to note here that light uniformity­—uniform light distribution across the top surface of the crop—is equally as important as light intensity.

Read More: A Beginner's Guide to Calculating Light Needs

The way a horticulturist sets up their lights will depend on the wattage of the LED light fixture, its distance from the plant canopy, and the type of optics the unit uses. A general rule of thumb is to observe the LED lighting system’s footprint (area of light that is projected from the LED fixture onto the plant canopy) and then, at the edge of those footprints, combine the footprints of multiple reflectors.

With most LED lighting systems, an overlap of a couple feet is more than enough to create a proper cross-pattern. Some fast-growing plants, such as tomatoes or peppers, require higher amounts of light energy and will require a more intense cross-pattern. For these types of plants, the LED fixtures should be spaced closer together.

With taller plants, light needs to be directed deeper into the canopy, which is another key reason a more focused field of illumination makes sense. Other plants that require less light, such as lettuce or culinary herbs like sage, parsley, thyme, etc., will perform just fine with less aggressive cross-patterns.

One of the biggest differences between cross-patterns for LEDs verses cross-patterns for HIDs is that LEDs do not use the same type of reflectors as an HID system. The reflector of an HID system is the main determining factor of the lighting system’s footprint.

Although some LED systems have a reflector built in to the system, the primary and secondary lenses are mainly responsible for an LED system’s footprint. This is why it is so important for growers to closely examine the specifications of an LED lighting system before purchasing.

Comparing horticultural LEDs to one another is a lot like comparing apples to oranges. They are similar in some ways, but so different in other ways. Regardless of the lighting technology being used, some of the main principles, such as the inverse square law and how light energy combines in cross-patterns, still apply. The same benefits growers have received from utilizing cross-patterns of HID lighting systems are also applicable to LED lighting systems.

As with many aspects of indoor horticulture, experimenting with light placement and positioning is a key to unlocking a garden’s full potential, but taking advantage of a value-added lighting plan from a manufacturer will save time and maximize efficiency.

Horticulturists with multiple LED lighting systems who use cross-patterns for maximizing their light energy will be rewarded with better yields and a higher return on investment.