Successful grow lighting begins with a carefully considered light plan. A light plan calculates the best possible coordination of grow lights in terms of their distance and orientation from one another, as well as the crops. The trick lies in determining the ideal combination of these factors to ensure that you will achieve optimum light yields—with as few fixtures as possible.

What Combination of Lighting is Best?

The accuracy of the light plan depends on a thorough analysis of several factors—including the desired light level, the light distribution, the most effective mounting height, environmental conditions, and the construction of the growing facility.

The target light level (at the crop canopy) should always be the starting point of any light plan. All the other factors that will affect the layout and performance of the grow lights should be considered to determine how to achieve the desired light level with optimal uniformity and efficiency.

These factors include the dimensions of the facility, physical objects that stand between the lighting and your plants, the type of plants you are growing, and any additional energy and temperature considerations.

Dimensions of the Grow Facility

The dimensions of the grow facility, with the most critical dimensions being the length, width, and ceiling height of the space, as well as the anticipated finished crop height, are dimensions that need to be used to establish the number, spacing, and orientation of grow lights required.

The anticipated crop height is important in calculating the available distance between the mounted grow lights and the surface of the crop canopy. This distance will determine what grow light and/or reflector will deliver the optimal performance in that particular application.

For a facility with a low ceiling height for example, a low-profile system—that is, designed for optimal thermal management—will allow for a greater distance between the light source and the crop. Similarly, a reflector that delivers a wide distribution of light can enable growers to achieve optimum light intensity, evenly across the surface of the crop, with fewer grow lights.

Tip: When calculating the anticipated crop height, don’t forget to factor in the bench height.

Physical Objects that Obstruct Grow Lights from Reaching Your Plants

Physical obstacles like AC units, fans, heating/irrigation pipes, framing, shade cloth, etc. can not only create a physical obstacle to the mounting of the lights, but also create shadowing on the crops below.

A good light plan will always factor these obstacles into the plan to ensure that the spacing and orientation of grow lights is designed to deliver optimal light intensity—uniformly across the entire surface of the crop. Another consideration that is often overlooked in a light plan is the lighting in relation to walls and walkways.

The symmetric light distribution from a typical grow light results in a lot of light being projected onto the outer walls or walkways where it is wasted. A carefully considered light plan should include lights with optics that are designed to direct light only where you need it—the plant canopy. So, consider a light/reflector with an asymmetric distribution for these areas.

Type of Crop Being Grown

The type of crop will also impact the light plan significantly in terms of optimal light intensity, light distribution on the canopy, and eliciting a particular plant response. For taller crops, directing light deep within the canopy to reach the lower leaves is typically a challenge.

For these types of applications, the light plan should specify a luminaire/reflector that delivers a more focused field of illumination between zero and 45 degrees in the lower hemisphere of the distribution curve, producing uniformly deep light penetration into the canopy below.

When including LED grow lights in a light plan, the crop and the desired plant response will determine the spectral recipe of the grow lights. Blue light, for example, inhibits stem elongation and can enhance leaf pigmentation, whereas red light promotes stem elongation and is essential for flowering.

Energy and temperature considerations are also something to take into account when developing a light plan. In many applications, LED grow lights consume significantly less electrical energy than traditional lighting technologies, and also produce less heat so they can be placed closer to the plants, enabling higher light intensities without excessive heat.

Oftentimes, however, two (or more) LED grow lights will be required in a particular application to produce the same light output as a single traditional 1,000W HPS grow light. Growers should always do their due diligence when comparing light plans based on a variety of solutions (traditional/LED/hybrid) before deciding on which is the best fit for their particular application.

Look at the cost per micromole (μmol) of light delivered (based on the total number of grow lights that will be required to achieve the required light level in the space) for a true comparison.

Reputable lighting manufacturers will guarantee the light levels presented in their light plans—provided the grow lights are installed in accordance with the recommended plan, of course. If you are comparing light plans from multiple lighting suppliers, be sure that the data being presented to you is a true indication of the actual performance you can expect.

A new lighting system is a big investment and can significantly impact your yield, so ask questions. Is the proposed light output of the grow lights based on the lamp’s maintenance factor (end of life) for the best indication of actual performance? Does the light plan include reflections? If it does, it can be used to artificially boost the projected light levels.

Additional Tips for Planning Out Your Grow Light Layout

  • A true horticultural lighting supplier will always specify light levels in μmol/m2/s (PAR photons delivered per square meter per second), which is a measure of the light that plants react to, rather than lumens, lux, or foot candles, which are measurements based on the wavelengths that the human eye can detect.
  • Light uniformity (an indication of how uniformly/evenly the light intensity is distributed across the surface of the crop) is a key metric to consider in a light plan. Always look at the “min/max” ratio on the light plan for a true indication of light uniformity, rather than the “min/average” uniformity.
  • Although you should always consider the cumulative optometric performance of the grow light in a given application, the polar distribution curve of a grow light can provide a good initial indication of the type of distribution expected from the light. Look at the ratio of highest (A) to lowest (B) illumination points on the curve. The closer the ratio is to one, the more uniform the distribution.
  • An added advantage of lighting systems that deliver high-uniformities is that they enable wider row spacing, meaning you can achieve the desired light levels with fewer light fixtures.

For a better idea of how to set up your grow lights, check out the charts and graphs supplied by P.L. Light Systems in the PDF version of this article (Maximum Yield USA, Sept.)