When you compare different lighting designs, make sure to compare apples with apples

September 29, 2022  By Pascal Van Megen

As a grower, you invest in supplemental LED grow lights because they power the yield and quality of your crops. In fact, the rule of thumb is that 1% light output equals 1% crop yield. So, it’s vital that your LED lighting investment delivers the full performance you paid for.

The performance is determined by the light intensity and light uniformity of the LED grow lights you use. If the light intensity produced by the grow lights installed is lower than what you expected from the design, there will be less yield. If the uniformity is inconsistent, individual plants will grow and develop at different rates and there will be uneven production in your greenhouse.

Before you start When you compare different lighting designs, make sure to compare apples with apples. This means taking three things into account:

The most important input parameters to check are:

Has the right product been specified? Check if the exact product that you have selected for your project has been used in the lighting design, with the right light output (PPF in µmol/s) and spectrum (blue/red/white/far red/…).

Have your specific design values been used, like the average light level at your crop (PPFD in µmol/m2/s) and the overall light uniformity?

What are the standard settings? The height of the grow light and crop (free height), reflection factors, and the size and position of the area that is used in the uniformity calculation all have an impact on the average light level and overall uniformity achieved.

1. Check the free height The first input parameter to check is the free height that specifies the distance between the LED module and the head of the crop. The free height can seriously impact the overall uniformity value. For high-wire tomato crops that have a limited free height of 1.50 to 2.50 m, for example, it can be a challenge to achieve a good overall uniformity value. Using an optimistic free height or calculating uniformity at floor level as if there is no crop, will positively impact the overall uniformity value.

The free height is calculated by measuring the distance between the eventual top of the crop and the mounting height of the LED grow light.

2. Check the reflection factors Another important parameter to check are the reflection factors used in the lighting plan. A reflection factor indicates the amount of light that is reflected by walls and other objects in a space. DIALux calculation software has originally been designed for indoor spaces like offices, where you will get reflection off the walls, ceiling and floors that impact the light level on your desk. To avoid being too optimistic about the outcome, the reflection values in DIALux are set at 0% for a greenhouse lighting design, because the glass in a greenhouse does not reflect the light from the grow lights.

3. Check the area that has been used to make light intensity calculations. The next thing to check is the calculation surface that has been defined. The size of the calculation area and the position of the grow lights within that area will seriously impact the average amount of active photons that reaches the surface of the crop (PPFD value in µmol/m2/s). When comparing lighting design results from different manufacturers, make sure that the calculation surface shows an equal number of maximum (peaks) and minimum (dips) light intensity values. This is the only way to calculate a realistic average PPFD value in the lighting design.

In the example below, you see two positions of a calculation area within the same light plan. Let’s assume that the light intensity peaks are located perpendicular to the grow light and the light intensity dips in between two grow lights. The values in the B scenario will generate far better average light intensity values, because the light intensity is calculated over an area that shows four peaks and one dip. The A scenario shows four peaks and four dips, and consequently, far better represent the reality after installation.

4. Check the size of the calculation surface Another important factor is the size of the calculation surface that will impact the overall uniformity value. When comparing lighting design results from different manufacturers, make sure the same calculation surface has been applied.

To represent a real-life situation, a calculation for the full compartment area should be made that includes the edges of the greenhouse. A calculation of the smaller centered area, which represents a typical production area should also be made. The calculation for the full compartment will generate lower light uniformity levels, because of the lower light intensities at the edges. So, make sure you always compare either calculations of full compartments or smaller areas within the full compartment.

5. Check the type of uniformity used The final parameter to check is the type of uniformity being used. Uniformity can be expressed in different ways. When you run the DIALux lighting design software, it provides you with different types of uniformity. When comparing lighting design results from different manufacturers, make sure the same type of uniformity calculation is applied. For a horticulture application we prefer to express uniformity as the average light intensity divided by the maximum light intensity, which best represents a real-life situation.

In Summary When comparing lighting designs there are lots of tweaks that suppliers can potentially make to their plan. If you want to make a proper comparison, you have to take a few parameters into account.

Pascal van Megen is an application engineer at Signify with a background in mechanical engineering. As an application engineer, Pascal ensures that growers are provided with high end horticulture lighting designs. He is providing internal and external training to customers and partners in the application of the Philips LED Horticulture products and systems. He acts as a consultant to customers and engineers, to drive continuous improvement of Philips LED lighting solutions.

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Five things to check when comparing lighting designs for LED grow lights. When you compare different lighting designs, make sure to compare apples with apples. https://www.greenhousecanada.com/five-things-to-check-when-comparing-lighting-designs-for-led-grow-lights/

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