In the final installment of our series on measuring horticultural lighting performance, we examine other factors that play a role in horticultural lighting.
If you’d like to read the previous articles, you can find them here:
Electrical Energy Input (Fixture Input Wattage)
The input wattage of horticultural lighting systems is one of the most incorrectly-used metrics to describe the lighting capabilities of a fixture. The input wattage of a fixture doesn’t tell you anything about the fixture’s performance, as two fixtures with same input wattages can, and likely, will have drastically different light outputs (PPF). If a fixture tech spec sheet only shows you the input wattage, then you are still left wondering how many photons that fixture can produce. For example, if a 1000-watt fixture and 500-watt fixture have the same PPF, what would you buy?
Photon Efficacy (PE)
Horticultural lighting systems convert electrical energy into light energy (measured in PPF) just like a fan converts electrical energy into kinetic energy. Photon Efficacy (PE) indicates how efficiently a certain fixture can convert input electrical energy into light energy in the form of PAR (µmol/Joule or µmol/s/watt). In addition to driving the luminaires, input electrical energy may also be used to power drivers, fans, and any other moving parts in a fixture. These extra components decrease Photon Efficacy.
LED performance and lifetime is highly dependent on efficient heat management. As a consequence, efficient passive heat-dissipation techniques are superior to active cooling techniques, because active cooling requires electrical energy input, thereby reducing photon efficacy. Additionally, active cooling lowers the reliability of the lighting fixtures due to the nature of extra components and complex designs that increase the likelihood of component failure.
Fixtures with higher photon efficiencies reduce the total energy required for a grow-operation, reducing electrical operating expenses. They also help grow-operations qualify for energy efficiency rebates, depending on the local power company.
The Coefficient of Utilization (CU) is the fraction of photons reaching the canopy from a fixture. CU depends on the distance from the fixture to the canopy, fixture light distribution pattern (beam angle), and fixture form factor. A proper lighting simulation will provide a quantitative measure about the CU. Higher CUs reduce light loss and thus are considered more economical. However, there is no magical beam angle that fits all horticultural lighting applications. Depending on the cultivation setup and structural arrangement, optimum lighting layouts must be designed uniquely to achieve maximum CU.
LED fixtures that have proper thermal management techniques differ from HID fixtures in that HID fixtures radiate significantly more heat. This makes HID fixtures unfavorable for close-proximity lighting, as the plant canopy will experience excessive heating and growers will need to compensate with additional cooling units and fans to accommodate for the extra heat on the canopy. This is why most HID fixtures require significant distance between the fixtures and the plant canopy and are not typically suitable for high-density, vertical farming applications.
The ability to modulate the PPF of a fixture is a useful tool for precision agriculture. Properly designed LED lighting fixtures provide the ability to control PPF. Light levels can be altered based on crop growth stage and dimming an LED fixture increases its lifespan.
Safety is another important consideration when it comes horticultural lighting fixtures. Fixtures that are properly protected from power surges, excessive dust, and high humidity can save lives and money. Similarly, avoiding the use of harmful chemicals, such as mercury, protect workers and consumers alike.
A lighting fixture is just one item of the total grow-operation equation. The lighting recommendations should tie in with the environmental conditions, growing techniques and financial goals. The right horticultural lighting system is a partnership between lighting manufacturer and the grower that ensures mutual success.
Before selecting your lighting system, do some homework on your crop and business goals. Thoroughly understand lighting requirements in conjunction with other environmental factors. Review all of the aforementioned lighting metrics to select the proper lighting system that meets your goals. Remember, great lighting also comes with great customer support. Reach out and call as many lighting solution providers as you can to get a feel for their knowledge, support, and willingness to help you succeed.
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About the Author
Tharindu Weeraratne, PhD, is a horticulture scientist and a plant physiologist working with Fluence Bioengineering.