Specialty crop growers have typically used high-pressure sodium lamps for supplemental lighting and incandescent and compact fluorescent lamps for photoperiodic lighting in greenhouses. In plant propagation facilities for tissue culture and research, sole-source electric lighting has typically been used, with fluorescent tubes as the main light source that is occasionally supplemented with incandescent lamps. Supplemental lighting typically requires high intensities (at least 50–100 µmol·m−2·s−1) because it is intended to increase photosynthesis and thus plant growth, whereas photoperiodic lighting only requires a low intensity (1–3 µmol·m−2·s−1) to regulate flowering of photoperiodic crops.
As LED technology developed and became less expensive, LED lamps have been used for a wide range of lighting applications, including for horticultural crop production. LED lamps have several advantages over traditional lamp types: 1) LEDs can be manufactured to emit a variety of narrow wave band colors, which facilitates generating a specific (sometimes adjustable) spectral output, 2) LEDs produce less radiant heat per photon, 3) LEDs can have a longer operating life, 4) individual LEDs are low power and small in size, allowing for small, low-wattage lamps (arrays) that can be placed closer to the plants, and 5) LEDs can be designed to produce light with a narrowly focused beam angle, potentially increasing the light-use efficiency by maximizing the capture of light by plant surfaces.
When prices started to drop and LED intensity and reliability increased through improved technologies and manufacturing processes, several companies started supplying LED lamps for plant growth applications. Despite a few early adopters, many growers were unsure of the financial benefits of these new lighting systems. In addition, the novelty of LED technology, the myriad possibilities for operational strategies, the lack of research data, and the often significantly higher investment costs caused trepidation among growers. Although many plant scientists are conducting research on best practices for LED applications (e.g., Mitchell et al., 2015) and some of the key performance metrics for horticultural lighting have been characterized (e.g., Nelson and Bugbee, 2014), so far little has been done to standardize and report the primary performance metrics of LEDs that are important for plant growth applications.
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