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Arend-Jan Both, Bruce Bugbee, Chieri Kubota, Roberto G. Lopez, Cary Mitchell, Erik S. Runkle, and Claude Wallace

. Photobiological safety. Like plants grown inside, people working in greenhouses and controlled environments are exposed to the radiation emitted by electric lamps. Depending on intensity and duration, ultraviolet, blue, and IR radiation exposure can have a

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Charles Barnes, Theodore Tibbitts, John Sager, Gerald Deitzer, David Bubenheim, Gus Koerner, and Bruce Bugbee

Photosynthesis is fundamentally driven by photon flux rather than energy flux, but not all absorbed photons yield equal amounts of photosynthesis. Thus, two measures of photosynthetically active radiation have emerged: photosynthetic photon flux (PPF), which values all photons from 400 to 700 nm equally, and yield photon flux (YPF), which weights photons in the range from 360 to 760 nm according to plant photosynthetic response. We selected seven common radiation sources and measured YPF and PPF from each source with a spectroradiometer. We then compared these measurements with measurements from three quantum sensors designed to measure YPF, and from six quantum sensors designed to measure PPF. There were few differences among sensors within a group (usually <5%), but YPF values from sensors were consistently lower (3 % to 20 %) than YPF values calculated from spectroradiometric measurements. Quantum sensor measurements of PPF also were consistently lower than PPF values calculated from spectroradiometric measurements, but the differences were <7% for all sources, except red-light-emitting diodes. The sensors were most accurate for broad-band sources and least accurate for narrow-band sources. According to spectroradiometric measurement, YPF sensors were significantly less accurate (>9% difference) than PPF sensors under metal halide, high-pressure sodium, and low-pressure sodium lamps. Both sensor types were inaccurate (>18% error) under red-light-emitting diodes. Because both YPF and PPF sensors are imperfect integrators, and because spectroradiometers can measure photosynthetically active radiation much more accurately, researchers should consider developing calibration factors from spectroradiometric data for some specific radiation sources to improve the accuracy of integrating sensors.

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Gregory D. Goins, Neil C. Yorio, and Lynn V. Lewis

Various electric lamp sources have been proposed for growing plants in controlled environments. Although it is desirable for any light source to provide as much photosynthetically active radiation (PAR) as possible, light spectral quality is critical in regard to plant development and morphology. Light-emitting diodes (LEDs) and microwave lamps are promising light sources that have appealing features for applications in controlled environments. Light-emitting diodes can illuminate a narrow spectrum of light, which corresponds with absorption regions of chlorophyll. The sulfur-microwave lamp uses microwave energy to excite sulfur and argon, which produces a bright, continuous broad-spectrum white light. Compared to conventional broad-spectrum sources, the microwave lamp has higher electrical efficiency, and produces limited ultraviolet and infrared radiation. Experiments were conducted with spinach to test the feasibility of using LEDs and microwave lamps for spinach production in controlled environments. Growth and development comparisons were made during 28-day growth cycles with spinach grown under LED (at various red wavelengths), microwave, cool-white fluorescent, or high-pressure sodium lamps. Plant harvests were conducted at 14, 21, and 28 days after planting. At each harvest under all broad-spectrum light sources, spinach leaf growth and photosynthetic responses were similar. Major differences were observed in terms of specific leaf area and weight between spinach plants grown under 700 and 725 nm LEDs as compared to plants grown under shorter red wavelengths.

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Gioia D. Massa and Jeff Norrie

) and Charles and Francis Darwin (1880) . During the same period, the use of electric lamps for plant growth began in the 1860s and expanded as lighting technologies developed. Wheeler (2008) provides a brief history of the development and adoption of

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Raymond M. Wheeler

Electric lamps have been used to grow plants for nearly 150 years with some of the earliest references being the work of Mangon (1861) and Prilleux (1869) (cited in Pfeiffer, 1926 ). As might be imagined, plant lighting technologies closely

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Elisa Solis-Toapanta and Celina Gómez

strategic locations within office, residential, and classroom environments, indoor light intensities using common electric lamps [e.g., fluorescent or light-emitting diode (LED) bulbs] can range from 5 to 1500 µmol·m ‒2 ·s ‒1 , depending on the time of day

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C. Michael Bourget

. 2. A comparison of the lifetimes of selected electric lamps. This is two to three times better than fluorescent or HID lamps and a 50-fold increase over typical incandescent lamps ( U.S. Dept. of Energy, 2005 ). LEDs are very rugged

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Allison Hurt, Roberto G. Lopez, and Joshua K. Craver

Lopez, 2014 ). To remedy this issue, greenhouse operations use high-intensity electric lamps to provide SL, with a standard target PPFD of 70 to 90 µmol·m –2 ·s –1 ( Lopez et al., 2017 ). Although HPS lamps are the current industry standard, LEDs have

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Gary W. Stutte

application in horticulture. Carl Wilhelm Siemens tested electric carbon-arc lamps on plant growth in the mid-to-late 1880s and coined the term “electro horticulture” to define this application of electric lamps. He postulated that “the horticulturist will

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Gioia Massa, Thomas Graham, Tim Haire, Cedric Flemming II, Gerard Newsham, and Raymond Wheeler

and research potential of these systems. Electric lamps such as fluorescent, high-pressure sodium (HPS), and metal halide (MH) have been used for decades to grow plants in controlled environments ( Sager and McFarlane, 1997 ; Wheeler, 2008