Compared to conventional lighting technologies, LEDs have several advantages:
- “High electrical efficiency: converting electrical energy into photons of light, (micromoles of photosynthetic light (µmol/m2s) per watt of energy per area (Wm2).
- High ‘relative quantum efficiency’ (RQE): the efficiency of particular wavelengths for driving photosynthesis. Red light has the highest RQE, blue light is about 70 to 75 per cent as effective, and green even less.
- Uniform light distribution.
- Durability and reliability.
- Compact size (reduces shadow).
- Acceptable cost.
- Effect of light quality on plant morphology, e.g. plant height and chlorophyll concentration. Design flexibility allows customized LEDs for specific crops.”1
INTEREST IN PLASMA INCLUDES RESEARCH AT HARROW
Others are working on a different kind of lamp: plasma. Dr. Xiuming Hao’s work at Harrow was reported previously in Greenhouse Canada. Plasma systems are not exactly new. Sager and Wheeler (NASA) note that plasma lamps were first demonstrated by Nikola Tesla, circa 1894.3 A plasma lamp contains an electrode-less design and an excitation source (a magnetron [microwave generator] or other radio frequency [RF] generator). Unlike fluorescent induction lamps with mercury and a phosphor coating, plasma lamps generate a continuous spectrum by exciting sulfur or halide molecules. Radiative efficiency is high: up to 70 per cent of power supplied is emitted as visible light.
Sulfur plasma lamps were developed by Ury and Wood (1980) and they commercialized several versions from 1995 to 1999. The development was supported by NASA, and in 1997 Fusion Lighting was contracted to develop an RF excited plasma lamp. The contract was not completed. “Sulfur plasma lamps are usually in the range of 700 – 3000W and have a photon efficacy of ~1.3 μmol/J. RF excited plasma lamps containing halides range from 250 to 500W and have a photon efficacy of ~1.0 μmol/J. Latest ceramic metal halide lamps yield ~1.9 μmol/J. and LEDs have reached 2.0 μmol/J.”3
Optex Lighting of Vancouver markets the ‘Plasma Lumixo Solsi,’ available in two forms: Diffuse light and Collimated beam. 4
DIFFUSE LIGHT RESEARCH LOOKS ENCOURAGING
Given the virtues of diffuse light glass (research from Wageningen UR) a diffuse plasma lamp might be extra interesting.
Like most plasma light units, i-Giant’s EcoPlasmaTM 900 has a spectrum “very similar to that of the sun and compared to other lighting technologies the ultraviolet and infrared is very low.”5 According to its website, “the fan inside the plasma lamp utilizes a high temperature motor ensuring a long lifetime of at least 20,000 hours” and “the technical grade magnetron used has a conversion efficiency of over 75 per cent.”5 They go on to say that “the EcoPlasma’s generally have 20 per cent higher light efficiency than similar lighting products” and “the inverter can run for more than 100,000 hours before first failure.” A step-less light dimming function (30 per cent to 100 per cent) is another benefit over HPS and units can even be adjusted remotely. With no “need for lead, arsenic or mercury vapour” there are also claims for environmental benefits.
Prof. Dickson Despommier, in his book “The Vertical Farm,” refers to “Organo LEDs” from General Electric. He describes these as organic compounds spread in thin – even flexible – plastic films so they can be physically manipulated to be configured around individual plants. What’s next – tricorders?
- Adapted from Erik Runkle, Michigan State University.
- J.C. Sager and R.M. Wheeler, Kennedy Space Centre, 4th International Meeting of UK CEUG, NCERA-101 & ACEWG, ‘Controlled Environments’, 2012, UK., accessed at http://www.controlledenvironments.org/minutes/2012/ceug_programe%202012.pdf