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Lighting Science: Artificial Light Sources; the Strengths and Weaknesses

Artificial Light Sources

 Artificial lighting began with the inefficient use of fire for light, followed by the advent of the candle which delivers a measly 12 lumens. In comparison, today’s discharge light sources are capable of producing over 150,000 lumens, with many HID and LED systems boasting efficacies of over 100 lumens per watt.

With respect to plant growth, artificial light sources attempt to deliver as many photons as possible (measured in micro-mol) per energy consumed (watts). Additionally, some advanced artificial light sources are engineered to optimize the spectral output of these light sources in order stimulate and support other desired plant functions such as short intermodal spacing and aggressive flowering while driving photosynthesis.

Largely governed by their technology make-up, various light sources have distinct advantages over others with regard to their raw output, their ability to be spectrally altered and their initial cost.

High Pressure Sodium (HPS)

HPS comprises the lion’s share of the installed base for plant growth lighting, primarily due to its high efficacy, long life and migration over the years from greenhouse environments to home hobbyists.

Advantages:

  • Efficiency: Some HPS lamps produce over 2.1 µmol per watt
  • Raw power in a small area: 1,000 µmol/m2/sec and higher, pending system efficiencies
  • Long life: Ratings of 20,000+ hours (although many are re-lamped after 4,000 hours or so)
  • Good supplemental light source for greenhouses, especially for northern climates where the heat can be used to warm the air
  • Good for flowering: HPS lamps are naturally weighted in the red/orange/yellow end of the spectrum

Disadvantages:

  • Limited ability to alter spectrum: While some lamps are advertised as dual-spectrum, HPS lamps have an extremely low relative amount of blue and violet output
  • Stretching: The natural red/orange spectrum can cause stretching (intermodal spacing)
  • Heat and IR: HPS lamps produce a good amount of heat as well as IR energy and not able to be place in close proximity to plants

Metal Halide (MH) and Ceramic Metal Halide (CMH)

Halide lamps, invented years after HPS, have traditionally been utilized for their cooler color to support vegetative growth. More recently, ceramic metal halide lamps have significantly grown in popularity due to their increased efficiencies and broader, more uniform spectrum.

Advantages:

  • Full spectrum: The broadest spectrum available, much higher than HPS
  • Efficiency: Some High CMH lamps deliver over 1.9 µmol of balanced light per watt
  • Long life: Most metal halide lamps have a rated life of 5,000 hours or more while many CMH lamps boast 25,000 hours
  • Good for vegetative growth: Very strong blue and violet output promotes strong vegetative growth and balanced, healthy growth
  • Also good for full cycle: Certain full spectrum MH and CMH lamps are used from veg through flower

Disadvantages

  • Heat and IR: Halide lamps produce a good amount of heat and emit IR energy, so are not able to be place in close proximity to plants

LED

Within the last five years, Light Emitting Diode systems have significantly improved in efficiency while coming down in price. This trend is expected to continue over the next several years, although potentially at a slightly slower pace.

Advantages:

  • Long life: LED systems are typically rated for 30,000 to 50,000 hours of life. Note, L70 is an industry standard to express the useful lifespan of an LED whereby it indicates the number of hours before the light output drops to 70% of the initial output.
  • Energy Efficiency: LED lighting systems can decrease energy consumption within greenhouses by as much as 40% when compared to HPS systems.
  • Dimmability: LED drivers can incorporate 0-10V dimming which allows them to be dimmed during peak daylight hours.
  • Ability to tailor spectrum: Individual LED’s can be selected with the design of LED fixtures to deliver light in specific areas of the spectrum.
  • Low heat: LEDs can be located very close to or within the plant canopy as they produce very little heat.

Disadvantages:

  • While technical advancements continue to be made, many LED systems lack the pure punch of HID systems, especially when compared to higher (1000W) HID systems.
  • Initial cost of ownership: In most cases, LED lighting systems cost $2 to $3 per watt compared to HID systems that cost as low as $0.50 per watt.

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