Figure 1.
The photon distribution of four fixtures with similar photon efficiency.
Each line represents a cross section of the photon intensity below the fixture. The LED fixture (Lighting Sciences Group) uses optics to achieve a narrow distribution, with the majority of the photons falling in a concentrated pattern directly below the fixture. Conversely, the Cycloptics ceramic metal halide fixture is designed for even light distribution, and therefore casts uniform radiation over a large surface area. Since the area increases exponentially as the distance from the center increases, an equal photon flux farther from the center represents a larger quantity of total photons.
Figure 2.
Canopy photon capture efficiency.
As the plant growth area under the fixture gets smaller, wasted radiation often increases. This figure illustrates the concept of canopy photon capture efficiency. Two meters was chosen as a typical mounting height, but this can be scaled as a unit-less ratio. Multiple overlapping fixtures are typically used to minimize PPF variation over a large area.
Table 1.
Efficiency of individual LEDs at a drive current of 700
Figure 3.
Effect of wavelength on relative photosynthesis per incident photon for a single leaf in low light (less than 150 µmol m−2 s−1) [4].
Figure 4.
Effect of drive amperage and color on photon efficiency of LEDs.
Data for Philips Lumileds LEDs (May 2014), courtesy of Mike Bourget, Orbitec.
Table 2.
Efficiency of fixtures using integrating sphere measurements compared with flat-plane integration.
Figure 5.
Effect of electricity price on average annual cost over five years for two capture scenarios.
(A) When all radiation is assumed captured, the most efficient HPS fixture (Gavita) has a lower average annual five-year cost per photon than the most efficient LED fixture (Red/Blue fixture, Lighting Sciences Group). (B) When only a narrow region below the fixture (68°) is considered to be captured (e.g. on benches), the LEDs can have a lower cost per photon then HPS fixtures, but the cost per photon increases for both fixtures.
Table 3.
Photon efficiency and cost per mole of photons, assuming all photons (180°) are captured by plants.
Figure 6.
Effect of canopy capture efficiency on average annual cost over five years.
The cost per mole of photons for LEDs (Red/Blue LED from Lighting Sciences Group) becomes more favorable than the best HPS fixtures (Gavita) when the lighting area is less than 68° from center, assuming $0.11 per kWh cost of electricity and 3000 hours per year use (approximate cumulative operation time at latitudes from 40 to 50 degrees).
Table 4.
Cost per mole photons for four capture assumptions.