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From physics to fixtures to food: current and potential LED efficacy

Open access
Kusuma, Paul, Pattison, P. Morgan, Bugbee, Bruce·2020·Horticulture Research
DOI 10.1038/s41438-020-0283-7PMC PMC7105460
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Abstract

Light-emitting diodes (LEDs) have enabled a historic increase in the conversion of electric energy to photons, but this is approaching a physical limit. The theoretical maximum efficiency occurs when all input energy is converted to energy in photosynthetic photons. Blue LEDs can be 93% efficient, phosphor-converted “whites” 76% efficient, and red LEDs 81% efficient. These improvements open new opportunities for horticultural lighting. Here we review (1) fundamental physics and efficiency of LEDs, (2) the current efficacy of LEDs, (3) the effect of spectral quality on crop yield, and (4) the potential efficacy of horticultural fixtures. Advances in the conversion of photons to yield can be achieved by optimization of spectral effects on plant morphology, which vary among species. Conversely, spectral effects on photosynthesis are remarkably similar across species, but the conventional definition of photosynthetic photons (400–700 nm) may need to be modified. The upper limit of LED fixture efficacy is determined by the LED package efficacy multiplied by four factors inherent to all fixtures: current droop, thermal droop, driver (power supply) inefficiencies, and optical losses. With current LED technology, the calculations indicate efficacy limits of 3.4 µmol J −1 for white + red fixtures, and 4.1 µmol J −1 for blue + red fixtures. Adding optical protection from water and high humidity reduces these values by ~10%. We describe tradeoffs between peak efficacy and cost.

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Fig. 1 The general relationship between color temperature on percent blue photons (left axis), and the effect of color temperature on photon efficacy (right axis). Exact values vary among manufacturers. Photon efficacy in this graph is presented at a junction temperature of 25°C and 150mA. The efficacy values will shift if these inputs are changed, see below
Fig. 1 The general relationship between color temperature on percent blue photons (left axis), and the effect of color temperature on photon efficacy (right axis). Exact values vary among manufacturers. Photon efficacy in this graph is presented at a junction temperature of 25°C and 150mA. The efficacy values will shift if these inputs are changed, see belowCC BY · PubMed Central
Fig. 2 Effects of drive current on photon efficacy at a junction temperature of 25 °C. The dashed lines in this graph represent inadequate test data at low drive currents. However, low drive current (e.g., 65 mA) is used in LED fixtures. Blue photons have a lower theoretical maximum efficacy than red photons, based on Planck’s equation, which states that energy is inversely proportional to wavelength (E = hc/wavelength). Blue photons centered at 450nm can provide 3.76µmolJ−1, and red photons centered at 660nm can provide 5.52µmol J−1. This is more of a characteristic of the photons than of the LEDs that make them
Fig. 2 Effects of drive current on photon efficacy at a junction temperature of 25 °C. The dashed lines in this graph represent inadequate test data at low drive currents. However, low drive current (e.g., 65 mA) is used in LED fixtures. Blue photons have a lower theoretical maximum efficacy than red photons, based on Planck’s equation, which states that energy is inversely proportional to wavelength (E = hc/wavelength). Blue photons centered at 450nm can provide 3.76µmolJ−1, and red photons centered at 660nm can provide 5.52µmol J−1. This is more of a characteristic of the photons than of the LEDs that make themCC BY · PubMed Central
Fig. 3 Effects of junction temperature on photon efficacy. Note that higher drive currents increase the junction temperature. The dashed lines in this graph represent temperatures below 25 °C, and therefore temperatures below ambient conditions. Reducing the temperature below ambient would be an energy requiring process
Fig. 3 Effects of junction temperature on photon efficacy. Note that higher drive currents increase the junction temperature. The dashed lines in this graph represent temperatures below 25 °C, and therefore temperatures below ambient conditions. Reducing the temperature below ambient would be an energy requiring processCC BY · PubMed Central
Fig. 4 Historical, current, and projected LED package combination efficacy of a 20/80% ratio of blue and red LEDs. This is a weighted average of the two LEDs. The figure constrains LED performance to specific current and temperature operating conditions as discussed above. In practice, the current technology point is not constrained to these conditions so it can be higher than what is shown in the figure
Fig. 4 Historical, current, and projected LED package combination efficacy of a 20/80% ratio of blue and red LEDs. This is a weighted average of the two LEDs. The figure constrains LED performance to specific current and temperature operating conditions as discussed above. In practice, the current technology point is not constrained to these conditions so it can be higher than what is shown in the figureCC BY · PubMed Central
Fig. 5 Example long-term depreciation of LEDs based on temperature. LEDs will depreciate slower when operated at lower temperatures. Specific rates of depreciation for LEDs will depend on color, current operation, package type, and LED caliber as well as temperature. Colors on the graph represent temperature and not LED color
Fig. 5 Example long-term depreciation of LEDs based on temperature. LEDs will depreciate slower when operated at lower temperatures. Specific rates of depreciation for LEDs will depend on color, current operation, package type, and LED caliber as well as temperature. Colors on the graph represent temperature and not LED colorCC BY · PubMed Central
Fig. 6 A suggested fixture for high efficacy. The Y-axis assumes one watt of input power
Fig. 6 A suggested fixture for high efficacy. The Y-axis assumes one watt of input powerCC BY · PubMed Central

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References 48 · Cited by 283

Research works printed but not linked: 1

  1. Shchekin, O. LED technology considerations for high luminance sources. DOE SSL R&D Workshop. https://www.energy.gov/sites/prod/files/2017/02/f34/shchekin_considerations_longbeach2017.pdf (2017).

Metadata

Authors
Kusuma, P., Pattison, P. M., & Bugbee, B.
Affiliations
Utah State University (US)
Corresponding author
Bugbee, Bruce
ORCID
Venue
Horticulture Research
Publisher
Oxford University Press (OUP)
Published
2020-03-30
Volume
7
Issue
1
Pages
56-56
ISSN
  • 2052-7276 (ISSN-L)
  • 2662-6810
DOI
10.1038/s41438-020-0283-7
PMC
PMC7105460
OpenAlex
W3013613785
Cited by
283
References
48
Citations by year
2026: 44 · 2025: 59 · 2024: 51 · 2023: 41 · 2022: 34 · 2021: 39 · 2020: 15
FWCI
2.73
Type
journal article
Language
en
Open access
Licence
Locations
  • Horticulture Research (publishedVersion, cc-by)
  • Europe PMC (PubMed Central) (publishedVersion, cc-by)
  • PubMed Central (publishedVersion, cc-by)
  • PubMed (publishedVersion)
  • OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information) (submittedVersion)
  • Digital Commons - USU (Utah State University) (submittedVersion, cc-by)
Funders
  • Utah Agricultural Experiment Station (9260)
  • United States Department of Agriculture | National Institute of Food and Agriculture (2018-51181-28365)
  • National Aeronautics and Space Administration (NNX17AJ31G)
  • NASA-CUBES: https://cubes.space/
Topics
  • Life Sciences › Agricultural and Biological Sciences › Plant Science › Light effects on plants
  • Physical Sciences › Physics and Astronomy › Condensed Matter Physics › GaN-based semiconductor devices and materials
  • Life Sciences › Agricultural and Biological Sciences › Plant Science › Greenhouse Technology and Climate Control
Keywords
light-emitting diodes, plant growth lighting, light quality, blue LEDs, red LED, phosphor-converted white LEDs, optical loss, crop yield, plant morphology, photosynthesis

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CoChat. (2026). Citation check: From physics to fixtures to food: current and potential LED efficacy. https://app.cochat.ai/papers/10.1038/s41438-020-0283-7
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Kusuma, P., Pattison, P. M., & Bugbee, B. (2020). From physics to fixtures to food: current and potential LED efficacy. Horticulture Research. https://doi.org/10.1038/s41438-020-0283-7