Measuring Photosynthesis & Consumer Interpretation: PAR, Lumens, PPFD & DLI

Words and images by Richard Jones.

Part 3: PPFD

Photosynthetic Photon Flux (PPF) is the total amount of Photosynthetically Active Radiation (PAR) emitted by a light. Measured in micromoles per second (µmol/s), PPF provides a general understanding of a light’s full output, but it does not measure the photons that actually reach your specimen. To rationalize the true utility of natural and artificial light, the Horticulture Industry relies on a narrow interpretation of photon delivery by gauging Photosynthetic Photon Flux Density (PPFD). Expressed as micromoles, per square meter, per second (µmol/m2/s), PPFD is an evenly weighted measurement of PAR that falls on a specific spot. As detailed below, the Apogee SQ-500 Quantum Sensor and the LI-COR LI-190R Quantum Sensor employ a balanced testing profile to assess PPFD across the entire PAR spectrum.

Consumer Interpretation of PPFD

When exploring the Grow Light Market, access to objective PPFD data is critical to understanding a grow light’s general utility, comparing brands and making an informed purchase. Therefore, all grow light sellers should publish some sort of PPFD assessment in association with a logical context. If a seller posts a single PPFD number, the height of the light is required to provide context. Or, if a seller posts an average PPFD value, the height and area is required to provide context. For example:

  • At a height of 1 foot on center, SH16 peak PPFD is  1,920 µmol/m2/s.
  • At a height of 2 feet on center, SH16 peak PPFD is  1,970 µmol/m2/s.
  • At a height of 3 feet on center, SH16 peak PPFD is  1,070 µmol/m2/s.
  • At a height of 3 feet over an area of 9 square feet (3′ x 3′), SH16 average PPFD is  805 µmol/m2/s.

When PPFD is observed in combination with an associated spectral image, more can be revealed about a grow light’s performance. For example, the spectral profile of some grow lights is fully contained within the limits of PAR (400 to 700nm), so any PPFD assessment approximates 100% photon delivery. Alternatively, the bandwidth of some grow lights exceed the limits of PAR, in which case a PPFD assessment will understate the actual photon count. As detailed below, the Solar Hemisphere 16 (SH16) induces the Emerson Effect by emitting Far Red (700 to 750nm). This range of light is outside the PAR spectrum, therefore typical PPFD is underestimated. Functional photon output may be as high as 10% greater:

  • At a height of 1 foot on center, SH16 peak PPFD is  2,112 µmol/m2/s.
  • At a height of 2 feet on center, SH16 peak PPFD is  2,167 µmol/m2/s.
  • At a height of 3 feet on center, SH16 peak PPFD is  1,177 µmol/m2/s.
  • At a height of 3 feet over an area of 9 square feet (3′ x 3′), SH16 average PPFD is  886 µmol/m2/s.

Part 4 explores DLI.

Happy growing from Zenith LED Grow Lights.

Words and images by Richard Jones.

Copyright © 2016   Zenith LED Grow Lights & ActionGroove Media Limited.

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