Two-minute explainer: Horticultural lighting metrics

One of the underlying benefits of LED technology is its ability to produce light sources tuned to almost any spectral demand.

If you’ve recently strayed into the world of horticultural lighting then it is likely that you’ve found yourself immersed in an entirely new alphabet soup.

This is because our lighting technology has been designed, unsurprisingly, with only people in mind. But plants don’t work in the same way that we do, so a new set of metrics is needed to understand if a lighting environment is suitable for the plants in question.

Here are some guidelines:

PAR: Photosynthetic active radiation

These are the wavelengths of light within the (human) visible range of 400nm – 700nm that drive photosynthesis. But PAR is not actually a measurement; it defines the nature of the light spectrum that specific plants need for photosynthesis to occur.

The metrics that concern horticultural illumination come from the establishment of the PAR environment.

PPF: Photosynthetic photon flux

PPF is expressed in μmol/s (micro moles per second). It is the total amount of PAR produced by a lighting system per second. It is not an indicator of the effectiveness of the system – how much light is actually falling on the plant, but is a good indicator of the system’s efficiency.

PPFD: Photosynthetic photon flux density

PPFD is expressed in μmol/m2/s (micro moles per square metre, per second). It measures the amount of PAR that actually falls on the plant. It equates to a luminance reading in the human sphere and is a ‘spot’ measurement on a specific location. As with an architectural luminaire, an assessment of performance can not be provided by a single figure (beam centre, for example). A polar diagram or equivalent is needed to understand how the luminaire actually performs.

Photon efficiency

The Photon efficiency of a system tells you how efficient that system is in converting electrical energy into PAR light.

Micromole = number of photons emitted from a luminaire. It enables growers to know how much light is falling onto a given area (PPFD), and how much light a plant may receive in a day i.e the daily light integral (DLI).

A mole is defined as the number of atoms in a 12 gram pure sample of carbon-12, which happens to be 6.022141 x 10 23 atoms.

The introduction of LED into horticultural lighting is challenging some of the conventional ways of looking at horticultural metrics, chiefly because LED lighting can be ‘tuned’ to be so much more efficient than traditional light sources. We can expect the LED industry to introduce yet more new metrics into the alphabet soup very soon.

  • A revolution is happening in horticulture. It’s a seismic shift that will change fundamentally how we grow plants – and it’s all down to lighting. Lux's Horticulture Lighting Conference  USA will take place in Denver on 17 October 2017. To find out more information and to register to attend  please click here

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