Luminaire datasheets from seven horticultural lighting manufacturers are considered to look for trends across reported performance metrics. Metrics to look for are explained below, along with those lacking relevance in a horticultural setting. This information applies to any luminaire regardless of light source type, though both HPS and LED fixtures are considered.
Look for: Spectral Graph, PPF (µmol/s)
Photosynthetic Photon Flux (PPF) is a measurement of the amount of photosynthetically active radiation a luminaire will produce per second. A limitation inherent in PPF is that all photons are weighted equally. A more appropriate metric would be the Yield Photosynthetic Flux (YPF), which weights photons based on the photosynthetic response of the particular plant. Currently, this metric is not reported for luminaires, as more research must be done to determine which wavelengths are optimal for growing cannabis (further refined by strain and growing objective). Until more research is done, PPF comes in a close second.
A spectral graph shows the distribution of photons across the active spectrum. An example is shown below, courtesy of Fluence.
While much debate remains regarding the ideal spectrum for cannabis, a spectral graph is important to rule out any missing wavelengths. Make sure the graph is filled in, without any valleys dropping below 20% normalized PPF.
What to avoid: Lumens (lm), color temperature (K)
Both lumens and color temperature are not representative of the energy used by a plant. Lumens measure the quantity of light seen by the human eye; plants can utilize light outside the visible spectrum. Color temperature is another characteristic of visible light.
Look for: Efficacy (µmol/J), Power Factor, L90
Efficacy (sometimes reported as efficiency, with equivalent units of µmol/s/W) is merely a measurement of how well the luminaire produces light. Values of at least 2 µmol/J are considered acceptable.
Power factor is another measure of efficiency. The lower the power factor, the more current is wasted by the luminaire within the circuit. Look for a power factor of at least 0.9.
The “L” rating defines the useful life of the fixture and is usually reported in hours. L90 is the amount of time the luminaire can operate before it's light output drops below 90% of its initial out-of-the-box value. This equates to the time at which a light fixture should be replaced. For HPS fixtures, L90 is usually around 10,000 hours – or about 18 months of usable life at 18 hours on per day. For LEDs, L90 is substantially higher. Look for an L90 of at least 50,000 hours. For a veg room operating at 18 hours per day, this equates to a lifetime of approximately seven years. For flower rooms operating at 12 hours per day, an LED fixture could last up to 11 years.
What to avoid: L70
Once a luminaire reaches L90, it will no longer be effective at producing consistent crop yields. L70 is a poor indicator of performance – effectively conveying a useless piece of information, as the luminaire will need to be replaced long before this point. The time for replacement is unknown when L70 is reported.
Look for: Light distribution (angle), Uniformity plot
Light distribution (sometimes reported as beam angle) refers to how the light intensity diminishes on a plane parallel to and below the luminaire. In practical applications, this plane would be representative of the crop canopy. If the maximum light intensity is directly below the center point of the luminaire, one-half of the beam angle in any direction connects a line from the luminaire’s center point to the parallel plane where the light intensity will be 50% of maximum. A wide angle of at least 120° is considered appropriate for cannabis when growing using 4’x4’ or 5’x5’ tables.
A uniformity plot is a visual representation of both PPF and beam angle and provides more details as to the fixture layout within a grow space. The following uniformity plot illustrates this, courtesy of Gavita.
The colored bar to the right denotes PPF values; the square grid on the plot indicates approximate 4’x4’ tables.
High PPF uniformity is one factor which contributes significantly to consistency in crop quality. Most luminaire manufacturers do not publicly display uniformity charts; this should be requested and verified before making a purchase decision.
What to avoid: PPFD (µmol/s/m2)
Photosynthetic photon flux density is a measurement of the average PPF over the lighting plane. This used to be a commonly-reported metric, though lighting manufacturers are shying away from this as PPFD says nothing about light uniformity. A high PPFD could be reported, but if the distribution is highly concentrated at the luminaire’s center, plants will be scorched in the middle of the canopy and stunted around the edges.
Look for: UL listing
Underwriters Laboratory is an independent testing laboratory dedicated to product safety. UL has no vested interest in any product but instead tests to nationally-recognized standards and vets product performance claims.
What to avoid: Unlisted luminaires
While final judgment is left to each cultivation facility, worthy of note is the significant time and money investment required by a company to obtain the UL mark on their luminaires.
Other necessary data includes the input power (W) and voltage (V) of the luminaire. While not a critical selection criteria, this data will be used by engineers and contractors during installation and for heat load calculations.