Hi everyone,
I know this topic has been brought up countless times, but I hope this post can help clarify things a bit—and maybe, for once, I can contribute some useful info!
It seems like whenever light levels are discussed online, there’s a lot of confusion due to the many units of measurement involved, and often the guidance you find is downright contradictory.
Let's start with some conversions and handy links.
1 Foot Candle = 10.7639 Lux | 1 Lux = 0.092903 Foot Candle
To convert Lux and Foot Candle into PPFD - photosynthetic photon flux density - (measured in µmol/s/m˛), there’s no set formula because it depends on the light spectrum of the source. Here’s an approximate converter:
LUX to PPFD.
However, it’s important to look at how much light a plant receives throughout the day, not just at any one moment. For this, we use DLI - Daily Light Integral, which is simply the total amount of light a plant gets over a day. Here’s another converter:
PPFD to DLI.
In a greenhouse lit only by artificial lights with no natural sunlight, you calculate this based on the PPFD for the time the lights are on. With natural light, which changes throughout the day, you need to average it out, as explained well on
Ray’s website.
Since PPFD measures the light that’s actually available to the plant, not just the total light, it’s the key metric when talking about light in plant care. So, if you find references in Foot Candles or Lux, it’s probably assuming sunlight as the source, which means you’ll need to convert that to PPFD if you’re supplementing with artificial light.
Also, based on Ray’s point that Foot Candle values should be divided by two because the measure reflects peak daylight, meaning you can approximate it as half that level for the full day, I often find recommendations that seem, from my experience, way too high.
For instance, following AOS’s light recommendations, you could calculate:
Oncidium recommended 5000 Foot Candles → Divided by 2 = 2500 Foot Candles → 2500 Foot Candles = 26,909.78 Lux → 26,909.78 Lux of natural light = 618.92 µmol/s/m˛ → 618.92 µmol/s/m˛ x 12 hours of light = 26.74 DLI (at 30 DLI you can grow cannabis...)
I grow aroids (Monstera, Philodendron, Anthuriums, Alocasia) in an isolated greenhouse, and if I push the daily light above 5 DLI, the plants start to fade and droop. Since many orchids share similar habitats, I’d be surprised if they could handle such a high DLI.
For the aroids, after endless research and trials, I eventually just gave up on specific light levels and simply relied on observing the plants. In this case, if I burned a leaf, I could count on fast new growth (generally, I get a new leaf per plant every one to two weeks), but I’d hate to damage an orchid with its slower growth rate.
Have you found any reliable guidelines?
Also… Because of my work, I have access to professional light meters and spectrophotometers to analyze light, so I’ve started doing some spectrometric analysis of the lights I have.
I’m attaching a few graphs here showing the actual spectrum of a Mars Hydro TS1000 (a fairly pricey light), compared to a Chinese quantum board with Samsung LM283B LEDs (which costs less than a quarter of the price of the other one). I took measurements for the Mars Hydro 1000TS, both at half power and full power, and for the Chinese quantum board at maximum and minimum power, with the red LEDs both on and off. Honestly, the Chinese one actually seems better...
If you’re interested, I’ve also analyzed other light sources (like Sansi LEDs and other non-plant-specific LEDs). I’m working on the graphs and hope to share them soon. Plus, I’m working on a Python script to calculate actual PPFD values based on the correct reference spectrum measured by the spectrophotometer, without relying on approximations from online converters.
Hope this is helpful!
Talk soon,
Ale
Light Levels Madness
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