Growing houseplants and orchids under lights has opened up an incredible world of potential plants to growers that previously were just stuck with the windowsills their house came with. The decreased cost and increased availability of an incredible variety of growlight fixtures has led to an explosion of new houseplant growers. Unfortunately, the research about how to grow houseplants under lights hasn't quite kept up with how fast the technology is advancing. Simple questions about artificial light growing have gone largely unanswered by the scientific community and lighting companies as a whole.
One question we hear ALL THE TIME is, how much light do my plants need?
Turns out this isn't so simple to answer. Most light bulbs and lower-end grow lights that are available on the market today will give you their brightness values in lux, lumens, foot candles and sometimes even just watts (not even a brightness measure, yikes). The problem with these measurements is that they are measurements of how our human eyes see light. Plants "see" light a little differently than we do. Lux, lumens, and foot candles all measure light in a way that weights it towards the wavelengths that human eyes see best. This is called luminous flux.
Take a look at this curve above (Fig 1.) The x axis is the visible light spectrum, 400-700 nm. You will notice the single peak in the green/yellow range. Those wavelengths happen to be the colors that we see best. Lights and light meters that advertise their brightness by using luminous flux are essentially advertising their brightness weighted in the way that a human eye would interpret it.
In Fig. 2, you can see the curve for Ideal PAR plotted in black. (This curve has a peak in the red spectrum; this is because red photons are less energetic than blue ones, so it takes more of them to give the same amount of power.) Unlike the luminous flux efficacy, plotted in green; PAR measures the entire spectrum.
The third line that I'd like to point out is the absorptance, plotted in blue. This line is based on a classic study by McCree (1971), which measured the light absorbed by leaves of various crop plants, such as of beans, corn, and potatoes. The authors used an apparatus that shot specific wavelengths of light into a spherical cavity and measured the intensity of the reflected light. They performed this measurement twice: once with the cavity empty (in order to get a baseline value), and once with a leaf present in the cavity. The difference in the measurements told them how much light the leaf had absorbed.
Their experiment produced two notable results. First, plants are really good at absorbing light: over the 400-700nm range, they absorb between 74% and 93% of of the light they are given. Second, they absorb slightly less light in the green and yellow spectrum (525-575 nm). This is the reason that plants appear green. They are absorbing more blue and more red light, and they reflect back the green and yellow light to our eyes. And this is also why we see green and yellow so well. We evolved to search forests full of leaves looking for predators. You can see this very visible dip in the absorptance line around the green and yellow wavelengths. If we go back to the green Luminous Efficacy line for just a moment, you will notice that that line happens to PEAK in the 500-550nm range. In conclusion, by using lumens, lux, and foot candles to measure the light your plants are getting, you are ignoring a huge fraction of the photosynthetic light, and only looking at light that humans see best.
The last line on the graph (yellow) is looking at the actual photosynthesis rate of plants in response to different wavelengths of light. This is based on the same 1971 study by McCree. They calculated the photosynthesis rate by illuminating leaves inside a chamber with air flowing through it, and measuring the difference in CO2 concentration between the incoming and outgoing air. As you can see, the rate of photosynthesis over the 400-700nm wavelength range pretty closely tracks the original Ideal PAR line. Therefore, we believe PAR to be a really great measure of how much "Plant Light" your plants are getting. Forget lumens, buy a bulb that will tell you its brightness in PPFD.
So after all that, I still haven't told you how much light you need. Here are the PPFD Values we have our LED bulbs set to:
Low light plants (Phalaenopsis, Begonia, Ferns, Jewel orchids, some Paphs, African violets) 40-60 umol/m2/s
Medium light plants (Oncidium, Phrags, some Dendrobium, Pothos, foliage plants) 100-150 umol/m2/s
High Light Plants (Cattleya, Brassavola, Rupiculous Laelia, Some brighter-growing Aroids) 200-250 umol/m2/s
We use a 12-hour light cycle with these intensity values.