I have a question about the trade-off of light intensity and duration. I'm growing mostly phals, paphs, and phrags under LEDs. Given the specs from the manufacturer (confirmed, FWIW, by an iPhone app that seems pretty good!), the PAR readings are higher than suggested here. Given the setup, I can't conveniently move the lights higher. But I can keep them on for less time, getting the same DLI as described here. Is that a bad idea? One other note: the paphs have strong leaves, but lots of red, which I've heard can be from too much light.A typical PAR meter measures light (photons) in the 400-700 nm wavelength range that is most effective at stimulating photosynthesis. Light outside this range may still have some beneficial effect on orchid culture, but focusing on PAR is reasonable for most hobbyists. Most moderately priced ($200-300) PAR meters aren’t perfect, usually with a measurement dip at one end of the range or another, but they get a good assessment of this most important part of light for orchids. I use an Apogee meter that is simple to use and has been reliable for over 3 years. I consider the investment well worth it considering the money invested in my plants, lights, and other cultural supplies and equipment. Getting light correct is often the difference between poor growth with no blooming and attractive leaves with flowers.
These moderately priced meters typically output the PAR measurement as the instantaneous “photon density” in micromoles/meter squared/sec. With outdoor or greenhouse lighting from the sun, the photon density is changing continuously through the day as the sun rises and sets and as clouds or other obstructions appear and disappear. It takes a more sophisticated meter to record all the densities over the course of a day to result in the “daily light integral” (DLI). With steady artificial light it is easier because you just multiply the number of seconds in the day by the measured photon density per sec. A growing number of orchid culture and blooming experiments are reporting DLI as well as the peak PAR.
The following chart lists five different peak PAR densities that might typically be used with artificial lights and gives the DLI achieved with a photoperiod of 13.5 (second column) and 11.0 hours (third column). The peak PAR should probably be measured in the upper half of the leaf zone.
50 2.4 2.0
100 4.9 4.0
150 7.3 6.0
200 9.7 8.0
250 12.2 9.9
To put these DLI values in perspective with a few published studies, a Dendrobium experiment in a greenhouse in Texas had DLI that varied from 2.0 in the winter to 10.0 in the summer and achieved good growth and blooming. A Miltoniopsis experiment in another location had DLI that varied from 2.0 - 6.0 over the year and a Phalaenopsis blooming experiment used a range of 2.5 - 4.0 over a year for successful growth and flowering. Thus, our typical lower light orchids like Phalaenopsis and Paphiopedilums can be well served with PAR meter readings between 50 -100 and a photoperiod between 11.0 and 13.5 hours per day. Intermediate light examples might be Phragmipediums and Miltoniopsis which should be good at 100, maybe up to 150. Higher light orchids like Cattleyas and some Dendrobiums would then get the 150 - 250 levels, depending on the species. By the numbers, an average of 300 might even be excessive for some higher light orchids.
I’m wondering what Par meter you are using? The Apogee Quantum 200 accurately measures the LED spectrum (5,000 K daylight) the Quantum100 does not (it under reads the level), and I have to be careful at 300 PAR at top of leaf canopy on high light Cattleyas such as Dowiana, percivilana, L. purpurata, Semontiana and warscewiczii, even though they are recommended to take that light. I can get the beginning of burning if I’m not diligent watching them. My multi-florals are at much lower PAR (closer to 100) and bloom well.I've taken light-loving multiflorals as high as 400.