The OsmoBot Hydroponic test unit ships with a light sensor capable of monitoring Lux and PAR. Now that the algorithm for PAR is operational, OsmoBot users will be able to calculate PPFD. According to photobiology, one photon excites one electron during photosynthesis. In horticulture, we want to measure the number of usable photons hitting an area every second.
Written by Mark Calvarese.
The range of plant response to light is also ~400 – 700 nanometers wavelength, the same as the human eye. However, the peak sensitivities of plant pigments and human eyes are very different. Plant sensitivity peaks at blue and red wavelengths.
Chlorophyll and its accessory carotenoid pigments (carotenes and xanthophylls) harvest primarily blue and red photons for photosynthesis. Much of the green wavelengths are reflected, giving plants their green color.
PPFD is measured in micromoles per meter squared per second (μmol/m2/s). There are 1,000,000 micromoles in a mole, so if PPFD is known, you can multiply μmol/m2/s by the daily photoperiod (in seconds) and divide by 1,000,000 to calculate your DLI (Daily Light Integral).
If you know the spectrum of your lights, you can even modify μmol/s values by weighting specific wavelengths. This will give you a quantity called Yield Photon Flux (YPF).
Being that these terms are all so closely related, it’s hard to get them mixed up. Remember when shopping for lights to take into consideration the important units of light.
(Mark Calvarese is a Hydroponic/Aquaponic enthusiast currently living in Southern Oregon)