Which statement about PPFD is true?

• A. It measures photosynthetically active radiation delivered per minute.
• B. It measures photosynthetically active radiation delivered per second per area in micromoles per square meter per second.
• C. It measures only the spectral quality of light.
• D. It measures the total daily light integral.

Answer: (B)

PPFD measures how many photons in the photosynthetically active range are hitting a square meter each second, expressed as micromoles per square meter per second (μmol m^-2 s^-1). This is an instantaneous flux density—a momentary snapshot of available PAR over an area. That’s why the statement that best describes PPFD is the per-second, per-area measurement in μmol m^-2 s^-1. It isn’t per minute, and PPFD doesn’t tell you spectral quality by itself—that would be about the wavelengths present. It also isn’t the total daily amount of light; that comes from the daily light integral, which you get by integrating PPFD over the entire light period.