Foot Candles Measurement Limitations on lighting
What's Happening
Foot-candles (and Lux) are photometric units designed to measure light as perceived by the human eye, which is most sensitive to green and yellow light (555nm peak sensitivity). Plants, however, utilize light through photosynthesis using pigments like chlorophyll a and b, which absorb maximally in the blue (430-450nm) and red (640-680nm) spectrum. A foot-candle meter calibrated to human vision will give misleading readings for plant growth—showing high values for 'bright' green/yellow light that plants cannot use efficiently, while under-reporting dim red/blue light that drives photosynthesis.
How to Fix It
- 1
Switch to PAR/PPFD measurement using a quantum sensor meter for accurate plant light assessment
- 2
Understand that foot-candle readings work only for natural sunlight and broad-spectrum white grow lights as rough estimates
- 3
Avoid relying on foot-candle meters for red/blue LED grow lights—these will give wildly inaccurate low readings despite adequate plant-usable light
- 4
When converting estimates: Full sunlight ≈ 10,000 fc = 2,000+ PPFD; Bright indirect ≈ 500-1,000 fc = 100-250 PPFD
How to Prevent It
Use PAR meters (measuring PPFD in μmol/m²/s) for all plant light measurements. If only a foot-candle meter is available, recognize it as a rough proxy ONLY for natural sunlight or full-spectrum white light—not for colored LEDs.