PPFD or Lux: What Is the Difference?
An explanation of which measurement to choose for what
Both PPFD and Lux are units used to measure the intensity of light. Lux is universally used to measure and compare the brightness of visible light that reaches a surface, also referred to as illuminance. Foot-candle measures the same light as Lux does, but it measures the intensity in candela per square foot whereas Lux measures in candela per square meter. PPFD on the other hand is referring to the amount of light suitable for active photosynthesis of plants.
Photosynthesis is the process by which plants grow from light and air. Photosynthesis takes place in special cells which get excited by a certain spectrum, or color, of light. This spectrum is described as photosynthetically active radiation or PAR for short. For the vast majority of green plants, the photosynthesis is mostly taking place in the red and blue spectrum, which is the reason why most LED grow lights provide a purple spectrum instead of white light to reduce the waste of unnecessary light energy. PPFD measures the amount of light in the PAR range that hits one square meter.
When To Use What
Whenever dealing with lighting conditions of plants, Lux might not accurately represent the real light intensity suitable for photosynthesis. It is therefore strongly suggested to measure and compare plant lighting conditions in PPFD which is exactly measuring the light as the plant utilizes it.