A point of confusion for some people entering Controlled Environment Agriculture (CEA) is how to measure a light’s output (flux) and efficacy. Common questions we are asked by those who have knowledge of traditional lighting metrics include: “What is your lumen output?” and “What are your lumens per watt?” Our answer to both questions is, “Lumens are only useful in measuring the amount of light in general illumination for people.” The metric that counts when it comes to growing plants is photons, not lumens.
Plants and people react differently when exposed to the same light. Lumens are a flux unit related to the human eye’s sensitivity to light. When testing lumen output, the total quantity of visible light emitted by a source is measured. Human eyes are less efficient at detecting the wavelengths that correspond to red and blue light, while the peak sensitivity of the eye is in the region of wavelengths corresponding to green light. The lumen measurement takes this into account by mimicking the eye’s sensitivity to various wavelengths.
Plants, on the other hand, do not see light like humans do. In fact, red and blue wavelengths happen to be the most important for driving photosynthesis and plant growth. The way lumens are measured is the reason why they are not a meaningful metric for CEA and determining if a light source can drive plant growth.