7 Myths About Grow Lights

Silly grower… lumens are for humans! That lumens are an appropriate way to measure light produced by a grow light is the all-time number-one indoor gardening myth. Measuring light intended for photosynthesis in lumens is just plain stupid. Let’s be clear: a lumen (scientific symbol: lm) is a measurement of how much light the human eye perceives. It does not, in any way, measure the light that drives photosynthesis. Period. Simply put, lumens measure the total amount of human visible light that comes from a particular light source.

Plants and humans evolved under the same light, coming from the sun. But humans and plants use this light very differently. Humans use most of the “visible light range” between 400nm and 700nm, but our eyes are focused on 500-600nm, mostly the green and yellow portions of the spectrum. Plants have a completely different response to light, focusing their absorption around 400nm-500nm (blue) and 600nm-700nm (red). They also absorb some light in the rest of the visible spectrum as well as non-visible light in the ultraviolet and infrared bands.

Measuring grow light output in lumens is an artifact of the lighting industry itself. Since light bulb manufacturers focus mainly on illumination for humans, they 射燈 publish their lamp specifications in lumens. Some countries require light bulbs to rated according to lumen output. Indoor gardeners have adopted this method for measuring the brightness of their grow lights since it’s generally available from the lamp manufacturers (at least up until LEDs came on to the scene).

When it comes to garden lighting, it’s time to stop thinking in lumens and start thinking about “photosynthetic photon flux density” (PPFD), which describes the density of photons reaching a particular surface area. PPFD is measured in “micromoles (μmol) per meter2 per second,” which is a more useful measurement for the light your plants receive than lumens. You need a quantum flux meter to measure how much photosynthetically active light energy is actually reaching your plants. When testing LED grow lights, make sure to pick a quantum flux meter that is specifically designed for LEDs, or your measurements will be off. Unfortunately, these devices are very expensive.