Lumens are for Humans
Now that we understand a little about the types of light, we can talk about how to measure it. For human applications, like lighting homes or workplaces, the intensity of visible light can be gauged by measuring radiant flux (or power), which is calculated as the sum of visible light (in Watts, or J/s). This metric, however, can be misleading because systems that fail to produce the full range of visible wavelengths can still produce high radiant flux values. For example, a lighting system that produces high levels of violet and red light might have a high overall radiant flux output, but since the human eye is more sensitive to yellow and green light, the light wouldn’t seem as bright as the radiant flux measurement suggested.
Instead, measurements of luminous flux are much more meaningful. This metric, which is expressed in lumens (lm), is similar to radiant flux but is weighted according to the sensitivity of the human eye to different wavelengths of light. Accordingly, light sources with higher lumen ratings are perceived as brighter. However, because the human eye is most sensitive to green/yellow light (550 nm), the metric is heavily biased toward these wavelengths. This means that measures of luminous flux underrepresent red and blue wavelengths and, consequently, that luminous flux is a poor indicator of the usefulness of light sources to plants, which mainly rely on red light for photosynthesis. For this reason, measures of luminous flux and lumen ratings are inadequate for assessing and comparing grow lights. Beware that lux, foot candles, and candelas are all additional measures of luminous flux. These units of measurement are based off the lumen (lm) and are equally inadequate.