Umol or micromole measures the amount of photons (light particles) that cover a specific area over a specific amount of time (per/s or per/day) when you can achieve maximum photons absorb-able by your plant species this is called light saturation point (LSP). Light saturation point is important because this allows your plant to take full advantage of nutrients, use less water and to maximize the C02 intake, provided the temperature and the humidity are at an optimal level. There are many metric units that are used for different purposes. The most basic way to measure light is in English units called, foot-candle. 0 foot-candle = darkness 100 foot-candles = your livingroom or your kitchen 1000 foot-candles = (June 21st) The longest day of the year at noon, at the equator with zero humidity. An instantaneous foot-candle measurement cannot take day-length into account but foot-candle measurements are certainly better than trying to make estimates of light with your eyes when analyzing light intensities (inside and outside) the growing space or greenhouse when using artificial light such as LED grow lights.