We will get to the complicated formal definition at the end of this article but for horticultural lighting it is quite simple – a micromole is a number of photons. Actually, it is a very large number of photons. 602 quadrillion to be exact. That’s 602 followed by 15 zeroes.
Obviously it difficult to work with such large numbers, so growers started using the more manageable term “micromole” (also written as μmole or micro-mole) rather than the cumbersome 602 quadrillion.