**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.

**Light output**in micromoles of photons per second.**Light intensity**in micromoles of photons per square meter per second, which is commonly, called PPFD (photosynthetic photon flux density).**Daily light integral**in total number of micromoles of photons per day.

Every lighting fixture emits a certain number of photons every second. For example, double ended HPS might emit 2,000 micromoles of photons per second and a fluorescent T5 might emit 50 micromoles of photons per second (similar to Transcend's LED T5).

Lets assume that a light fixture emits 1,000 micromoles of photons onto a two square meter area every second. The resulting PPFD would be 500 micromoles per square meter per second.

Great, now we know our PPFD but what about our DLI (daily light integral)? This is easy. Lets assume the same light is turned on for 16 hours per day. This is the same as 57,600 seconds. Then a one square meter area will receive:

- 500 micromoles per second x 57,600 seconds = 28,800,000 micromoles of photons per day

Now we are back in big number territory. To solve this growers usually use moles instead of micromoles when talking about DLI.

- 28,800,000 micromoles = 28.8 moles

Each type of plant has specific light intensity and daily light integral requirements. It is important to choose a lighting fixture that can provide the correct amount of light.

Now, as promised, the complicated definition. A mole is defined as the number of atoms in a 12 gram pure sample of carbon-12 which is 6.022141 x 10^23 atoms. A micromole is 0.000001 moles or 6.022141 x 10^17.