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The measure of photons per second produced by a light. Photons are measured in micro (10^-6) [https://en.wikipedia.org/wiki/Mole_(unit) moles] there are a huge amount of photons coming from any light source. One µmol is actually 602 quadrillion photons. Simply put, this metric is the intensity per second from a light.
=== PPFD ===
Photosynthetic Photon Flux Density: its unit is μmol/m<sup>2</sup>/s. It is a measure of light intensity within the [[Light#PAR|PAR]] intensity (light that can be used for photosynthesis)range over an area per second. ThisThe ismaximum important to note when buyingPPFD a grow light. Forcan example,produce lightis Aimportant mayto provideknow anbefore intensitybuying ofas 800well μmol/s in a 1m<sup>2</sup> area, while light B providesas the samesize intensity in a 4m<sup>2</sup> area. Light B provides the same intensity but over four timesof the area, so it can supportdeliver a larger plant. In this case, light B has a greaterthat PPFD.
=== μmol/J ===
The light intensity a horticultural light produces per Joule of power it uses. This is a measure of its efficiency. At the time of writing, the most efficient LED package available is the [https://www.samsung.com/led/lighting/mid-power-leds/3030-leds/lm301h/ LM301H] by Samsung. It achieves 3.10 μmol/J @ 65 mA, 25°C. Note the efficiency will change depending on the temperature and current supplied.
 
== Measuring light intensity ==
[[File:Relative Spectral Power Distrobution for color temp.jpg|thumb|310x310px|An SPD chart of common light spectrums]]
The amount of photosynthetically reactive light ([[Light#PAR|PAR]]) present can be measured using a PAR meterdedicated device known oras a Quantum sensor. They are very accurate, however, as most smartphones have a lux meter, combineda withmuch thecheaper chartalternative ofis to use a "PPFD meter" app. A lux value can be combined with the grow light's spectral power distribution (SPD) to calculate the amount of PAR radiation. This is necessary because lux meters measure light in the frequency relevant to human vision and not photosynthesis.
A dedicated device may provide the most accurate results, but a cheaper alternative is to use a PPFD meter app on a smartphone. These apps use the phone's brightness sensor to find a lux value and ask the user to input the spectrum of the light they are using.
 
Simply put, these apps use the phone's brightness sensor to find a lux value and ask the user to input the frequency of the grow light they are using. Often 3500K or 4000K for "full-spectrum lights", Samsung produces a number of LEDs in this range.
For example [https://play.google.com/store/apps/details?id=com.homestudio.ppfdmeter PPFD Meter] on Android or [https://apps.apple.com/us/app/photone-grow-light-meter/id1450079523 Photone] on iPhone (Better implementations may exist).▼
 
▲ForSome exampleexamples are [https://play.google.com/store/apps/details?id=com.homestudio.ppfdmeter PPFD Meter] on Android or [https://apps.apple.com/us/app/photone-grow-light-meter/id1450079523 Photone] on iPhone (Better implementations may exist).
 
A competant online implementation exists at [https://www.waveformlighting.com/horticulture/convert-lux-to-ppfd-online-calculator waveformlighting].
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