PAR Light | Understanding light metrics - PPFD, PPF and more

PAR light is quite a confusing term. Some use it as an acronym for photosynthetically active radiation, while others use it to describe parabolic aluminized reflector lights. In this article we’ll be talking about the former one - photosynthetically active radiation. We will also mention other light metrics, such as PPF, PPFD and light efficiency, and why it is important to keep them in mind when shopping for your next set of indoor grow lights!

What is PAR light?

As many of our readers may already know, light is nothing but a group of photons (light particles) flying through space. It exists on a electromagnetic spectrum, and depending on the wavelength of the light it falls into one of the following categories:

  • Gamma rays (< 0.01 nm)
  • X-rays (0.01 - 10 nm)
  • UV light (10nm - 400 nm)
  • Visible light (400 - 740 nm)
  • Infrared light (740 nm - 1 mm)
  • Radio waves (> 1 mm)

Wavelength is inversely proportional to the amount of energy which these particles emit. The higher the wavelength, the less energy they release. PAR light hits the sweet spot where it releases just the right amount of energy needed for plants to absorb to the fullest of their capabilities.

light spectrum wavelenghts
PAR light is also on this spectrum - however, it’s not marked as a category of it's own because it isn’t a specific type of light. It’s just a very specific wavelength, in fact it is located solely in the visible light spectrum - between the 400 nm and 700 nm wavelengths.

PAR light definition

Here’s the PAR light definition according to the Wikipedia article on the topic:

“PAR light designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis.”

If we were to interpret this definition in the most rudimentary way and explain it to people in simpler terms - we’d say PAR light is the amount of light usable by plants.


PPF, abbreviation of photosynthetic photon flux, is a light metric which measures the total amount of PAR light produced by a grow light each second.

It is probably the 2nd most important metric, reason for that being that it doesn’t actually show you the amount of light which lands on the plants - but rather the potential for it.

It comes in useful when you want to calculate the light efficiency of a certain light system at creating PAR. The units used to measure PPF are micromoles per second (μmol/s). Aside from that, it isn’t of much use to hobbyists and home growers, which is why manufacturers rarely place it on the packaging.


PPFD is short for photosynthetic photon flux density, and it measures the amount of PAR light which actually hits the canopy. This light metric is expressed in μmol/s.m², and it is included on the packaging of most lighting systems.

You can easily measure it by charting the light density over an area divided into sections, thus creating a socalled. If you measure the density in each section - you can add all the numbers together and divide them with the number of sections, and you’ll get the average PPFD of your growing area.

PAR map

PAR map
PAR maps usually have a wide range of numbers across the squares (which represent the μmol value). The numbers you want to get should be anywhere between 700 and a 1000 across the whole grid. If you get lower values, you should either bring the lights a bit lower down, or switch to a stronger set of lights.
PAR light chart
Lights which have a singular source will create hot spots in the middle of the grid and the light it releases will be uneven with low penetration as well. In order to avoid this, try using a grow light with even coverage. LED grow lights have shown to have amazing coverage and penetration compared to HID and HPS lights, so keep that in mind when doing your next round of shopping for grow supplies.

Grow light efficiency

Grow light efficiency is perhaps the most important lighting metric. This metric tells you the efficiency of a lighting system at converting electrical energy into PAR light.

Grow light efficiency is expressed in μmol/J. This metric is often included on the packaging, and when buying a lighting system always look for those with a higher μmol/J number. If a lighting system has good efficiency, you’ll be spending less electric energy while growing bigger buds!

Is lux and PAR the same?

No - they are both light metrics, however they don’t show the same type of information. Lux represents the intensity of light as perceived by the human eye.

1 lux = 1 lumen/m2

PAR vs lux

PAR can’t be expressed in lux or lumens because we wouldn’t be covering the full spectrum covered by certain light systems. It’s easier seen than explained as you can see  for yourself on the image above.

1 comment

Great article! Details all the relevant terms in an easy to understand explanation.


Ronald R Reppucci December 13, 2021

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