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Light Measurement: Lumens, Lux, PAR and Micromoles

Measuring Grow Lights: PAR, Micromoles and PPFD

Alex Grady

Growing indoors? You need grow lights.

But which grow light is best for your plants? When measuring grow lights, you might hear people talking about:

  • Lumens and lux (doesn't tell you much about grow lights)

  • Spectrum, PAR wavelengths, photons, µmol/W & PPFD (really helpful!)

Lux and lumen are great if you're looking at how well a light will illuminate your home. But they don't tell you much about grow lights.

When measuring the light you get from grow lights, you need to think about:

  • Spectrum - what type of light your grow light emits

  • Intensity - how much plant-usable light you get

  • Efficiency / Efficacy - how much light per W

For that, you need to look at:

  • Light Spectrum: the wavelengths a grow light can emit.

  • PAR Wavelengths: these are wavelengths of light that plants can use (400nm - 700nm)

  • Photons: particles of light from the PAR range

  • Micromoles (μmol): how many photons. 1 μmol = 602 quadrillion photons!

  • Grow Light Efficiency (µmol/W): how many micromoles (μmol) per W

  • Intensity - PPFD (Photosynthetic Photon Flux Density): μmol per second, emitted by your fixture

We'll talk more about each one below, and how they relate to one another.

Light Spectrum


You've seen a rainbow. It's clear to see that sunlight is made up of different colours – aka wavelengths. Wavelengths of light are measured in nanometres (nm).

  • Humans are sensitive to wavelengths of 500nm - 600nm.
  • Plants are sensitive to wavelengths of 400nm – 700nm (the PAR area).

This isn't to say that wavelengths outside the PAR range are useless. LED grow lights tend to emit a lot of infrared light, which really helps keep your room warm in winter.

PAR Wavelengths (400nm - 700nm)

These wavelengths of light are often described as the PAR area (Photosynthetically Active Radiation).

Specifically, plants respond well to:

  • Blue light (around 400nm-460nm). Encourages vegetative growth, strong root growth and intense photosynthesis.

  • Red light (around 580nm-700nm). Important for photosynthesis, stem growth, flowering, fruit production, and chlorophyll production.


Most grow lights today are full spectrum - they emit both blue and red wavelengths of light. This means that you can use the same grow light throughout your cycle.

However, there are times when you might want to use one type of light and not another. For instance, if you're keeping a mother plant, you can use a blue CFL to ensure she stays in vegetative growth.

Some growers also still use a blue grow light for veg, then switch to a red one in flowering. People doing this tend to:

  • During vegetative growth, use metal halide lamps (blue light)

  • During flowering, use a sodium lamp (red light)

Photons and Micromoles

To measure how much light from the PAR range is emitted, you look at the photon and micromole count.

Sound confusing?

Here's what you need to know:

A photon is a particle of light from the PAR range. There are two types of photons:

  • A blue photon: has a short wavelength
  • A red photon: has a long wavelength

Photons are counted in micromoles (µmol). One µmol is 602 quadrillion photons!

Photons and Micromoles

Your plant is only interested in the number of photons.

It takes 8 - 10 photons to bind to 1 molecule of CO2 for photosynthesis.

As it happens, red light produces more photons than blue light. This means it's better for photosynthesis.

Grow Light Efficiency - Micromoles per Watt (µmol/W)

A higher wattage doesn't always mean a higher light output.

Why? Because some lights emit more µmol per Watt than others - they have a higher efficiency.

This is what really drives running costs down. Since you can hit a higher light intensity on a lower wattage, you can get a greater amount of light and emit less heat. This is the main reason people switch to an LED grow light in summer.

A typical HPS grow light will be around 1.6 to 1.9µmol per W.

Yet the Omega Infinity LED light hits 3.0µmol per W. That is almost twice the amount of light as some grow lights - way more bang for your buck. This is especially true of LED lights, which are known to last around 10 years+.

Intensity - PPFD (Photosynthetic Photon Flux Density)

To measure your light intensity overall, you need to look at your PPFD. It tells you the amount of light your fixture emits overall.

This is really useful if you're trying to compare one grow light to another.

You can measure this with a quantum meter or integrating sphere (aka Ulbricht Sphere).

It'll tell you how many photons are emitted per second. This is known as the photosynthetic photon flux density (PPFD) and is measured in micromoles (µmol/s).

Quantum Meter

The only (slight) issue with micromoles and photons... that only photons from the PAR range are counted.

Photons from wavelengths lower than 400nm (e.g. UVA & UVB), and higher than 700nm (e.g. infrared & far red) aren't counted.

Wavelengths outside of the PAR range have been shown to play a key role in plant health:

  • Hormone signaling

  • Formation of beneficial substances (e.g. essential oils & phytochemicals for defense against pests).

Just make sure you check out the spectrum charts of a light you want to buy. Some growers buy supplemental CDM lights too, which are known for broadening your grow light spectrum. Though they have a wattage of 315W, our CDM lamps gives your plant types of light that other lamps can't produce. It'll transform your crop.


Most grow lights are 240V. However some high end HPS lights are 400V. these grow lights tend be more efficient and more consistent than a 240V equivalent would be. They'll always outperform a 230 - 240V light.

You'll need a 400V ballast or complete light system to use 400V grow lights.

Lux and Lumens

There's a reason why we don't use lux and lumens for measuring your grow light output.


Lumens measure light intensity. One lumen is equal to one candle. It's pretty simple.

Unfortunately, lumens only relate to human eye sensitivity – they don't tell you anything about plant grow lights.

They're great for telling you how well a lamp will let you see in the dark, but that's it.

Remember: Lumens are for humans, PAR is for plants.



Lux tells you how many lumens fall on each square metre of a surface. So, an illumination of 50,000 lux is 50,000 lumens falling on each square metre.

Lux measurements tell you how intense your grow lamps are...this'll help you:

However, lux still won't tell you how useful an indoor grow light is to your plant.


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