Light Measurement: Lumens, Lux, PAR and Micromoles
Growing indoors? A window sill just won’t cut it – you need grow lights.
But which one is best for your plants?
Most people pick the lamp with the highest lumens per watt…don’t do that. There are far more reliable measures… here’s what you need to know.
Lumens and Lux
People tend to talk about light in terms of:
- Lumens and lux (doesn’t tell you much about grow lights)
- Spectrum, Par, Photons and Micromoles (really helpful!)
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:
- Choose how far to mount your light from your plants (..more on mounting heights here)
- Get an idea of light degradation (...more on light degradation here)
However, lux still won’t tell you how useful a grow light is to your plant.
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).
PAR (400nm - 700nm)
Plants are sensitive to wavelengths of 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 growers use a dual spectrum light, which emits both blue and red light.
Other growers switch the lamp they're using for each stage of growth.
With this technique, plant growth is slower, but plants are better developed at the end.
- During vegetative growth, people tend to use metal halide lamps (blue light)
- During flowering, a sodium lamp is best (red light)
Photons and Micromoles
Don't go by lumens and lux - you'll learn more from the photon and micromole count in the PAR area.
Here's what you need to know:
A photon is a particle of light. There are two types of photons:
- A blue photon: has a short wavelength
- A red photon: has a long wavelength
Your plant is only interested in the number of photons.
It takes 8 - 10 photons to bind to 1 moleculed of CO2 for photosynthesis.
As it happems, red light produces more photons than blue light. This means it's better for photosynthesis.
Photons are counted in micromoles (µmol). One µmol is 602 quadrillion photons!
Remember, each photon is a particle of light from the PAR range. This means it's plant usable light.
Here are the typical readings for each light:
- A good 600W, 230V HPS lamp will emit around 1.8 µmol per Watt
- A good 600W, 400V HPS lamp will emit around <1.9 µmol per Watt
- A good 315W, 400V CDM (ceramic discharge metal-halide) produces around 1.9 µmol per Watt
- A good 1000W, 400V lamp registers around 2.1 µmol per Watt
The only (slight) issue with micromoles and photons...
...is that only photons from the PAR range are counted.
Photons from wavelengths lower than 400nm (e.g. UV-A & UV-B), 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 signalling
- Formation of beneficial substances (e.g. essential oils & phytochemicals for defence against pests)
What about Watts and Volts?
When measuring micromoles, you're counting how many are emitted per watt.
If a lower wattage lamp produces more micromoles per watt, a higher wattage lamp may still produce more overall.
However, watts aren't everything - make sure you pay attention to the spectrum of light lamps produce, too.
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 growers use a 230V - 240V light. However, 400V grow lights are more efficient and more consistent. They'll always outperform a 230 - 240V light.
You'll need a 400V ballast or complete light system to use 400V grow lights.
Measuring protons and micromoles
You can count the number of photons with a quantum meter or integrating sphere (aka Ulbricht Sphere).
They'll tell you how many photons are emitted from the lamp per second. This is known as the photosynthetic photon flux (PPF) and is measured in micromoles (µmol/s)