Pioneering Hydroponics

Since 1976

Light Measurement and Spectrum Explained

Light Measurement

We as humans cannot see the spectrum of light that plants use to promote growth and flowering. Lumens are an indication of the usability of light for plants, however other methods of measurement exist that are more suitable.


Natural sunlight consists of a wide spectrum of colours, although it may look orange/yellow/red to the human eye, the sun actually produces a much larger spectrum of light, including blue, green and purple. These become visible when a rainbow is present. Plants make use of a range of colours present in the spectrum. This broad spectrum of colour can also be separated by wavelength, which is measured in nanometers.

Light Spectrum


The ideal wavelength for plants to turn light into energy (photosynthesis) is between 400nm and 700nm.  This segment of wavelength is commonly referred to as PAR (Photosynthetically Active Radiation)

Plants favour two specific segments of light spectrum – between 400nm-460nm (blue) and between 580nm-700nm (red).

Micromoles (µmol)

A micromole is a measurement of the number of particles/photons that plants need for growth and foliage. Micromoles cause plants to react to light. PAR is measured in micromoles.


Lumens measure the light intensity output from lights.

One lumen = one candle

Lumens can be used to measure light levels and to assess how well you would be able to see in the dark based on output intensity. This is not relevant when measuring the spectrum output which is important when using grow lights, as you will need to know what properties your light is emitting that are of use to the plant.


Lux measures the number of lumens that fall on 1 square metre.

50,000 lux = 50,000 lumens per square metre.

You can measure the intensity level of the lights in your grow room using Lumens. Using this information you can ensure lights are not placed too close or too far away from plants

Lux is not the best measurement of light for growers who want to monitor the quality of their light.


Professional growers now prefer to measure light by the photon count in the PAR area rather than by Lux and Lumens.

Plants are interested in the number of photons (light particles). Plants need 8-10 photons to bind one molecule of CO2.

A blue 600W light is less efficient for photosynthesis than a red 600w light as it produces less photons. Photons, can be measured using a quantum meter.

PPF (photosynthetic photon flux) refers to the number of photons a light emits per second.