Understanding Photosynthetically Active Radiation (PAR) – The Yield Support Team

The Yield's Microclimate Sensing System (MCSS) utilises the Digital Output Quantum Sensor by Apogee to measure photosynthetically active radiation (PAR) and is measured by the amount of micro moles of light per square meter per second (μmol).

The sensor provides growers with accurate sensor readings every 15 minutes which are available via the The Yield Analytics App. The PAR sensor is installed on either port 8 or 9 of your node.

What is Photosynthetically Active Radiation (PAR)?

Photosynthetically Active Radiation (PAR) is the amount of light available for photosynthesis, which is light in the 400 to 700 nanometer wavelength range. PAR changes seasonally and varies depending on the latitude and time of day. Levels are usually greatest during the summer at midday.

Factors that reduce the amount of PAR available to plants include anything that reduces sunlight, such as cloud cover, shading by trees, and buildings. Air pollution also affects PAR by filtering out the amount of sunlight that can reach plants.

Why is PAR important?

PAR is needed for photosynthesis and plant growth. Higher PAR promotes plant growth, and monitoring PAR is important to ensure plants are receiving adequate light for this process.

Visibility of PAR captured on your site will enable you to better understand whether bushfire smoke and/or tunnel age may have affected yield; and when measured in the canopy, can help to better understand the impact of pruning decisions.

How is PAR used?

PAR is also referred to as Quantum light, because it is measured in units of moles striking an area over time. PAR light spectrum ranges from 400 to 700nm – which is the extent of spectrum for photosynthesis, and extends into shorter wavelengths above violet into UV – the long-wave UV or UV-A ultraviolet, and longer wavelengths below red into near-IR – near infra-red.

Each crop species has an optimal light intensity that maximises photosynthesis and plant growth. Photosynthesis occurs mainly in the high frequency violet and UV and low frequency red and near IR of the PAR spectrum. This is why many plant species are green – green and mid-frequency light is reflected – i.e. the colour seen, whereas the PAR edge frequencies are absorbed for photosynthesis. Measuring PAR can indicate whether a crop is getting a sufficient amount of usable light.

Daily Light Integral

Adding each PAR measurement over a day’s sunlight hours can give deeper insights into a crop’s growth across the growing season. The daily measurement is called DLI – Daily Light Integral. Integral, because it adds up the full day’s PAR amounts, and because PAR varies over a day, the DLI is the area under the curve of the PAR. Currently The Yield app only shows PAR, with plans to add in DLI in the coming months.

How to spot a cloudy day and a clear sky day

During cloudy days, the PAR readings fluctuate as the clouds go over the sun, causing the readings to increase and decrease:

On clear sky days, you will see the PAR readings in a curve with very little disruption – the minimum, maximum and average readings are then all very similar values:

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On a sunny day in summer with no clouds, the typical maximum PAR reading will be around 2,000 μmol/m^2/s.

The Cloud Edge Effect – “Every cloud has a silver lining”

“A sudden increase in irradiance due to reflection of the passing cloud focusing more sunlight to the array”.

On cloudy days, you may notice higher than expected maximum readings compared to the average. These high readings are caused on partly cloudy days, where the edges of the clouds reflect additional sunlight back down towards the earth’s surface, causing the PAR sensors to record values far higher than we would see on clear, cloudless days. The events are quick passing and can be affected by cloud speed, shape sun angle and time of day. Theses readings are known in the industry as the cloud edge effect.

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Should you wish to investigate this further, we suggest looking into photovoltaic installations and voltage regulation.

How does PAR readings get used at The Yield?

When forecasting weather for the next 7 days, The Yield can use different inputs to predict what the weather will look like. PAR readings are used for wind direction and speed predictions.

Useful Resources

How photosynthesis coverts photons (light) into yield, by Bruce Bugbee.