Measuring PAR inside the greenhouse, alongside global radiation outside, gives you a more realistic and accurate picture of the amount of growth light your plants actually receive. This is especially valuable when natural and artificial light come together.
Measure PAR inside the greenhouse
What does your crop actually receive?
Working only with Joules or watts can provide an incomplete picture of the amount of growth light a crop receives during the day. The same amount of global radiation outside does not always result in the same amount of PAR inside the greenhouse.
For example, a clear day with a radiation sum of 1,000 J/cm² can result in different PAR sums depending on the season and the angle of the sun. When the sun is high in the sky, the amount of PAR per Joule is higher than when the sun is low.
Cloud cover also changes the relationship between PAR and global radiation. Cloudy days generally have a higher PAR-to-Joule ratio than clear days. Measuring PAR inside the greenhouse therefore provides a more accurate picture of the light that is actually available to the crop for photosynthesis.
PAR makes life easier
Especially when using artificial lighting, working with PAR is easier and more accurate than relying on Joules alone.
For many years, formulas have been used to convert the output of lighting installations, measured in µmol/m²/s, into Joules. This allows sunlight and artificial light to be added together. With HPS lighting this is already complex, but with hybrid or full-LED installations it becomes even more difficult.
Using PAR makes this much simpler. The same unit can be used for both natural and artificial light, making it easier to understand the total amount of photosynthetically active radiation the crop receives.
Better light data, better growing decisions
More accurate light information can also help growers determine the appropriate daytime temperature in the greenhouse. Instead of basing decisions on the total amount of radiation outside, growers can work with the PAR that actually contributes to photosynthesis and crop growth.
The relationship between temperature and light is fundamental to maintaining a strong assimilate balance. The amount of light influences how many assimilates the plant can produce, while temperature strongly influences how quickly those assimilates are consumed for growth, development and respiration.
By measuring the light that actually reaches the crop, growers gain a stronger basis for matching temperature to available light. This helps support a more balanced crop, more consistent production and greater resilience.
Measure where it matters
Outdoor radiation measurements remain valuable, but they do not tell the whole story. The greenhouse covering, screens, season, solar angle, clouds and artificial lighting all influence the amount of growth light that ultimately reaches the crop.
Measuring PAR inside the greenhouse helps close that information gap. It gives growers a clearer picture of the light available for photosynthesis and a better basis for decisions about temperature and crop steering.
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