What is Photosynthetically Active Radiation (PAR)?

The ALTA® by Monnit Photosynthetically Active Radiation (PAR) Light Meter monitors the spectrum or wavelengths of light most plants need for photosynthesis—PAR. The PAR light spectral range is between 400 to 700 nanometers (nm).

The ALTA PAR Light Meter uses an advanced optical detector to provide highly accurate PAR measurements in 389 to 692 nm ± 5 nm under all light sources, including all modern colored LEDs.

This article will focus on PAR. For more about the two essential measurements that our PAR Light Meter captures, see this article about photosynthetic photon flux density (PPFD), and this one focused on daily light integral (DLI).

Knowing about PAR improves grow light selection and optimization.

Photosynthetically active radiation or PAR can help growers determine the type and volume of light they need to optimize plant health, flowering, and yield. Imitating sunlight for an ideal amount of time with PAR light can also help fine-tune light solutions with grow lighting density, crop canopy application, and measurements.

There are some important questions to answer when researching, setting up, and optimizing horticulture lighting systems. Among them are:

  • How much PAR does a grow lighting fixture produce?
  • How much PAR is available to the crop canopy?
  • How much energy are lights consuming to produce PAR?

Ultimately, understanding PAR can help growers realize a better return on their lighting investment, layout, and use. Knowing how much PAR each light produces, the amount of energy they use to deliver PAR, and how much PAR reaches plants is critical for lighting selection and optimization.

To better understand the need to measure PAR, let’s go back to when we used lux or lumen levels to measure light in various agriculture and commercial growing operations such as greenhouses. Humans can see lux and lumens as our eyes react to primarily white light. We have three receptors for light which help us visualize blue, green, and yellow light.

PAR to lux range

The figure above shows the lumen wavelength range compared with the PAR spectrum. As you can see, the spectral ranges are similar except for the wavelengths of blue and red light included in the PAR range, where photosynthesis is most potent with blue and red photons. As horticulturists, plant biologists, botanists, and more have come to recognize the value in measuring PAR for plants rather than lux, grow lighting manufacturers have followed suit.

PPFD and DLI—The two key measurements of PAR light

The PAR Light Meter monitors photosynthetic photon flux density (PPFD) and daily light integral (DLI). The PPFD measurement indicates PAR light intensity and its amount that reaches a square meter of a crop canopy each second. Learn why PPFD is so essential to determining PAR from this article.

The DLI measurement is also critical to assessing the effects of PAR from lighting on your crops. It’s important to PAR light optimization because it indicates the amount of PAR that lands on a square meter of a crop canopy over 24 hours. Discover more about the value of measuring DLI in this article.

Leveraging PAR for a grower’s advantage

Every crop needs to grow under optimal PAR light conditions. Plants suffer the natural consequences when there isn’t enough light or when there’s too much of it for their ideal growth and production processes. However, there are several tactics growers can take to manage lighting throughout the day and growing seasons.

Managing and measuring PAR should be at the center of your lighting operations. Optimizing photosynthesis, growing facility temperatures, irrigation management, photoperiod controls, and mitigating crop stress and energy costs can all be strengthened by focusing on PAR management.

Learn more about our PAR Light Meter in its user guide and datasheet.
Click here to view the ALTA PAR Light Meter.



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