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Is the McCree Curve outdated?

Plants need light to grow, so naturally, professional and industrial growers have spent a lot of time focusing on what type of light is best used by plants.

In the 1970s, scientific research on light absorption in plants resulted in the creation of the McCree Curve, named after the late Dr. Keith McCree. The McCree Curve, also known as the PAR Curve, shows the average light absorption across PAR wavelengths.

McCree Curve - PAR Curve

The Importance of PAR and How it Affects Plants.

What is PAR? PAR, short for Photosynthetically Active Radiation, is the range of light used by plants during photosynthesis.

This range of light generally begins at 400 nanometers and ends at 700 nanometers, akin to the range of light visible by humans. In outdoor settings, PAR is highest in the summer months, which prompts plants to grow, flower, and germinate due to the abundance of energy.

As PAR levels decrease in the autumn months, growth is slowed, and plants become dormant. In a controlled indoor garden outfitted with LED grow lights, this is no longer a concern, and year-round indoor plant growth is possible.

Professional growers measure the amount of Photosynthetically Active Radiation their plants receive as DLI, or Daily Light Integral. DLI is a measurement of moles of light per square meter per day. Monitoring DLI is important for Northern growers, who face extreme variance in seasonal DLI values.

How New Technology is Changing the McCree Curve.

PAR and the McCree Curve originated in the 1970s and formed the foundation of modern grow lighting.

Now, fifty years later in 2021, what has changed? Is this research still relevant? 400 nm to 700 nm light is still very important for photosynthesis—that will never change. However, Dr McCree’s research was bound by the technology of his time.

Prisms and rudimentary filters were unable to give the fine-tuned control found in today’s LED grow lights. Still, the horticulture industry has remained content to treat PAR as unchallengeable and absolute, which has led to gaps in effective grow lighting.

ILUMINAR Lighting is challenging the McCree Curve with their line of iLogic™ LEDs with optional spectrums from Full Spectrum to Full Spectrum plus UV+Far-Red.

ILUMINAR Lighting isn’t alone in believing that Photosynthetically Active Radiation (PAR) does not capture the full spectrum of light necessary for optimal plant growth.

Dr. Bruce Bugbee of Utah State University, a contemporary photobiologist, has conducted research that challenges old assumptions. Bugbee’s research shows that the range of light used by plants should be expanded to 380 nm to 760 nm and that some of the light in this expanded range is beneficial for plants.

This expanded range, also known as ePAR, contains Ultraviolet (UV) light and Far-Red light. Using LED grow lights to provide light outside of the PAR spectrum has been shown to increase disease resiliency and boost plant biomass.

Modern photobiological research has inspired researchers and companies to challenge old assumptions like the McCree Curve.

ILUMINAR Lighting is one of those companies, and they continue to push past PAR with their energy efficient iLogic™ LED family. By using Ultraviolet (UV-A) and Far-Red lighting, you can experience more complete plant growth.

Check out our full line of iLogic™LEDs at and follow ILUMINAR Lighting for updates on the future of LED and full-spectrum lighting.


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