Do you know Why grow lights are purple?

by | Mar 22, 2024 | blogs, Grow Lights | 0 comments


Grow lights shine purple because they are arrays of bluish and reddish LEDs. Plants are most efficient at absorbing blue and red light to conduct photosynthesis, which results in sugars used for many of the plant’s metabolic processes.

Chlorophyl in thylakoid membrane

Fig 1 Photo by Kara Eads on Unsplash

Why blue and red light?

Visually, plant leaves are mostly green, which means they reflect that color – but as you can see, blues and reds are not reflected – they are absorbed by the leaf, and used in photosynthesis.

Chlorophylls love blue and red light.

Chlorophyll are molecules that are buried in a plant cell and are used to absorb light and convert it into chemical energy (eventually used to produce sugar).

There is Chlorophyll-b, whose peak  absorbance is blue light.  There is also Chorophyll-a whose peak absorbance is red light. Figure 2 is an absorbance chart that shows this. Note that sensitivity to green colors, although less absorbed, still can contribute to light absorption.

Fig 2 Chlorophyll absorption chart.

But why blue light?  Bang for the buck.

The wavelength frequency of a blue photon is very short and, therefore, highly energetic. Even though a blue photon, pound for pound, doesn’t directly increase chlorophyll activity compared to the red photon, blue photon energy catalyzes many of the biochemical reactions surrounding the entire process. In fact, farmers will add supplemental blue light to stimulate crop production (blue light supplemental effect).

It’s most likely an evolutionary feature that photosynthesis takes advantage of blue photon energy as the best bang-for-buck energy source for contributing to photosynthesis.

Fig 3 Blue light has shorter frequencies and is more energetic

But why red light? Extending the range.

Chlorophyll-a’s and to a lesser extent Chlorphyll-b’s ability to absorb red light simply extends the range of colors that contribute to photosynthesis.  Another evolutionary feature.

Actually, Chlorophyll-a peak absorption further widens this range in both directions compared to Chlorphyll-b (Fig 4).


Fig 4 Chlorophyll-a extends the range of colors available for photosynthesis.

Where is Chlorophyll-a and Cholorphyll-b? (deep dive.)

Chlorophyll-a and Chlorophyll-b are buried deep in plant cells, particularly in the green leaves.  Inside the cell, there are pancake-like structures called Thylakoids (figure 5).  These Thylakoids have a membrane, which contains the factory where light is absorbed and processed into chemical energy for making sugar.

Figure 6 shows this factory and two important stations: PS-II and PS-I. PS-II contains more Chlorophyll-b light-absorbing molecule, while PS-I contains more Chlorophyll-a.

Fig 5 Chlorophyll resides in Thylakoid structures, deep in a plant’s cells

Fig 6 Chlorophyll-b in PS-II, Chlorophyll-a in PS-I

How do you know if your grow lights are targeting the right colors.

It’s important to ensure you hit the “sweet spot” when purchasing grow lights or adjusting the colors on your Dynamic LED lights.  This is done using “Spectral” PAR meters to measure the color spectrum and compare it to a absorption curves.

Top-end indoor farmers and agriculture researchers employ this device as a must-have tool for optimizing efficiency and productivity.

LED light color and Photosynthesis alignment

Fig 7 Grow Light Spectrum with Chlorophyll absorption curve.

Purple grow lights make sense, but full spectrum lights?

It makes sense for grow lights to shine purple with blue and red LED bulbs.  They are adapted to efficiently target Chlorophyll molecules highly absorptive in those color ranges, as proven through experimentation and data.

It’s a feature of nature’s marvelous evolutionary consequence – Survival of the fittest.

However, aside from Blue-Red Grow Lights, there is another camp that promotes full-spectrum lighting.  In the next article we will explore that.


Fig 8 Full Spectrum Grow Lights

PG200N Spectral PAR Meter

Spectral PAR  measurement allow farmers and researchers the advantages of expert lighting, measuring peak color wavelengths to ensure grow lights are hitting the sweet spots for efficient photosynthesis.

PG200N product description

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United Power Research and Technology

UPRtek (est. 2010) is a manufacturer of portable, high-precision light measurement instruments; Handheld Spectrometers, PAR meters, Spectroradiometers, Light Calibration Solutions.

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