Effect of Frequency and Duty Cycle of a Full-Spectrum Pulsed LED

Light Source on Plant Photosynthetic Rate

Wei Wei

, Guoyi Zhang

Jintian Lin

, Lei Chen

, Min Wang

and Yuxin Tong

5,*

Jiangsu Intelligent Optoelectronic Devices and Measurement and Control Engineering Research Center,

Yancheng Teachers University, Yancheng, Jiangsu 224007, China

Dongguan Institute of optoelectronics, Peking University, Dongguan, Guangdong 523808, China

Xuyu Optoelectronics Co., Ltd, Shenzhen 518101, China

Beijing Lvneng Jiaye New Energy Co., Ltd, Beijing 100085, China

Institute of Agricultural Environment and Sustainable Development, Chinese Academy of Agricultural Sciences,

Haidian District, Beijing 100081, China

Keywords: Full Spectrum, Photocarbon Ability, Photosynthetic Rate, PPFD, Pulse.

Abstract: In this study, we reviewed the research progress on the use of pulsed lighting for plants and presented the

concept of a full-spectrum pulsed LED light source panel. The changes in photosynthetic photon flux density

(PPFD) at 25 cm below the lamp plate in response to changing light source frequency and duty cycle were

measured. Furthermore, changes in photosynthetic rate with frequency were studied under a fixed duty cycle

of the light source. The photosynthetic rate was greatest at 100 Hz. Additionally, the photosynthetic rate with

a fixed light source frequency was highest at a duty cycle of 40%. Since PPFD cannot be maintained at a fixed

frequency or duty cycle alongside a change in duty cycle or frequency, the concept of photo carbon ability is

proposed here (photo carbon ability = photosynthetic rate/PPFD, which refers to the number of carbon dioxide

molecules that can be induced into photosynthesis by a photon). The changes in the optical carbon ability with

varying frequency were studied under a fixed duty cycle of the light source. The optical carbon ability was

greatest at 100 Hz. Compared with traditional continuous light sources, this pulse frequency is expected to

save 40% energy.

1 INTRODUCTION

The market scale of plant lighting is huge, and there

is large potential for specific uses in the field. The

total global area of greenhouses was equivalent to

2.835 million hectares, of which China accounts for

approximately 86% (Qichang et al., 2009).

Furthermore, it was estimated that a 200 kW high-

pressure sodium lamp is required on average per

hectare, although it is also estimated that 50–60% of

the energy used could be saved by switching to LEDs

instead. The market scale for LEDs purposed for plant

lighting was approximately 283 billion watts, and

considering this, appropriate plant lighting conditions

can be adjusted to reduce energy waste while also

promoting production (Haishan et al., 2021; Jianzhao

et al., 2022; Runa et al., 2021; Wang et al., 2021;

Zheng et al., 2022). The efficiency of light energy

utilization was generally low, at approximately 1%

for plants in the field, and can reach as high as 3% for

rice (Haxeltine et al., 1996; Ma et al., 2020;

Middleton et al. 2009; Nichol et al., 2002; Yao et al.,

2017). Various reasons contributed toward the

challenges pertaining to this efficiency, including

nutrient composition, light parameters, temperature,

and humidity (Angmo P et al., 2021; Nomura K et al.,

2011; Qichang, 2008, 2011a, 2011b, 2011c; Qichang

et al., 2011; Zhang et al., 2011, 2021). One important

challenge with light parameters is that the solar

lighting is continuous, and not pulsed.

Currently, the majority of plant lighting fixtures

utilize continuous lighting, including LEDs,

incandescent lamps, fluorescent lamps, and high-

pressure sodium lamps. However, plant pulse lighting

optimizes the efficiency of photosynthesis and

improves the utilization of light by adjusting the

frequency and duty cycle of the pulse light source,

thereby improving the output while simultaneously

saving energy.

144

Wei, W., Zhang, G., Lin, J., Chen, L., Wang, M. and Tong, Y.

Effect of Frequency and Duty Cycle of a Full-Spectrum Pulsed LED Light Source on Plant Photosynthetic Rate.

DOI: 10.5220/0011948800003536

In Proceedings of the 3rd International Symposium on Water, Ecology and Environment (ISWEE 2022), pages 144-149

ISBN: 978-989-758-639-2; ISSN: 2975-9439

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

Previous studies have investigated the impacts of

pulsed LED lights on plants and found that this can

lead to an improved photosynthetic rate (Cinq-Mars

M et al., 2021;

Kanechi M et al., 2011; Kozai et al.,

1999; Miliauskienė J

et al., 2021; Shimada et al.,

2016). For example, Kozai et al. (1999) in their study

on the effects of LED pulse lighting on

photosynthesis and growth in lettuces, found that at

the pulse lighting period shorter than 100 µs, the

growth of lettuce was 20% higher than that under

continuous lighting. In addition, Shimada et al.

(2011) showed that pulse light improved the levels of

a few plant indicators by more than 30% when

studying plant cultivation under a 180° phase

difference between red and blue light.

These studies have used plants for long-term

verification, with lengthy experimental periods, high

costs, several uncertain variables, and small

optimized pulsed light parameter conditions. To solve

these problems, the present study proposes

optimizing the parameters of the pulsed light source

through rapid measurements using a photosynthesis

instrument, LI-6400.

The experimental temperature was 23 °C, CO

concentration was 600 PPM, and the atmospheric

pressure was 1 atm. The LED lamps used consisted

of 2835 lamp beads of red, white, and yellow light

mixed into a full spectrum with a power supply that

had an adjustable frequency and duty cycle power

supply. The light spectral structure of the lamps after

mixing is shown in Figure 1, while the solar spectrum

is shown in Figure 2. It was found that the spectrum

after LED mixing highlighted the blue and red

wavelengths absorbed by chlorophyll, which can

promote the absorption of light by chlorophyll to a

certain extent.

Figure 1: Light spectral structure of full-spectrum LED

beads.

Figure 2: The light spectral structureof sunlight.

Figures 3–5 show the variation curves for

photosynthetic photon flux density (PPFD) 25 cm

under the lamp panel at varying frequencies. Figure 3

shows that in the range of 0–2000 Hz, PPFD

increased with increasing frequency, and Figure 4

shows that between 2000–20000 Hz, PPFD remained

relatively stable. Furthermore, Figure 5 revealed that

in the range of 2000–100000 Hz, PPFD decreased as

frequency increased.

Figure 3: Variation curve for PPFD data 25 cm below the

light plate with frequency 0–2000 Hz

Figure 4: Variation curve for PPFD data 25 cm below the

light plate with frequency 2000–20000 Hz.

Effect of Frequency and Duty Cycle of a Full-Spectrum Pulsed LED Light Source on Plant Photosynthetic Rate

145

Figure 5: The variation curve for PPFD data 25 cm below

the light plate with frequency 20000–100000 Hz.

For the experimental procedure, the appropriate

frequency and duty cycle were first selected, before

turning on the light for 30 min to allow both the LED

lamp and plant to stabilize. Then, the photosynthetic

rate of the plant was measured using LI-6400.

Figure 6: Variation curve for PPFD data and frequency at

25 cm below the lamp panel.

Figure 7: Variation curve for photosynthetic ability data and

frequency at 25 cm below the lamp panel.

In total, 150 full-spectrum LED beads composed

of 2835 patch mixed beads were used. The

experiments were divided into two groups. The first

group was for the fixed duty cycle and frequency

conversion and the second group was for a fixed

frequency with a conversion duty cycle. Finally, the

optimized optical parameters were obtained.

Frequency was varied from 1 Hz, 10 Hz, 100 Hz,

1000 Hz, 10000 Hz, and 100000 Hz, with a duty cycle

of 50%. Owing to the use of pulse width modulation

(PWM) pulse light, the photosynthetic photon

illuminance of the pulse light at different frequencies

and the same duty cycle both changed because of the

non-zero opening voltage. The PPFD data

corresponding to the above frequencies were 170

µmol·m

-2

-1

, 90 µmol·m

-2

-1

, 150 µmol·m

-2

-1

, 170

µmol·m

-2

-1

, 150 µmol·m

-2

-1

, and 65 µmol·m

-2

-1

respectively. It can be seen from Figure 7 that with the

variation in pulse light frequency, the resultant

photosynthetic rates were 2.55 µmol(CO

)·m

-2

-1

, 1.07

µmol(CO

)·m

-2

-1

, 2.98 µmol(CO

)·m

-2

-1

, 3.24

µmol(CO

)·m

-2

-1

, 2.68 µmol(CO

)·m

-2

-1

, 0.88

µmol(CO

)·m

-2

-1

, respectively. It can also be seen

from the figure that the highest photosynthetic rate for

plants was recorded at 1000 Hz.

Figure 8

Variation curve for PPFD data alongside duty

cycle at 25 cm below the lamp panel.

Figure 9 Variation curve for photosynthetic rate data

alongside duty cycle at 25 cm below the lamp panel.

ISWEE 2022 - International Symposium on Water, Ecology and Environment

146

Next, we studied the effects of pulse lighting with

varying duty cycles on plant photosynthetic rate

under a fixed frequency of 100 Hz. Figure 9 shows

that for these conditions the PPFD results associated

with the above frequencies were 59 µmol·m

-2

-1

, 113

µmol·m

-2

-1

, 194 µmol·m

-2

-1

, and 281 µmol·m

-2

-1

respectively. For pulse lighting with a duty cycle of

20%, 40%, 60%, and 80%, the respective

photosynthetic rates of plants were 0.90

µmol(CO

)·m

-2

-1

, 2.52 µmol(CO

)·m

-2

-1

, 3.83

µmol(CO

)·m

-2

-1

, and 5.20 µmol(CO

)·m

-2

-1

. When

frequency was 100 Hz, the photosynthetic rate of the

plants was highest at a duty cycle of 60%.

Figure 10 Variation of photocarbon ability alongside

frequency at 25 cm below the light plate.

To better measure the photosynthetic rate under

the unit PPFD, we propose a new concept here:

Photocarbon ability = (photosynthetic rate)/PPFD,

that is, the normalized treatment of photon

illuminance by photosynthetic rate (i.e., how many

carbon dioxide molecules a photon can convert).

Frequencies of 1 Hz, 10 Hz, 100 Hz, 1000 Hz,

10000 Hz, and 100000 Hz were applied, with a duty

cycle of 50%. Considering that PWM pulse light was

used, the photosynthetic photon illuminance of pulse

light with different frequencies and the same duty

cycle changed as a result of the non-zero switching on

voltage. Therefore, the plant photosynthetic rate was

normalized by the same amount of photosynthetic

photon illuminance, and the plant utilization rate of

light was measured using the normalized treatment of

photosynthetic rate on photon illuminance, that is, the

light carbon ability. It can be seen from the figure that,

with the changes in pulse light frequency to 1 Hz, 10

Hz, 100 Hz, 1000 Hz, 10000 Hz, and 100000 Hz, the

respective optical carbon ability was 0.015

µmol(CO

) µmol

-1

(photon), 0.012 µmol(CO

) µmol

(photon), 0.020 µmol(CO

) µmol

-1

(photon), 0.019

µmol(CO

) µmol

-1

(photon), 0.018 µmol(CO

) µmol

(photon), and 0.014 µmol(CO

) µmol

-1

(photon).

Figure 10 also shows that the optical carbon ability

was highest with a pulse frequency of 100 Hz.

Figure 11 Variation of photocarbon ability alongside duty

cycle at 25 cm below the light plate.

Subsequently, we investigated the effects of pulse

lighting with a varying duty cycle on plant

photosynthetic rate under a fixed light pulse

frequency of 100 Hz. Figure 11 shows that under

these conditions, the light carbon ability of plants

were 0.015 µmol(CO

) µmol

-1

(photon), 0.022

µmol(CO

) µmol

-1

(photon), 0.020 µmol(CO

) µmol

(photon), 0.019 µmol(CO

) µmol

-1

(photon) with

respective duty cycles of 20 %, 40 %, 60 %, and 80

3 CONCLUSIONS

To conclude, by first fixing the duty cycles and

varying the frequencies, we found that the

photosynthetic rate of plants was greatest at a

frequency of 1000 Hz. Furthermore, at a fixed pulse

frequency of 1000 Hz, the highest photosynthetic rate

of plants was observed at a duty cycle of 40%. To

better measure the ability of photons to convert CO

a new concept of optical carbon ability is proposed

here. That is to say, under the same conditions,

varying the pulse light frequency to 1 Hz, 10 Hz, 100

Hz, 1000 Hz, 10000 Hz, 100000 Hz. This study

showed that the light carbon ability of plants was

greatest when frequency was 100 Hz. Then, at a fixed

pulse frequency of 100 Hz, the light carbon rate of

plants was optimal at a duty cycle of 60%. Compared

with traditional continuous light sources, this pulse

frequency is expected to save 40% energy.

Effect of Frequency and Duty Cycle of a Full-Spectrum Pulsed LED Light Source on Plant Photosynthetic Rate

147

ACKNOWLEDGEMENTS

This work was supported by the Jiangsu Province

"Mass Entrepreneurship and Innovation Doctor"

Project (JSSCBS20211145) in 2021 and the Jiangsu

Intelligent Optoelectronic Devices and Measurement

and Control Engineering Research Center open

project in 2022.

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