Conversion Relationship Between Illuminance LX and PPFD in Plant Growth Lights

Plant growth lights are important lighting equipment in facility agriculture, used to provide light required for plant photosynthesis. In the testing and evaluation of plant growth lights, illuminance LX (lux) and PPFD (Photosynthetic Photon Flux Density) are two important photometric parameters. PPFD is a core parameter in plant growth light evaluation, used to assess the impact of plant growth lights on plant photosynthesis. Recently, customers have been asking: how to convert illuminance LX to PPFD? Is there a simple calculation formula? Many friends must also want to know this. First, it is clear that there is a simple calculation formula, which needs to be based on the XD factor. The XD factor is a key parameter in the conversion relationship between illuminance LX and PPFD in plant growth lights, compliant with T/SZFAA01-2018 standard requirements. Below we provide a detailed introduction to the conversion relationship between illuminance LX and PPFD, hoping to help solve the problem.

Quick Answers: Common Questions About Plant Growth Light LX and PPFD Conversion

Core Concept Comparison Table

Parameter	Illuminance LX	PPFD	XD Factor
Definition	Luminous flux received per unit area (human visual response)	Number of photosynthetically effective photons received per unit area per second	Key parameter for LX to PPFD conversion
Unit	Lux (lx)	μmol/(m²·s)	lm/(μmol/s)
Spectral Response	Human visual function V(λ), 380-780nm	Plant photosynthetic response, 400-700nm (PAR)	Depends on light source spectral distribution
Application	General lighting, human visual evaluation	Plant growth light evaluation, facility agriculture	LX to PPFD conversion calculation
Measurement Equipment	Illuminance meter, integrating sphere + spectrometer	Integrating sphere + spectrometer, goniophotometer	IMS-2021 integrating sphere testing system
Standard Reference	CIE 15:2018, GB/T 5700-2008	T/SZFAA01-2018, ASABE S640	T/SZFAA01-2018 Standard Page 11

Part 1: Conversion Relationship Between Plant Growth Light Illuminance LX and PPFD

1.1 Basic Concepts and Definitions

Illuminance LX (Lux) is a traditional parameter for measuring light intensity, representing the luminous flux received per unit area, unit: lux (lx). LX is based on human visual function V(λ), applicable to 380-780nm visible wavelength range, mainly used for evaluating light intensity perceived by human eyes. In plant growth light applications, LX values can be used for preliminary light intensity assessment, but cannot directly reflect plant photosynthetic effects.

PPFD (Photosynthetic Photon Flux Density) is a core parameter in plant growth light evaluation, representing the number of photosynthetically effective photons received per unit area per second, unit: μmol/(m²·s). PPFD is based on plant photosynthetic response curve, applicable to 400-700nm Photosynthetically Active Radiation (PAR) wavelength range, directly reflecting effective light intensity for plant photosynthesis. The PPFD value directly affects plant photosynthesis efficiency and growth rate, and is an important reference indicator for plant growth light design and application.

Key Difference: LX and PPFD cannot be directly converted because they are based on different spectral response curves. LX is based on human visual function V(λ), peak at 555nm (green light); PPFD is based on plant photosynthetic response, more sensitive to red light (600-700nm) and blue light (400-500nm). Therefore, different spectral distribution light sources with the same LX value may have significantly different PPFD values.

1.2 Standard Basis and Authoritative Sources

In the Shenzhen Facility Agriculture Industry Association Group Standard T/SZFAA01-2018 "Plant Growth Light Testing Method" page 11, the conversion relationship between plant growth light illuminance LX and PPFD is clearly specified. This standard is developed by Shenzhen Facility Agriculture Industry Association, providing scientific basis for the testing and evaluation of plant growth lights, helping users accurately evaluate the lighting performance and PPFD values of plant growth lights.

Related International Standard References:

• ASABE S640 (American Society of Agricultural and Biological Engineers Standard): Defines PAR and PPFD measurement methods
• CIE 15:2018 (International Commission on Illumination Standard): Specifies basic methods for illuminance measurement
• GB/T 5700-2008 (Chinese National Standard): Lighting measurement methods
• ANSI/IES LM-79-19: LED luminaire electrical and photometric testing standard

The conversion relationship between illuminance LX and PPFD in plant growth lights is an important content in facility agriculture lighting evaluation, and T/SZFAA01-2018 standard provides standard methods and calculation formulas for plant growth light PPFD testing.

1.3 XD Factor Conversion Formula

From the above figure, we know the conversion relationship between plant growth light illuminance LX and PPFD. The XD factor is related to the spectral shape of the radiation source, and the spectral shape of the radiation source is obtained through spectrometer testing. According to T/SZFAA01-2018 standard, the conversion formulas are as follows:

The XD factor is a key parameter in the conversion relationship between illuminance LX and PPFD in plant growth lights, compliant with T/SZFAA01-2018 standard requirements. Plant growth lights with different spectral distributions have different XD factor values, typical ranges are as follows:

• Full-spectrum white LED plant growth lights: XD factor typically 0.015-0.025 lm/(μmol/s)
• Red + Blue combination LEDs: XD factor typically 0.010-0.020 lm/(μmol/s)
• High red ratio LEDs: XD factor typically 0.008-0.015 lm/(μmol/s)
• Traditional fluorescent lamps: XD factor typically 0.020-0.030 lm/(μmol/s)

Measurement Process: From the calculation formula, we first measure Photosynthetic Photon Flux (PPF) and Luminous Flux (LM) through the IMS-2021 Yiming integrating sphere testing system, then calculate the XD factor, and finally calculate PPFD based on the XD factor and illuminance LX. By accurately measuring the XD factor, the PPFD value of plant growth lights can be accurately calculated, thereby evaluating the impact of plant growth lights on plant photosynthetic photon action, compliant with T/SZFAA01-2018 standard requirements.

Part 2: Calculation and Application of Plant Growth Light XD Factor

2.1 Physical Meaning of XD Factor

The XD factor is a key parameter in the conversion relationship between illuminance LX and PPFD, reflecting the impact of light source spectral distribution on plant photosynthesis. A smaller XD factor value indicates that the light source's spectral distribution is more favorable for plant photosynthesis (i.e., higher PPF value for the same luminous flux). The XD factor is an important parameter in plant growth light testing and evaluation, used to accurately calculate PPFD values, compliant with T/SZFAA01-2018 standard requirements.

Factors Affecting XD Factor:

• Spectral Distribution: Higher proportion of red light (600-700nm) and blue light (400-500nm) results in smaller XD factor
• Color Temperature: Low color temperature (warm white) typically has larger XD factor, high color temperature (cool white) has smaller XD factor
• Color Rendering Index: High CRI light sources typically have larger XD factor because they contain more green and yellow light
• PAR Efficiency: PAR efficiency = PPF / Total radiant power, higher PAR efficiency results in smaller XD factor

2.2 XD Factor Measurement Method

To calculate the XD factor, two key parameters need to be measured first:

1. Luminous Flux (LM): Total luminous flux emitted by the light source, unit: lumens (lm). Luminous Flux LM is a basic parameter in plant growth light testing and can be accurately measured through the IMS-2021 integrating sphere testing system. Measurement method complies with CIE 15:2018 and GB/T 5700-2008 standard requirements.
2. Photosynthetic Photon Flux (PPF): Photosynthetically effective photon flux emitted by the light source in the 400-700nm wavelength range, unit: μmol/s. Photosynthetic Photon Flux (PPF) is an important parameter for evaluating the photosynthetic photon action capability of plant growth lights, compliant with T/SZFAA01-2018 standard requirements, and can be accurately measured through the IMS-2021 integrating sphere testing system.

Test Equipment Requirements (T/SZFAA01-2018 Standard):

• Integrating Sphere: Diameter ≥1.5m, inner wall reflectance ≥95%, compliant with CIE 84:1989 requirements
• Spectrometer: Wavelength range 380-780nm, wavelength accuracy ≤1nm, spectral resolution ≤5nm
• Test Environment: Temperature 25℃±2℃, humidity ≤85%RH, no external light interference
• Warm-up Time: LED light source warm-up 30 minutes to ensure stable light output

These two parameters can be accurately measured through the IMS-2021 Yiming integrating sphere testing system, and then the XD factor value can be calculated according to the XD factor formula, compliant with T/SZFAA01-2018 standard requirements. A smaller XD factor value indicates that the light source's spectral distribution is more favorable for plant photosynthesis. Accurate measurement of the XD factor is crucial for PPFD calculation and photosynthetic photon action evaluation of plant growth lights, compliant with T/SZFAA01-2018 standard requirements.

2.3 Practical Application Cases

Case 1: Full-Spectrum LED Plant Growth Light XD Factor Measurement

• Test Sample: 100W full-spectrum LED plant growth light, color temperature 4000K
• Measurement Results: LM = 8500 lm, PPF = 180 μmol/s
• Calculate XD Factor: XD = 8500 ÷ 180 = 47.2 lm/(μmol/s) = 0.0472 lm/(μmol/s)
• Application: On-site measurement LX = 500 lx, calculate PPFD = 500 ÷ 0.0472 = 10,593 μmol/(m²·s)

Case 2: Red + Blue Combination LED Plant Growth Light XD Factor Measurement

• Test Sample: 50W red + blue combination LED, red 660nm, blue 450nm
• Measurement Results: LM = 3200 lm, PPF = 200 μmol/s
• Calculate XD Factor: XD = 3200 ÷ 200 = 16 lm/(μmol/s) = 0.016 lm/(μmol/s)
• Application: On-site measurement LX = 300 lx, calculate PPFD = 300 ÷ 0.016 = 18,750 μmol/(m²·s)

Part 3: Calculation and Application Scenarios of Plant Growth Light PPFD

3.1 PPFD Parameter Details

PPFD (Photosynthetic Photon Flux Density) is a core parameter in plant growth light evaluation, representing the number of photosynthetically effective photons received per unit area per second, with units of μmol/(m²·s). The PPFD value directly affects plant photosynthesis efficiency and growth rate, and is an important reference indicator for plant growth light design and application, compliant with T/SZFAA01-2018 standard requirements.

Relationship Between PPFD and Plant Growth:

• Light Compensation Point: PPFD = 20-50 μmol/(m²·s), balance between plant photosynthesis and respiration
• Light Saturation Point: PPFD = 500-1500 μmol/(m²·s), varies significantly by plant species
• Optimal PPFD Range: Typically 70-90% of light saturation point, ensuring efficient photosynthesis

PPFD Requirements for Different Plant Growth Stages (Reference Values):

• Seedling Stage: 100-300 μmol/(m²·s), promoting root development and leaf expansion
• Vegetative Growth Stage: 300-600 μmol/(m²·s), promoting stem and leaf growth and biomass accumulation
• Flowering Stage: 400-800 μmol/(m²·s), promoting flower bud differentiation and flowering
• Fruiting Stage: 600-1000+ μmol/(m²·s), promoting fruit development and quality improvement

PPFD is a key parameter for evaluating the impact of plant growth lights on plant photosynthetic photon action. By accurately measuring PPFD values, the lighting performance of plant growth lights and their promotion effect on plant growth can be assessed, compliant with T/SZFAA01-2018 standard requirements.

3.2 Method 1: Plant Growth Light PPFD Calculation Based on Illuminance LX and XD Factor

The PPFD obtained above is only for the case where the testing equipment only has a spectrometer testing system and an illuminance meter. We know that different test points have different illuminance values. Therefore, the PPFD at this time is a test value at a certain point, and this situation can only be used for on-site testing of plant growth light installation, compliant with T/SZFAA01-2018 standard requirements.

Calculation Formula:

Measurement Steps:

1. Measure illuminance LX (unit: lx) at plant canopy height using an illuminance meter
2. Obtain XD factor value from laboratory test report or product specification
3. Calculate PPFD value according to the formula
4. Measure at multiple points to assess PPFD distribution uniformity

Advantages:

• Simple and easy to use, only requires an illuminance meter
• Low cost, suitable for on-site rapid assessment
• Fast measurement, can be completed in minutes

Limitations:

• Requires prior knowledge of XD factor value, different light sources have different XD factors
• Can only measure single-point PPFD value, cannot obtain spatial distribution
• Measurement accuracy affected by illuminance meter accuracy and XD factor accuracy
• PPFD values at different positions may differ, requires multi-point measurement

Applicable Scenarios:

• On-site rapid assessment of plant growth light installation
• Daily PPFD monitoring and maintenance
• Preliminary verification of whether PPFD meets plant requirements

To obtain more accurate plant growth light PPFD evaluation results, it is recommended to use professional testing equipment for measurement, compliant with T/SZFAA01-2018 standard requirements.

3.3 Method 2: Plant Growth Light PPFD Measurement Based on Goniophotometer

PPF is determined by the luminaire, and PPFD is related to the installation height and position of the luminaire. To obtain satisfactory results, we recommend using a color goniophotometer for testing. At this time, we can measure PPFD values at different heights and different angles, compliant with T/SZFAA01-2018 standard requirements.

Measurement Principle:

The goniophotometer measures luminous intensity distribution at different angles by rotating the luminaire and detector, combined with spectrometer measurement of spectral distribution, can calculate PPFD values at any spatial position. By simulating different installation heights and spacing through software, can predict PPFD distribution of plant growth lights in actual applications.

Test Equipment Requirements:

• Goniophotometer: Type C plane goniophotometer, angle accuracy ≤0.1°, compliant with CIE 70:1987 requirements
• Spectrometer: Wavelength range 380-780nm, spectral resolution ≤5nm, compliant with T/SZFAA01-2018 requirements
• Test Distance: Typically 5-10 times the luminaire luminous surface size
• Test Angles: Vertical angles 0°-90°, horizontal angles 0°-360°, angle interval ≤5°

Advantages of Using Goniophotometer to Measure PPFD:

• Spatial Distribution Measurement: Can measure PPFD values at different heights and angles, understanding the spatial distribution characteristics of light, comprehensively evaluating the PPFD distribution of plant growth lights, compliant with T/SZFAA01-2018 standard requirements. Can generate PPFD contour maps, 3D distribution maps and other visualization data.
• Comprehensive Evaluation: Can evaluate the light uniformity of plant growth lights in the entire planting area, assessing the overall impact of plant growth lights on plant photosynthetic photon action, compliant with T/SZFAA01-2018 standard requirements. Can calculate PPFD uniformity, minimum/maximum PPFD ratio and other indicators.
• Design Optimization: Provides scientific basis for the installation height and angle design of plant growth lights, optimizing the PPFD distribution of plant growth lights to improve plant photosynthesis efficiency, compliant with T/SZFAA01-2018 standard requirements. Can simulate different installation schemes and select optimal configuration.
• Standard Compliance: Generate test reports compliant with T/SZFAA01-2018 standard, including complete PPFD distribution data and contour maps, can be used for product certification and quality control.

Application Case:

A facility agriculture company used Yiming Technology GMS goniophotometer testing system to conduct PPFD distribution testing on 100W LED plant growth lights. Test results showed: at installation height 1.5m, average PPFD in planting area (2m×2m) was 450 μmol/(m²·s), PPFD uniformity (minimum/maximum ratio) was 0.75, meeting PPFD requirements for lettuce vegetative growth stage (300-600 μmol/(m²·s)). By optimizing installation height to 1.2m, average PPFD increased to 580 μmol/(m²·s), uniformity increased to 0.82, significantly improving light utilization efficiency.

Summary and Key Points

Core Conclusions

The conversion relationship between illuminance LX and PPFD in plant growth lights is an important content in facility agriculture lighting evaluation. The Shenzhen Facility Agriculture Industry Association Group Standard T/SZFAA01-2018 clearly specifies the conversion relationship between plant growth light illuminance LX and PPFD, where the XD factor is a key parameter.

Measurement Method Selection Guide

Measurement Method	Application Scenario	Accuracy	Cost
Illuminance Meter + XD Factor	On-site rapid assessment, daily monitoring	±10-15%	Low
Integrating Sphere + Spectrometer	Laboratory precise measurement, XD factor calculation	±3-5%	Medium
Goniophotometer + Spectrometer	Spatial distribution measurement, design optimization	±2-3%	High

Standard Compliance

To calculate the XD factor, it is necessary to first measure Luminous Flux (LM) and Photosynthetic Photon Flux (PPF) through the IMS-2021 Yiming integrating sphere testing system, compliant with T/SZFAA01-2018 standard requirements. Then, the PPFD value can be calculated based on the XD factor and illuminance LX. For cases where only a spectrometer testing system and an illuminance meter are available, the PPFD value at a certain point can be measured, suitable for on-site testing of plant growth light installation, compliant with T/SZFAA01-2018 standard requirements. To obtain more comprehensive PPFD distribution data, it is recommended to use a color goniophotometer for testing, which can measure PPFD values at different heights and angles, providing scientific basis for the design and installation of plant growth lights, compliant with T/SZFAA01-2018 standard requirements.

Related Standard List:

• T/SZFAA01-2018 "Plant Growth Light Testing Method" (Shenzhen Facility Agriculture Industry Association Group Standard)
• ASABE S640 (American Society of Agricultural and Biological Engineers Standard)
• CIE 15:2018 (International Commission on Illumination Standard)
• GB/T 5700-2008 (Chinese National Standard)
• ANSI/IES LM-79-19 (LED Luminaire Testing Standard)

Professional Testing Solutions

As a professional LED lighting testing equipment manufacturer, Yiming Technology deeply understands the importance of plant growth light testing and has developed the IMS-2021 integrating sphere testing system and GMS goniophotometer testing system, which can accurately measure all parameters required for plant growth lights, including Luminous Flux (LM), Photosynthetic Photon Flux (PPF), XD factor, illuminance LX, PPFD, etc., and generate test reports that comply with T/SZFAA01-2018 standard requirements.

Yiming Technology Testing System Advantages:

• High Precision Measurement: PPFD measurement accuracy ±3%, XD factor measurement accuracy ±2%
• Standard Compliance: Fully compliant with T/SZFAA01-2018, ASABE S640 and other international standards
• Automated Testing: One-click test report generation, including PPFD distribution maps and contour maps
• Professional Services: Provide testing method training, on-site technical support, customized testing solutions

Yiming Technology's testing systems fully comply with plant growth light testing standard requirements, helping enterprises accurately evaluate the PPFD performance and lighting performance of plant growth lights, providing reliable basis for facility agriculture lighting design. Yiming Technology is committed to providing professional plant growth light testing services and facility agriculture lighting testing solutions for customers, helping users accurately evaluate the impact of plant growth lights on plant photosynthetic photon action, jointly promoting the development of the facility agriculture lighting industry.