Understanding the Conversion from Lux to PPFD

Grow lights are essential for indoor gardening. Two terms often come up when it comes to measuring light intensity: lux and PPFD. Today, PPFD is most commonly used to evaluate grow lights. However, some growers still use lux when planning their setups. To help meet different needs, we will provide a simple guide on how to convert lux to PPFD in this blog. 

What Is Lux and PPFD

Lux and PPFD are both terms used to measure light, but they differ in their focus and purpose.

What Is Lux

Lux measures light intensity as perceived by the human eye. It quantifies the amount of luminous flux (in lumens) falling on one square meter of surface area. One lux equals one lumen per square meter. Lux is useful for evaluating brightness in homes, offices, and other general lighting applications. However, it doesn’t consider the specific light wavelengths that plants need for photosynthesis.

What Is PPFD

PPFD, or photosynthetic photon flux density, measures the number of photons in the 400–700 nm photosynthetically active radiation (PAR) range that reach one square meter each second and is expressed in micromoles per square meter per second (µmol/m²/s). Unlike lux, PPFD focuses on the light that plants can use for photosynthesis. Higher PPFD levels generally mean more energy is available for plant growth.

What Is the Difference

Lux accounts for our eyes’ sensitivity to different wavelengths of light. In contrast, PPFD is tailored to plants, measuring the light energy they can truly use to grow. For growers, PPFD is a far more accurate and reliable method for evaluating grow lights for horticultural use. 

How to Convert Lux to PPFD

To convert lux to PPFD, simply multiply the lux value by the appropriate conversion factor for your light source. For example, full sunlight has an intensity of about 108,000 lux. Using a conversion factor of 0.0185 for sunlight, the PPFD would be: 

108,000 × 0.0185 ≈ 2,000 µmol/m²/s

We also provide a conversion factor for converting PPFD to lux. You can get lux from PPFD using the same method.

Keep in mind that these conversion factors vary depending on the type of light source. You can refer to the table below for specific values.

Factors Affecting Conversion Accuracy

When converting lux to PPFD, it’s important to consider several factors that can impact accuracy.

Spectrum Distribution

The spectrum distribution of a light source plays a critical role in plant growth and photosynthesis. It influences the conversion factor because it determines how many photosynthetically active photons are emitted.

Angle of Light Distribution

The angle at which light is distributed affects how much light reaches the plant canopy. This is especially important for LEDs, which often have fixed, narrow beam angles with limited spread. This can affect the intensity plants receive and influence conversion accuracy.

Light Loss Factors

Light can be lost through reflection, absorption, and transmission before reaching plants. These losses reduce the actual PPFD delivered and can cause discrepancies when converting from lux.

Inaccuracy of Lux Data

Spectrometers and quantum sensors provide more precise measurements but can be expensive for many growers. As a result, most growers use affordable lux meters. However, lux readings from uncalibrated lux meters may be inaccurate.

Conclusion

In summary, converting between lux and PPFD depends on factors like the light spectrum, so the conversion coefficients provided here are meant as general guidelines. For the most accurate information, it’s best to refer to the data supplied by the manufacturer. If you have any questions, feel free to leave a comment below.