Why Discuss Light?

The tools available for growing our plants include testing a variety of parameters including specific ion nutrient testing, pH, salts, crop height, pathogen analysis, leaf tissue testing, and substrate testing to name a few. One overshadowed parameter is the amount of light we give our crops. Tracking light levels is important, but there are various ways to measure light, each with different implications for understanding plant growth.

Understanding Light for Nurseries and Greenhouses

Lighting is fundamental to plant growth in nurseries and greenhouses, where choosing the right measurement can help growers optimize production. Two common ways to measure light intensity are foot-candles and Photosynthetic Photon Flux Density (PPFD). Sometimes our reference materials like the Ball Redbook and journal articles refer to foot-candles or their metric counterpart, lux, but newer and more accurate light measurements use this upgraded term (PPFD). 

What Are Foot-candles?

Foot-candles measure illuminance based on how bright light appears to the human eye. Our eyes are most sensitive to green and yellow light around 555 nm, and a light meter using foot-candles uses a mathematical formula that gives increased brightness towards this yellow-green light. 

This means that foot-candles readings are biased toward those wavelengths and do not evenly sample the full spectrum of light that plants use for photosynthesis. Ideally, we’d want to measure all the photons plants use equally. 

Upgrade to PPFD

Photosynthetic Photon Flux Density (PPFD) measures the number of photons in the photosynthetically active radiation (PAR) range that reach the plant canopy each second. The photosynthetic range is (400-700nm), which overlaps with the range detected by foot-candle measurements. However, unlike foot-candle measurements, which are weighted preferentially toward green light based on human eye sensitivity, PPFD counts all photons within this wavelength range equally. This avoids the measurement bias (or skewing) present in foot-candle readings and provides a more accurate assessment of the light relevant to plant growth. 

PPFD is expressed as micromoles per square meter per second (μmol∙m−2∙s−1), which quantifies the amount of light that a plant can utilize for photosynthesis. This unit is more accurate and meaningful for growers who want to maximize crop growth.

You can still use foot-candles

Because foot-candles were a common unit for light intensity, it is possible to convert between foot-candles and PPFD, but the conversion factor depends on the light source. Table 1 shows some commonly used conversion factors between foot-candles and PPFD, depending on light source. And more can be found at Hort Americas conversion guide. Keep in mind that these conversions provide convenience but sacrifice spectral accuracy and are estimates of what a standalone PAR meter would measure. 

Quantum Sensors

Quantum sensors, sometimes called PAR sensors or meters, measure PPFD directly. These devices, priced between $100 and $1,000, are essential tools to accurately assess light intensity and distribution. They also enable calculation of Daily Light Integral (DLI), a key metric representing the total light received by plants over a day and directly correlated with growth. However, DLI is a whole other topic, deserving its own detailed discussion in future articles. The main takeaway from this discussion is to understand how different light meters measure light and how you can use those readings to maximize lighting and plant growth for your production setting.

Resources 

• Stallknecht, E. (2025). Calculating and Using Daily Light Integral (DLI): An Introductory Guide. Virginia Cooperative Extension, School of Plant and Environmental Sciences, Virginia Tech. SPES-720NP.​
• Runkle, E., & Bugbee, B. (2013, September). Problems with Foot-candles, Lux and Lumens. GPN Magazine, 78
• Hort Americas foot-candle to PPFD conversion guide: https://hortamericas.com/grower-resources/conversion-ppfd-to-foot--candles/

Photo Credit:
Fig 1. Ric Mann / MediaStudies Trust, from lightcolourvision.org, used with permission. Caption created by author.