Photosynthetically Active Radiation (PAR) is the specific portion of the light spectrum that plants utilize to drive photosynthesis, the process that fuels their growth. It is not a measure of general brightness visible to the human eye, but a scientific metric focused purely on plant biology. Understanding this measurement is important for optimizing plant environments in commercial agriculture, greenhouses, or indoor gardening. By quantifying the light available for growth, growers can make informed decisions about light fixtures, placement, and energy use.
The Specifics of Photosynthetically Active Radiation
Photosynthetically Active Radiation is defined by the wavelength range of light between 400 and 700 nanometers (nm). While this range corresponds closely to the visible light spectrum, the measurement focuses specifically on photons that interact with plant pigments. Light outside this range, such as ultraviolet or infrared, is generally not used for photosynthesis or may even be damaging.
This specific range is important due to the absorption peaks of chlorophyll, the primary pigment for photosynthesis. Chlorophyll \(a\) and chlorophyll \(b\) strongly absorb light in the blue region (around 430–470 nm) and the red region (around 640–670 nm). Accessory pigments like carotenoids also harvest light, mostly in the blue-green range, and pass this energy to the chlorophyll.
Although plants absorb blue and red wavelengths most efficiently, photons within the entire 400 to 700 nm range contribute to the process. Green light is largely reflected by chlorophyll, giving plants their characteristic color, but a significant portion still penetrates the canopy and contributes to photosynthesis.
Translating PAR into Measurable Units (PPFD)
Scientists use Photosynthetic Photon Flux Density (PPFD) to practically measure PAR. PPFD quantifies the number of photons within the 400–700 nm range that arrive at a specific surface area each second. This metric is far more accurate for plant growth than traditional measurements like Lux or Lumens, which are weighted for human sight.
The standard unit for PPFD is micromoles per square meter per second (\(\mu \text{mol} \text{m}^{-2} \text{s}^{-1}\)). The micromole (\(\mu \text{mol}\)) represents the number of photons, while the remaining terms define the area and time of the measurement. Growers take this measurement directly at the plant canopy level using a quantum sensor.
PPFD must be distinguished from Photosynthetic Photon Flux (PPF). PPF measures the total light output of a fixture in all directions, expressed in micromoles per second (\(\mu \text{mol}/\text{s}\)). PPFD, conversely, measures the density of light at a specific distance and location—what the plant actually receives. A fixture may have high PPF, but its PPFD decreases significantly as the distance from the plant increases.
Practical Application of PAR Data
PPFD readings are the most practical way for growers to evaluate their lighting setup. The required light intensity changes depending on the plant species and its stage of development. Adjusting the distance between the light source and the plant canopy is the most direct way to manipulate the PPFD level.
Delicate seedlings and low-light houseplants generally thrive with a lower PPFD, typically ranging from 100 to 300 \(\mu \text{mol} \text{m}^{-2} \text{s}^{-1}\). Plants in the vegetative stage require a moderate increase, with optimal PPFD levels often falling between 400 and 600 \(\mu \text{mol} \text{m}^{-2} \text{s}^{-1}\). High-light, fruiting, and flowering plants, such as tomatoes or peppers, demand the highest intensity, with recommended PPFD values ranging from 600 to 900 \(\mu \text{mol} \text{m}^{-2} \text{s}^{-1}\) for maximum yield.
While instantaneous PPFD is useful, the Daily Light Integral (DLI) is a more comprehensive metric for successful growth. DLI is the cumulative amount of PAR a plant receives over a full 24-hour period, expressed in moles per square meter per day (\(\text{mol} \text{m}^{-2} \text{d}^{-1}\)). This measurement combines light intensity (PPFD) with the duration of the light cycle (photoperiod).
DLI is important because plant growth is driven by the total daily dose of light energy, not just the peak intensity at any single moment. Low-light plants may only require a DLI between 5 and 10 \(\text{mol} \text{m}^{-2} \text{d}^{-1}\), while high-light crops often need a DLI exceeding 15 \(\text{mol} \text{m}^{-2} \text{d}^{-1}\) to thrive. Tracking DLI helps growers prevent both light starvation and light stress.