Photosynthetically Active Radiation (PAR) refers to the specific range of light wavelengths (400 to 700 nanometers) that plants use for photosynthesis. Measuring PAR is fundamental for optimizing plant growth in controlled environments like indoor gardens, greenhouses, or aquariums. Accurately quantifying the usable light reaching the plant canopy allows growers to fine-tune lighting systems to maximize health, yield, and energy efficiency. This data-driven approach ensures plants receive the precise energy dose required for their specific growth stage.
Understanding the Key Units of PAR Measurement
Two primary units quantify Photosynthetically Active Radiation. The first is Photosynthetic Photon Flux Density (PPFD), which is an instantaneous measurement of light intensity expressed in micromoles per square meter per second (\(\mu mol/m^2/s\)). PPFD indicates how many light photons within the PAR range strike a specific square meter of surface every second. This metric is useful for real-time adjustments, such as setting the height of an LED fixture.
The second unit is the Daily Light Integral (DLI). DLI measures the total cumulative amount of photosynthetically active light delivered to the plant over a full 24-hour period. It is expressed in moles per square meter per day (\(mol/m^2/day\)) and integrates both light intensity (PPFD) and the duration of exposure. Since plant growth responds directly to the total light dose received daily, DLI is a more relevant metric for long-term health and yield than a single instantaneous PPFD reading.
Essential Equipment for Accurate Measurement
Accurately capturing light energy requires specialized instruments designed to count photons within the 400 to 700 nm range. The most common and accessible tool is the Quantum Sensor, often called a PAR meter. This device uses a sensor to integrate and count all photons in the PAR spectrum, providing a PPFD reading directly in \(\mu mol/m^2/s\). While high-end sensors are used in research, affordable handheld models are widely available for growers.
For specialized applications, such as analyzing the exact color components of a light source, a spectroradiometer is the professional-grade tool. Unlike a quantum sensor, which only measures the total quantity of PAR photons, a spectroradiometer measures the number of photons at each individual wavelength. This allows for analysis of light quality, or the spectral distribution, but comes with a much higher cost.
Avoid using tools designed for human vision, such as traditional lux meters or smartphone apps that claim to measure PAR. These devices measure illuminance, which is light perceived by the human eye, and our eyes are most sensitive to green and yellow wavelengths. Since grow lights emphasize red and blue light for photosynthesis, converting a lux reading to a true PPFD value is inaccurate and unreliable.
Step-by-Step Guide to Taking Readings
The first step in obtaining an accurate reading is positioning the quantum sensor correctly at the plant canopy level. The sensor should be placed horizontally where the leaves are receiving light, not above the light fixture or on the floor. For a newly planted area, place the sensor at the anticipated height of the mature canopy.
To account for inconsistencies in light distribution, a single reading is insufficient. Employ a technique called grid mapping, taking multiple PPFD measurements across the entire growing area. For a typical grow space, divide the area into a grid (e.g., a 3×3 or 4×4 matrix) and record the PPFD reading at the center of each square. This process reveals hot spots and low-light areas, allowing you to calculate a true average PPFD for the entire canopy.
If using a basic quantum sensor that only provides instantaneous PPFD, you can calculate the DLI for artificial lights with constant output. Multiply the average PPFD (\(\mu mol/m^2/s\)) by the number of seconds in the photoperiod, then divide by one million to convert micromoles to moles. A simplified conversion factor is multiplying the PPFD by the hours of light and then by 0.0036. Meters with integrated logging capabilities automatically sum readings throughout the day to provide a direct DLI value.
Interpreting Your Data and Setting Light Targets
Once the average DLI is established, compare it against the targets for your specific plant type. Providing the correct DLI manages growth rate and morphology. Shade-tolerant plants, such as many common houseplants or aquarium plants, require a low DLI, typically ranging from 5 to 10 \(mol/m^2/day\).
Most common vegetables and flowering annuals, including lettuce, basil, and herbs, thrive in a medium light environment. Their optimal DLI range falls between 12 and 17 \(mol/m^2/day\). Fruiting crops like tomatoes, peppers, and high-yield flowers demand more light energy for optimal development. These crops require a high DLI, often needing between 20 and 40 \(mol/m^2/day\) to maximize fruit set and yield.
If the measured DLI is below the target, two primary adjustments are possible: increasing the instantaneous light intensity or extending the photoperiod. Intensity can be increased by lowering the light fixture closer to the canopy or by increasing the fixture’s output power. Conversely, if the DLI is too high and causing stress, raise the light or shorten the number of hours the light is active daily. Regular measurement and adjustment ensure the light environment matches the plant’s metabolic needs throughout its life cycle.