Providing adequate grow light is critical for thriving indoor plants and productive indoor gardens. However, determining precisely how much light your plants are receiving can be challenging without the right metering tools.
A PAR (photosynthetically active radiation) meter is an indispensable tool for indoor gardeners looking to optimize their grow light setups. These handheld devices can accurately measure light intensity from LED, HPS, CFL and others grow lights to help dial in ideal levels for your plants.
In this article, we will cover everything you need to know about using PAR meters for measuring grow light output. We’ll discuss what PAR means, how it relates to plant growth, key specs and features of PAR meters, and detail a step-by-step process for taking PAR readings of your indoor garden.
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What is a PAR Meter?
A PAR meter, short for photosynthetically active radiation meter, is a measuring device used to quantify the amount of usable light available to plants for photosynthesis.
It specifically measures radiation in the 400-700 nanometer range, which encompasses the wavelengths of electromagnetic spectra that plants can effectively absorb for growth. This includes radiation that human eyes perceive as orange, red, blue, violet, and some green light.
PAR meters capture light intensity measurements in units called photosynthetic photon flux (PPF). This measurement denotes the number of photons hitting a given surface each second that can fuel photosynthesis. PPF is typically displayed in micromoles per meter squared per second (μmol/m2/s).
So, a PAR meter can provide detailed, quantifiable data on the intensity and distribution of photosynthetically useful radiation emitted from grow lights across a garden canopy.
Unlike luminance or foot candle meters, which focus more on light intensity suitable for human vision, PAR meters take specialized readings tailored specifically to gauging light usable for plant cultivation. They filter out unusable wavelengths like most green or infrared light, which in differentiate standard light meters might incorporate.
This is critical for horticultural applications where optimizing photosynthetic activity for accelerating plant growth is the primary goal. So, for dialing in indoor gardens and maximizing yields, PAR metering the usable plant light available offers indispensable precision and control.
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What to Consider for a Proper Measurement?
Taking accurate PAR measurements requires more than just purchasing a quality meter and starting to take readings. To truly quantify the photosynthetically active radiation hitting your plants, you need to follow best practices that account for environmental factors and allow standardized comparisons across measurement sessions.
Configuring the Light Source
When taking PAR measurements, you first need to ensure your grow light is configured appropriately. This means properly hanging or mounting the light fixture at the desired height in fixed position over the canopy. Reflectors and any adjustable parameters on the lights themselves should be consistently set as well for uniform tests.
Shut off any other light sources in the grow area and allow your grow lamp to thermally stabilize for at least 30 minutes prior to readings. This will provide a steady-state light intensity output for accurate measurements.
Cleaning the Camera
It is also critical to thoroughly clean the PAR meter’s photodiode sensor window prior to measurements. Dust, residue or condensation can partially block the sensor, leading to falsely low readings. Carefully use isopropyl alcohol and lens wipes to gently remove any contaminants on the glass window without leaving fibrous material or lint behind. Allow the sensor glass to fully dry before testing. Perform sensor cleaning before each new measurement session for reliable, comparable data.
Applying Screen Protectors
Consider temporarily applying screen protectors to your PAR meter if taking readings directly under intense discharge lamps like high-pressure sodium or metal halide. The high energy ultraviolet radiation can damage the meter’s photosensor over time.
Transparent screen protectors block UV but permit transmission of photosynthetically active radiation, allowing standard PAR readings without sensor degradation. Just be sure to calibrate your meter with the screen on for accurate comparative readings.
Photons and Micromoles
PAR meters capture measurements in units called photosynthetic photon flux (PPF), denoted in micromoles per meter squared per second. This refers to the number of photons in the 400-700nm range hitting a given surface each second. Understanding photons and micromoles provides context on these units. A photon is a single particle of light energy. Micromoles represent the number of photons carrying energy for photosynthesis.
Typical PAR meter readings for indoor gardens range from 200 to 1000 PPF, with levels over 400 μmol/m2/s considered high light. But remember that PPF decreases exponentially with increasing distance from the bulb, so readings should factor in canopy height. Also note that photons in different wavelength bands carry different energy, so PPF does not directly convert to total wattage. But it does directly relate to photosynthetic capability.
Light Spectrum
The range of wavelengths that PAR meters capture are critical because plants absorb and utilize different spectra for various growth processes. Generally, the blue wavelengths spur vegetative leaf development, while red spectra drive flowering and fruiting. An ideal light source has a balance of blue and red photons.
Some PAR meters measure these bands separately, displaying the proportion of blue, green, red and far-red radiation in addition to total PPF. This allows further refinement of light sources and positioning to optimize different growth stages. Note that almost all grow lights emit minimal green photons, since plants reflect this wavelength.
PAR Wavelengths (400nm-700nm)
Photosynthetically active radiation designates the light spectrum range from 400 to 700 nanometers that plants can use for photosynthesis. This includes visible violet, blue, green, yellow, orange and red wavelengths. Each pigment within plant tissues has a unique peak absorption spectrum. Combining the absorption capacities across all plant pigments yields an effective photosynthetic range between 400-700nm.
Wavelengths in the PAR spectrum get absorbed by light-harvesting complexes within the plant leaves and converted into chemical energy carriers like ATP and NADPH. This fuels the Calvin cycle to fix carbon from carbon dioxide into glucose sugars. In this way, the PAR spectra provide energy for plants to grow and develop. Too little PAR light results in slow, weak growth, while abundant PAR fuels vigorous plant metabolism and maximum yields.
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How to Measure Grow Light with A PAR Meter?
Using a PAR meter to quantify light levels allows indoor gardeners to precisely dial in optimal lighting to maximize plant growth and yields. Follow these key steps to leverage PAR data for controlled environment plant cultivation:
- Determine outdoor daily light integral. Measure outdoor PAR levels using a calibrated quantum meter at canopy height in an open, unobstructed area, recording the highest midday readings in direct sun over several days while also noting exact geographic location and timezone. Calculate DLI by adding the integrated total PAR micromoles received over a 24-hour solar period.
- Calculate greenhouse light transmission. Take PAR measurements inside the greenhouse under natural light, then take simultaneous external readings with a second calibrated meter. Divide the internal reading by the external reading and multiply by 100 to determine glazing light transmission percentage. Perform measurements at several locations.
- Determine indoor greenhouse daily light integral. Record internal greenhouse PAR readings every hour from sunrise to sunset using fixed meter calibration and location. Integrate micromoles received over the day length to calculate DLI provided through daylighting. Subtract this from your target crop DLI to determine supplemental light required.
- Identify crop target daily light integral. Research published crop cultivation guides to find consensus DLI guidelines required for your particular crop, whether vegetative leafy greens with lower targets or fruiting plants and flowers requiring higher intensity supplementation. Define an optimal DLI target to sustainably drive crop quality and maximum yields.
- Calculate necessary supplemental daily light integral. First determine the natural Daily Light Integral provided inside your greenhouse over a 24-hour period by recording PAR levels every hour from sunrise until sunset with a calibrated quantum meter fixed at canopy height, integrating the total micromoles received. Then identify the optimal published DLI target required for your particular crop cultivar, whether vegetative leafy greens and herbs needing lower intensities or fruiting pepper plants and flowering orchids requiring much higher totals.
- Determine light output. Use your PAR meter to quantify usable micromole output of your specific grow lamps, reflecting the intensity hitting upper canopy leaves within your defined garden footprint. Take readings at the same fixed height and locations under each fixture after thermal stabilization, averaging multiple readings across the lamp. Factor in manufacturer lamp degradation guidance over time to determine actual lumen intensity and PAR levels in service.
- Convert light output into mols/m2/h. To determine the supplemental Daily Light Integral provided to the crop canopy zone by your grow lights, you need to convert quantified PAR readings into DLI units. Take the measured micromoles covering the garden footprint at the defined height and divide this by the square meter area to determine μmol per m2 per second. Then multiply this by the number of hours per day your electric grow lights are operational.
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Tips for Choosing and Measuring LED Grow Lights
Investing in advanced LED grow lights represents a major opportunity to enhance indoor garden results. However, accurately evaluating and leveraging LED technology poses some unique challenges. Follow these key tips when selecting and utilizing LED lighting:
- Examining light measurements. When assessing LED grow lights, carefully examine the reported performance metrics for the light spectrum, PPF, μmol/Joule, μmol/Watt, and warranty lifespan. These quantify intensity, efficiency and duration capabilities. Ask for third party verification with actual sampled fixtures, as marketed data can be misleading.
- Ask questions. Engage LED lighting manufacturers with questions on everything from fixture build quality, mounting configurations and UL certifications to driver electronics, dimming capabilities and protections from electrical surges. Clarify what specific spectrum balance and intensity was tested at what distance for stated PPF levels.
- Balance budget considerations. Getting the highest intensity LED fixtures available may seem ideal, but ease of achieving positive and predictable ROI also deserves weight, considering energy savings potential against capital costs. Well-built mid-grade LEDs often offer the optimal balance of intensity, utility savings and durable value. Assess options against budget and growth timelines.
- Identify dimming needs. Determine if specific dimming capabilities are required for a particular indoor garden installation, whether manual dimming knobs, automated intensity reduction timed for flowering, integration with HVAC and carbon dioxide injection or other advanced functionality. Not all LED systems offer these controls.
- Conduct on-site PAR testing. Do not rely solely on marketed performance specs and data sheet values when acquiring LED grow lights. Make purchases contingent on vendor supported, on-site PAR measurements using your own calibrated meter to verify intensity and uniformity over the actual garden canopy at intended mounting heights.
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Common Issues When Measuring PAR
While PAR meters provide invaluable data on horticultural-useful light levels, several common mistakes can lead to inaccurate readings that fail to reflect true conditions.
Be aware of these key issues:
- Inconsistent sensor cleaning. Failing to properly clean PAR meter sensors leads to residue buildup that can partially block photodiode receptors, resulting in lower readings that underestimate actual light intensity. Carefully use isopropyl alcohol and lens wipes to regularly remove contaminants without leaving lint or materials that degrade optical transmission.
- Position errors. Care must be taken to precisely locate the PAR sensor at the desired height, angle and location within the garden when measuring light intensity. Small changes in orientation relative to the light source can substantially impact readings. Mark fix points and use a tripod for consistent positioning.
- Lamp conditioning factors. Failure to properly thermally stabilize and season new grow lamps leads to intensity variation that skews PAR measurements due to temporal output changes. Operate lamps consistently for at least 30 minutes before measurements to allow bulb filaments, plasma gases and junctions to reach steady thermal emission.
- Mismatched meters. Using different PAR meters, even of the same consumer model, often yields variation in measurements for the same light intensity due to slight manufacturing calibration differences in the photodiode Responsivity and integration circuitry. Employ just a single standardized meter for all measurements.
- Environmental interferences. Stray light leakage, reflective surfaces or even positioning under AC vents or fans can indirectly affect PAR sensor readings. Take care to minimize external light and air currents.
- Grower shadow effects. If growers position themselves improperly when collecting close-proximity PAR measurements, they can cast shadows on the sensor, thereby blocking light source illumination and corrupting data accuracy. Train staff on proper metering posture.
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FAQ
Why aren't lumen and lux optimal for measuring grow lights?
Lumen and lux measurements quantify light in terms of human vision, rather than focusing on wavelengths usable for plant photosynthesis. Lumen gauges perceived brightness weighted to the sensitivity of human eyes, while lux measures illuminance on a surface. But plant chloroplasts and leaf pigments absorb different spectra than our eyes see. PAR specifically captures photosynthetic radiation from 400-700mm, which lumen and lux meters fail to differentiate.
What constitutes ideal PAR levels for plant growth?
Optimizing plant growth requires tailoring DLI through supplemental lighting to achieve species-specific daily light integral targets ranging from 10-12 mol/m2/day for lettuce up to 25-30+ mol/m2/day for tomatoes and other fruiting plants. Daily PAR levels achieving <10 mol/m2/day become growth-limiting. Levels up from 30-50 mol/m2/day may be excessive, depending on other environmental factors.
How is efficacy measured?
Photosynthetic photon efficacy measured in micromoles per Joule or micromoles per Watt quantifies light energy conversion efficiency into photons usable for plant growth. It helps compare cost efficiency of lighting technologies. Typical LED values range 80-100 μmol/J and 1.7 μmol/Watt. HPS may rate 150 μmol/J but only 1.0 μmol/Watt due to waste heat.
Why measure PAR-per-second?
Measuring instantaneous photons per second (as μmol/m2/s) provides intensity uniformity across a garden, while integrating over daily photoperiods yields total light integral crops actually receive. Low second-to-second variability should be targeted within +10% across the canopy to prevent localized effects.
What to do if you got not measuring what you expect?
Unexpected PAR measurements warrant sensor cleaning, position checks relative to interfering objects, examination for stray light, and confirmation of test consistency against past conditions and data. If issues persist, interrogate lamp spectra with a quantum flux viewer and reconsider PAR meter calibration technique. Each stage of testing should be scrutinized before concluding an anomaly based on any one unexpected result.
