Blogs Jan 20, 2020

The Best PAR Meters, Pyranometers, and Spectroradiometers

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What is a PAR Meter?

A PAR meter, also known as a “quantum sensor” measures the intensity of light, the amount of electromagnetic radiation below a single source, or multiple sources of light. The easiest way to think of a PAR meter is to think of measuring light intensity.  A quantum of light is the smallest measurable amount of solar radiation and is called a “photon”. PAR meters measure quantum flux (aka “photon flux”), the number of photons per square meter per second.

A PAR meter consists of two main components:

  1. The sensor itself that measures light (usually a photodiode)
  2. A meter that displays and often records measurements (either a handheld unit, or computer)

The handheld meter can usually average several light readings over a set length of time.

How to use a PAR Meter

Using a PAR meter to record instantaneous light (PPFD) allows you to optimize your lighting environment to your specifications.  In controlled environment agriculture (CEA), this means the ability to deliver the amount of light your crop requires at any moment, and over the course of a day. To accurately measure light with a PAR meter, it is important that:

  • the sensor is level
  • the sensor head is clean
  • there are no reflective objects near the sensor
  • you are not physically interfering with the light source

Remember that a level sensor free of dirt and other light-affecting obstructions will only provide an accurate reading with correct calibration. Sensors generally need re-calibration every two years. You can calibrate a PAR meter using an artificial light source (calibration lamp) emitting a known quantity of light, or sunlight.

Best PAR Meters (2020)

Below are our favorite PAR meters on the market today.


Apogee MQ-500 Sensor with Meter

Image result for MQ-500: Full-Spectrum Quantum MeterThe MQ-500 quantum meter is the tool of choice for scientists all over the world to accurately measure photosynthetically active radiation from all light sources. It has a hand-held meter, attached via cable, that displays and stores measurements.

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LI-COR LI-190R Sensor

li-cor par meter

The LI-190R measures Photosynthetically Active Radiation (PAR, in µmol of photons m-2 s-1). It provides accurate measurements—in the open, in greenhouses, under plant canopies, or in growth chambers—for most broad-spectrum light sources, including natural sunlight, artificial, or mixed sources.

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LI-COR 250A Light Meter

li-cor light meter

The LI-250A is a portable meter that displays the output from any LI-COR light sensor that has a cable terminated with a BNC connector. It shows instantaneous values or 15-second averages of values from a sensor.

The LI-250A automatically selects a sensitivity range to optimize accuracy and resolution for a given sensor. It provides a wide dynamic range for high-resolution measurements, even in low-light conditions.

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Apogee SQ-521 Quantum Sensor

SQ-521 SDI-12 Digital Output Full-spectrum Quantum Sensor Serial-digital interface (SDI) quantum sensors use a microprocessor, draw low amounts of energy, and allow for connecting several sensors to connect to a single data logger/meter. By connecting sensors in serial, you can record average light levels over a large area in your growing space.

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Apogee SQ-520 Quantum Sensor

Apogee Instruments SQ-520 USB Smart Full-spectrum Quantum SensorUniversal serial bus (USB) quantum sensors connect to a computer and log real-time data directly on the computer screen, or can connect to a power source (battery pack or AC USB adapter) and store data over several days. A computer application downloads data that the quantum sensor and logger collects.

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What is a Pyranometer?

Pyranometers can be placed on the roof of a greenhouse to sense solar light. They connect to the same greenhouse control system that many growers use to regulate the greenhouse microclimate. Unlike a PAR meter, which measures light intensity over a narrower spectrum than that of the sun, pyranometers typically sense almost the complete solar radiation spectrum (300-2800 nm). They measure radiant flux (W/m2), the amount of electromagnetic radiation in watts per unit area.

Pyranometers provide solar radiation data that informs the greenhouse control system of when to close or open automatic shade curtains, and when to turn lights on or off. “Threshold control” is a lighting strategy whereby fixtures turn off when a pyranometer senses a pre-specified radiant flux (e.g. 300 W/m2).

Considerations When Using a Pyranometer

Although a pyranometer provides valuable solar radiation data, the broad spectrum that it measures means that it considers UV, far-red, and other wavelengths to be of equal importance as photosynthetically active radiation (PAR, 400-700 nm). These non-PAR wavelengths may play a role in crop growth, but we know that PAR wavelengths are necessary for photosynthesis and the plant’s ability to sense light. It is always valuable to consider the photosynthetic photon flux density (PPFD) when assessing radiation available for plant growth.

Best Pyranometers (2020)

Below are our favorite PAR meters on the market today.


LI-200R Pyranometer

black pyranometerThe LI-200R Pyranometer is meant to be used outdoors under unobstructed natural daylight conditions. It measures global solar radiation—the combination of direct and diffuse solar radiation—in the 400 to 1100 nm range.

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Apogee Silicon-Cell Pyranometers

Apogee silicon-cell pyranometer with cable connector.Apogee offers two types of pyranometers; our original silicon-cell models and our new line of thermopile pyranometers, both of which are now ISO 9060:2018 Class C rated (previously known as second class).  Apogee Instruments’ silicon-cell pyranometers feature a silicon-cell detector. The sensors are accurate, stable, durable, and cost-effective, the results of continual refinement of design.

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Apogee Thermopile Pyranometers

Apogee thermopile pyranometer with cable connector.Apogee thermopile pyranometers feature a blackbody thermopile detector and have a larger spectral range, making them more accurate in all atmospheric conditions.

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What is a Spectroradiometer?

A spectroradiometer measures the intensity of light at different wavelengths within a spectrum.  Breaking down the word, “spectroradiometer” helps in comprehending its use. The prefix, “spectro-“ means that a “spectroradiometer” has to do with spectrum, that is, a range of wavelengths. While the suffix, “-radiometer” means that it measures the intensity of radiation.

Measuring light with a spectroradiometer makes it possible to understand the wavelengths your crop receives, and then adjust light ratios to produce the healthiest crop possible.

The spectrum of horticultural lighting fixtures and photosynthetic light fall mostly within the visible waveband (380-740 nm) and so a spectroradiometer that is sensitive to visible light is sufficient to determine the spectrum a crop receives.

Best Spectroradiometers (2020)

Below are our favorite PAR meters on the market today.


A field spectroradiometer in a lab settingApogee Field Spectroradiometers

Apogee offers two types of spectroradiometers. The Field Spectroradiometer was designed for field measurements as well as laboratory measurements. 

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The lab spectroradiometer complete package includes a USB drive with required drivers and software.Apogee Lab Spectroradiometers

Apogee Instruments is a value-added reseller of StellarNet spectrometers. All units are calibrated in absolute radiation units for wavelengths between 300 and 1000 nm.

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LI-180 Spectrometer

li-cor spectrometerPlant growth and reproduction are driven by photosynthetically-active radiation (PAR), which occurs at wavelengths between 400 and 700 nanometers. This interval covers three distinct “color” bands of light, and modifying the intensity of these red, blue, green wavelengths, and additional near-UV (380-400 nm) and far-red (700-780 nm) bands can provide control over a variety of plant characteristics.

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Measuring Light for Controlled Environment Agriculture

In controlled environment agriculture (CEA), growers must measure light to avoid the risk of wasting energy or damaging their plants.

For instance, growers must assure that sun-loving plants like roses, cannabis, corn, and cucumbers grow below high-light levels of (30-50 mol·m-2·d-1). Lower light levels will slow production.

More shade-tolerant plants like lettuce, heuchera, and cyclamen require medium-light levels of (10-20 mol·m-2·d – 1). Excessive light will kill cells and bleach leaves of these plants.

It’s also important to keep in mind that the spectrum and photoperiod of the light should be calibrated along with the light intensity in these environments.

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