PAR lighting meters can be incredibly useful tools when growing plants, but as with any tool it is important to understand their limitations. All PAR meters are made with a sensor that counts individual photons ("particles" of light). These sensors aren't perfect though, and record some wavelengths (colors) of light better than others.

Spectroradiometer-based PAR meters correct for this inaccuracy by breaking the light into its constituent wavelengths (colors) and measuring the number of photons it sees per wavelength, then applying a per-wavelength correction which was determined when the sensor was calibrated with a known light source. This is by far the most-accurate way to measure PAR, but spectroradiometer-based PAR meters are also much more expensive than most people are willing to pay.

Less-expensive PAR meters use a single light sensor and put a filter in front of it to try to make the sensor only see light in the 400-700nm PAR range. The accuracy of these meters depends on how good the filter material is at letting all 400-700nm light through (and nothing else), and how good the light sensor is at responding to all wavelengths equally. Better filters and sensors typically cost more and so increase the price of the PAR meter. Because no filter material or sensor is perfect, these meters are calibrated to "average" light spectra, such as natural sunlight. Some PAR meters of this type have two or more calibration settings for different light spectra, such as natural sunlight and a particular type of electric light.

These PAR meters are generally fairly accurate at determining the total PAR for most common light sources. However, if the light's spectrum is concentrated in areas where the filter material or light sensor is less accurate, the PAR mater can give very inaccurate readings.

One of the least-expensive and most commonly-used PAR meters are the Apogee quantum sensors, which are also resold under the Sun Systems brand and possibly others. Many of these come with calibration modes for sunlight and "electric" light; the manufacturer explains that this is calibrated for a cool-white T5 fluorescent bulb. These meters' filter and sensor is reasonably accurate in the 450-650nm range, but detects almost no photons in the 660-700nm range at the deep red end of the PAR spectrum. Because of this, Apogee (to their credit) warns that "LEDs that output a large proportion of radiation above approximately 660 nm will read very low and should not be measured with an Apogee quantum sensor/meter."

Comparison of PAR, Apogee Meter Sensitivity, and the Phyto-Genesis Spectrumâ„¢

25% of the photons Black Dog PhytoMAX LED grow lights emit are above 660nm, so using an Apogee quantum sensor/meter to measure PAR levels from our lights will give very inaccurate readings, whether the meter is in sunlight or electric light mode. Much to their credit, Apogee provides a spreadsheet to calculate the error their meters will have for a particular spectrum. When the Black Dog LED Phyto-Genesis Spectrum for PhytoMAX lights is entered, the spreadsheet indicates that the Apogee quantum sensor/meter is under-reporting the actual PAR level by 25.9%.

This means that if you use an Apogee quantum sensor/meter (or Sun Systems PAR meter) to take PAR measurements for a PhytoMAX LED light, you need to multiply the reading by at least 1.259 to get the actual PAR.

Because plant canopies absorb more red light and scatter deep red light, the proportion of light over 660nm will increase when taking measurements within or underneath the plant canopy. This means that the Apogee meters will under-report PAR levels from Black Dog LED lights even more within or underneath the plant canopy.

When evaluating PAR readings, it is also important to remember that plants use light outside of the PAR range; this is why we include ultraviolet and infrared in our Phyto-Genesis Spectrum.

We could make our lights have higher PAR numbers by leaving these critical spectra out, but we choose to make lights that grow plants better, rather than just to make lights that look better on paper.

In addition, single PAR readings are meaningless as many lights are designed with secondary lenses or reflectors to focus most of their output in a narrow cone for higher PAR readings. This makes the light look better on paper, but away from the center of the lighting footprint, PAR levels fall dramatically. Our lights are designed to evenly cover the entire footprint to maximize growth of all plants, not just the one directly under the light.

For more detailed information, see this comparison of PAR meter spectral response.