For most cases you will not need to use a correction factor when measuring PPFD, but you may want to use a correction factor to make the sensor report more accurately under certain conditions. For example, when you want to measure PAR when the sensor is immersed under water, and / or under artificial lighting and / or in certain field conditions. The correction factor you use will depend on the sensor head attached to your PAR sensor. Newer PAR sensors have a marine grade metal connector that attaches the sensor head to the main cable. These sensors use the Apogee SQ-100X sensor head. Older PAR sensors that do not have a marine grade metal connector use an older Apogee SQ-110 sensor head that is calibrated for use in sunlight.

Underwater

Vernier PAR sensors are calibrated in air not under water. The refractive index of water (1.33) is greater than air (1.00). This causes the sensors to read low when they are immersed in water. You can compensate for this by applying a correction factor.

The correction factor for the newer Vernier PAR sensors is 1.15 when measuring PPFD under water. If you have the older Verier PAR sensor the correction factor is 1.08 when measuring PPFD under water.

Incandescent Light Sources

If you have a newer Vernier PAR sensor you do not need to use a correction factor when measuring PPFD from typical incandescent light sources.

If you have an older Vernier PAR sensor you need to decrease the reading by 14% to accurately measure PPFD from typical incandescent light sources. So use a correction factor of 0.86 for this condition.

Field Conditions

Use the following correction factors with the newer Vernier PAR sensors under the following conditions.

LED-Based Light Sources

The correction factor you need to apply will depend on the spectra of the light source. Apogee has a link below that will tell you which correction factor you need to use many popular LED grow lights. The correction factors presented at the link below are only for the newer PAR sensors that have a metal connector for the sensor head. Make sure you use the correction factor listed for the SQ-100x (original) for your light source when using this link.

Apogee also has a great interactive tool that estimates the correction factor for many light sources and field conditions. The interactive tool works for both older and newer sensor heads. Simply select the sensor head you have and then select the light source or condition you are interested in. The interactive tool will display the spectra of the light source and / or condition and will tell you the spectral error and the correction factor. You can even tell it to display the immersion correction factor for the sensor head in question.

Remember, if you have a Vernier PAR sensor with a metal connector that attaches the sensor head to the main cable, select SQ-100x for your sensor head. If you have a Verier PAR sensor that does not have a metal connector for the sensor head, select SQ-110 (sun calibration – retired May 2021) when using the interactive tool.