For instructions on applying a correction factor to the PAR sensor readings, see
How do I apply a Correction Factor to Vernier PAR sensors?
For most cases you will not need to use a correction factor when measuring Photosynthetic Photon Flux Density (PPFD). However, you may want to use a correction factor to make the sensor report more accurately under certain conditions such as underwater, in artificial lighting, and/or in certain field conditions.
The correction factor you use depends on the sensor head attached to your PAR sensor. Newer PAR sensors (including the Go Direct PAR sensor) 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 have a removable sensor head, use an older Apogee SQ-110 sensor head that is calibrated for use in sunlight. For more information, see Which PAR Sensor (PAR-BTA) do I have?
Newer PAR Sensors
| Condition | Correction Factor |
|---|---|
| Underwater* | 1.15 |
| Incandescent Light Source^ | N/A |
| In the field: Cloudy Conditions | 0.97 |
| In the field: below Deciduous Canopy | 0.92 |
| In the field: below Conifer Canopy | 0.90 |
Older PAR Sensor
| Condition | Correction Factor |
|---|---|
| Under Water* | 1.08 |
| Incandescent Light Source^ | 0.86 |
| In the field: Cloudy Conditions | 0.99 |
| In the field: below Deciduous Canopy | 0.93 |
| In the field: below Conifer Canopy | 0.95 |
* Since the refractive index of water (1.33) is greater than air (1.00), the PAR sensors read low when they are immersed in water.
^ For the older Vernier PAR sensor you need to decrease the reading by 14% to accurately measure PPFD from typical incandescent light sources.
LED-Based Light Sources
The correction factor depends on the spectra of the light source. Use the link below to find the correction factor you need to use for many popular LED grow lights.
https://www.apogeeinstruments.com/how-to-correct-for-spectral-errors-of-popular-light-sources
The correction factors presented at the link below are only for the newer PAR sensors. Make sure you use the correction factor listed for the SQ-100x (original) for your light source when using this link.
Other Light Sources
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 reports the spectral error and the correction factor. You can even tell it to display the immersion correction factor.
https://www.apogeeinstruments.com/quantum-par-sensor-spectral-error-estimation-tool
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.