User Manual

Underwater Measurements and Immersion Effect
When a quantum sensor that was calibrated in air is used to make underwater measurements, the sensor reads
low. This phenomenon is called the immersion effect and happens because the refractive index of water (1.33) is
greater than air (1.00). The higher refractive index of water causes more light to be backscattered (or reflected)
out of the sensor in water than in air (Smith,1969; Tyler and Smith,1970). As more light is reflected, less light is
transmitted through the diffuser to the detector, which causes the sensor to read low. Without correcting for this
effect, underwater measurements are only relative, which makes it difficult to compare light in different
environments.
The SQ-400 series sensors have an immersion effect correction factor of 1.08. The immersion effect correction
factor is automatically applied to the sensor measurement when using the aM3!, aMC3!, aC3!, and aCC3!
commands. These commands return an electric light calibration PPFD value that has been multiplied by the
immersion effect correction factor of 1.08. The immersion effect correction factor will need to be multiplied to all
other measurement and concurrent commands when the sensor is used for underwater measurements.
Further information on underwater measurements and the immersion effect can be found at
http://www.apogeeinstruments.com/underwater-par-measurements/.