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-420X sensor has an immersion effect correction factor of 1.15. The
immersion effect correction factor can be automatically applied to SQ-420X
measurements by turning on the immersion setting in the settings option of the
ApogeeConnect software, as pictured. Once you are finished making your
underwater measurements simply uncheck the immersion setting in the software,
to turn it off, and continue making measurements in air like normal.
When making underwater measurements, only the sensor and cable can go in the
water. The USB connector is not waterproof and must not get wet.
Further information on underwater measurements and the immersion effect can
be found at
http://www.apogeeinstruments.com/underwater-par-
