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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-520 sensor has an immersion effect correction factor of 1.32. The immersion effect
correction factor can be automatically applied to SQ-520 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-measurements/
.
Correlation between photosynthetic
photon flux density (PPFD) and yield
photon flux density (YPFD) for multiple
different radiation sources. YPFD is
approximately 90 % of PPFD.
Measurements were made with a
spectroradiometer (Apogee Instruments
model PS-200) and weighting factors
shown in previous figure were used to
calculate PPFD and YPFD.