Spectral Errors for PPFD Measurements with Apogee SQ-100X Series Quantum Sensors
Radiation Source (Error Calculated Relative to Sun, Clear Sky)
PPFD Error [%]
Sun (Clear Sky)
0.0
Sun (Cloudy Sky)
0.2
Reflected from Grass Canopy
5.0
Reflected from Deciduous Canopy
7.0
Reflected from Conifer Canopy
7.3
Transmitted below Grass Canopy
8.3
Transmitted below Deciduous Canopy
8.4
Transmitted below Conifer Canopy
10.1
Cool White Fluorescent (T5)
7.2
Cool White Fluorescent (T12)
8.3
Metal Halide
6.9
Ceramic Metal Halide
-0.9
High Pressure Sodium
3.2
Blue LED (448 nm peak, 20 nm full-width half-maximum)
14.5
Green LED (524 nm peak, 30 nm full-width half-maximum)
29.6
Red LED (635 nm peak, 20 nm full-width half-maximum)
-30.9
Red, Blue LED Mixture (80 % Red, 20 % Blue)
-21.2
Red, Green, Blue LED Mixture (70 % Red, 15 % Green, 15 % Blue)
-16.4
Cool White Fluorescent LED
7.3
Neutral White Fluorescent LED
1.1
Warm White Fluorescent LED
-7.8
Quantum sensors can be a very practical means of measuring PPFD from multiple radiation sources, but spectral
errors must be considered. The spectral errors in the table above can be used as correction factors for individual
radiation sources.
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-100X series sensors have an immersion effect correction factor of 1.15. This correction factor should be
multiplied to measurements made underwater.
Further information on underwater measurements and the immersion effect can be found at
http://www.apogeeinstruments.com/underwater-par-measurements/
