Spectral Errors for PPFD Measurements with Apogee SQ-100x and SQ-500 Series Quantum Sensors
Radiation Source (Error Calculated Relative to Sun, Clear Sky)
SQ-100X Series
PPFD Error [%]
Sun (Clear Sky)
0.0
Sun (Cloudy Sky)
0.2
Reflected from Grass Canopy
5.0
Transmitted below Wheat Canopy
7.0
Cool White Fluorescent (T5)
7.2
Metal Halide
6.9
Ceramic Metal Halide
-8.8
High Pressure Sodium
3.3
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 LED (667 nm peak, 20 nm full-width half-maximum)
-56.7
Red, Blue LED Mixture (80 % Red, 20 % Blue)
-21.2
Red, Blue, White LED Mixture (60 % Red, 25 % White, 15 % Blue)
-29.7
Cool White LED
7.3
Warm White LED
-7.8
Quantum sensors can be a very practical means of measuring PPFD and YPFD 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 MQ-210X sensor has an immersion effect correction factor of 1.15. The immersion effect correction factor is
already accounted for in the MQ-210X meter firmware so there is no need to apply the correction factor to your
measurements. If you wish to use your meter to make measurements in air, simply divide the measured number
by the immersion effect (1.15).
When making underwater measurements, only the sensor and cable can go in the water. The handheld meter is
not waterproof and must not get wet. If the meter might get wet from splashing, we recommend placing it in a
plastic bag or other container to help protect it from accidentally getting wet.
Further information on underwater measurements and the immersion effect can be found at
http://www.apogeeinstruments.com/underwater-par-measurements/