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Spectral Error
Quantum sensors are the most common instrument for measuring PPFD, because they are about an order of
magnitude lower cost the spectroradiometers, but spectral errors must be considered. The spectral errors in the
table below can be used as correction factors for individual radiation sources.
Spectral Errors for PPFD Measurements with Apogee SQ-610 Series ePAR Sensors
Radiation Source (Error Calculated Relative to Sun, Clear Sky)
Sun (Clear Sky)
Sun (Cloudy Sky)
Cool White Fluorescent (T5)
Metal Halide
Ceramic Metal Halide
Mogul Base HPS
Dual-ended HPS
Cool White LED
Warm White LED
Blue LED (442 nm)
Cyan LED (501 nm)
Green LED (529 nm)
Amber LED (598 nm)
Red LED (666 nm)
Far Red LED (737 nm)
Immersion Effect Correction Factor
When a radiation sensor is submerged in water, more of the incident radiation is backscattered out of the diffuser
than when the sensor is in air (Smith, 1969; Tyler and Smith, 1970). This phenomenon is caused by the difference
in the refractive index for air (1.00) and water (1.33) and is called the immersion effect. Without correction for the
immersion effect, radiation sensors calibrated in air can only provide relative values underwater (Smith, 1969;
Tyler and Smith, 1970). Immersion effect correction factors can be derived by making measurements in air and at
multiple water depths at a constant distance from a lamp in a controlled laboratory setting.
Apogee SQ-610 series ePAR sensors have an immersion effect correction factor of 1.25. This correction factor
should be multiplied by PPFD measurements made underwater to yield accurate PPFD.
Further information on underwater measurements and the immersion effect can be found on the Apogee
webpage (http://www.apogeeinstruments.com/underwater-par-measurements/).
Smith, R.C., 1969. An underwater spectral irradiance collector. Journal of Marine Research 27:341-351.
Tyler, J.E., and R.C. Smith, 1970. Measurements of Spectral Irradiance Underwater. Gordon and Breach, New York,
New York. 103 pages
SQ-610 Series
PPFD Error [%]
2.5
2.6
-0.5
0.7
0.3
0.3
0.6
-1.0
-0.4
-2.7
-0.7
-0.7
-0.4
-0.3
4.5