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Spectral Error
The combination of diffuser transmittance, interference filter transmittance, and photodetector sensitivity yields
spectral response of a quantum sensor. A perfect photodetector/filter/diffuser combination would exactly match
the defined plant photosynthetic response to photons (equal weighting to all photons between 400 and 700 nm,
no weighting of photons outside this range), but this is challenging in practice. Mismatch between the defined
plant photosynthetic response and sensor spectral response results in spectral error when the sensor is used to
measure radiation from sources with a different spectrum than the radiation source used to calibrate the sensor
(Federer and Tanner, 1966; Ross and Sulev, 2000).
Spectral errors for PPFD measurements made under common radiation sources for growing plants were calculated
for Apogee SQ-100 and SQ-500 series quantum sensors using the method of Federer and Tanner (1966). This
method requires PPFD weighting factors (defined plant photosynthetic response), measured sensor spectral
response (shown in Spectral Response section on page 7), and radiation source spectral outputs (measured with a
spectroradiometer). Note, this method calculates spectral error only and does not consider calibration, directional
(cosine), temperature, and stability/drift errors. Spectral error data (listed in table below) indicate errors less than
5 % for sunlight in different conditions (clear, cloudy, reflected from plant canopies, transmitted below plant
canopies) and common broad spectrum electric lamps (cool white fluorescent, metal halide, high pressure
sodium), but larger errors for different mixtures of light emitting diodes (LEDs) for the SQ-100 series sensors.
Spectral errors for the SQ-500 series sensors are smaller than those for SQ-100 series sensors because the spectral
response of SQ-500 series sensors is a closer match to the defined plant photosynthetic response.
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. If desired, the spectral
errors in the table below can be used as correction factors for individual radiation sources.
Spectral Errors for PPFD Measurements with Apogee S2-141 PAR-FAR Sensors
Radiation Source (Error Calculated Relative to Sun, Clear Sky)
Sun (Clear Sky)
Sun (Cloudy Sky)
Reflected from Grass Canopy
Transmitted below Wheat Canopy
Cool White Fluorescent (T5)
Metal Halide
Ceramic Metal Halide
High Pressure Sodium
Blue LED (448 nm peak, 20 nm full-width half-maximum)
Green LED (524 nm peak, 30 nm full-width half-maximum)
Red LED (635 nm peak, 20 nm full-width half-maximum)
Red LED (667 nm peak, 20 nm full-width half-maximum)
Red, Blue LED Mixture (80 % Red, 20 % Blue)
Red, Blue, White LED Mixture (60 % Red, 25 % White, 15 % Blue)
Cool White LED
Warm White LED
Similar to PPFD spectral errors, far-red photon flux density spectral errors can be calculated from the far-red
sensor spectral response, if a range of far-red radiation is defined. However, a widely accepted definition of far-
red, analogous to equal weighting of all photons between 400 and 700 nm for PAR, does not exist. To provide an
indication of spectral errors for far-red photon flux density measurements with the Apogee S2-141, far-red
radiation had been defined as equal weighting of all photons between 700 and 760 nm. If desired, the spectral
errors in the table below can be used as correction factors for individual radiation sources.
S2-141 Sensor
PPFD Error [%]
0.0
0.1
-0.3
0.1
0.1
0.9
0.3
0.1
-0.7
3.2
0.8
2.8
-3.9
-2.0
0.5
0.2