11
OPERATION AND MEASUREMENT
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 reproduce the defined
plant photosynthetic response to photons (equal weighting to all photons between 400 and 700 nm), 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 different radiation sources were calculated for the 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, cosine, and temperature errors. Spectral error data (listed in table below) indicate errors
typically 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. Spectral errors for the SQ-500 series
sensors are smaller than those for SQ-100 series sensors because the SQ-500 spectral response is a closer match to the
defined plant photosynthetic response.
Spectral Errors for PPFD and YPFD Measurements with Apogee SQ Series Quantum Sensors
Radiation Source (Error Calculated Relative to Sun, Clear Sky)
SQ-100 Series
PPFD Error [%]
SQ-500 Series
PPFD Error [%]
Sun (Clear Sky)
0.0
0.0
Sun (Cloudy Sky)
1.4
0.5
Reflected from Grass Canopy
5.7
0.0
Transmitted below Wheat Canopy
6.4
1.1
Cool White Fluorescent (T5)
0.0
2.2
Metal Halide
-3.7
3.1
Ceramic Metal Halide
-6.0
1.9
High Pressure Sodium
0.8
2.2
Blue LED (448 nm peak, 20 nm full-width half-maximum)
-12.7
3.0
Green LED (524 nm peak, 30 nm full-width half-maximum)
8.0
5.2
Red LED (635 nm peak, 20 nm full-width half-maximum)
4.8
0.2
Red LED (668 nm peak, 20 nm full-width half-maximum)
-79.1
-1.9
Red, Blue LED Mixture (84 % Red, 16 % Blue)
-65.3
-1.2
Red, White LED Mixture (79 % Red, 21 % Blue)
-60.3
-0.8
Cool White Fluorescent LED
-4.6
2.2
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.
Federer, C.A., and C.B. Tanner, 1966. Sensors for measuring light available for photosynthesis. Ecology 47:654-657.
Ross, J., and M. Sulev, 2000. Sources of errors in measurements of PAR. Agricultural and Forest Meteorology 100:103-125.
Yield Photon Flux Measurements
Photosynthesis in plants does not respond equally to all photons. Relative quantum yield (photosynthetic efficiency) is
dependent on wavelength (blue line in figure below) (McCree, 1972a; Inada, 1976). This is due to the combination of spectral
absorptivity of plant leaves (absorptivity is higher for blue and red photons than green photons) and absorption by non-
photosynthetic pigments. As a result, photons in the wavelength range of approximately 600-630 nm are the most efficient.