Spectral Errors and Yield Photon Flux Measurements
The MQ-210 Apogee quantum meters are calibrated to measure PPFD for electric light. The sensors calibration
source is T5 cool white fluorescent lamps and will read approximately 12 % low in sunlight.
In addition to PPFD measurements, Apogee MQ series quantum meters can also be used to measure yield photon
flux density (YPF): photon flux weighted according to the plant photosynthetic action spectrum (McCree, 1972) and
summed. YPFD is also expressed in units of µmol m
-2
s
-1
, and is similar to PPFD, but is typically more closely
correlated to photosynthesis than PPFD. PPFD is usually measured and reported because the PPFD spectral
weighting function (equal weight given to all photons between 400 and 700 nm; no weight given to photons
outside this range) is easier to define and measure, and as a result, PPFD is widely accepted. The conversion factor
to calculate YPFD from the PPFD measurements displayed on the LCD is 0.89 for electric light measurements.
The weighting functions for PPFD and YPFD are shown in the graph below, along with the spectral response of
Apogee quantum sensors. The closer the spectral response matches the defined PPFD or YPFD spectral weighting
functions, the smaller spectral errors will be. The table below provides spectral error estimates for PPFD and YPFD
measurements from light sources different than the calibration source. The method of Federer and Tanner (1966)
was used to determine spectral errors based on the PPFD and YPFD spectral weighting functions, measured sensor
spectral response, and radiation source spectral outputs (measured with a spectroradiometer). This method
calculates spectral error and does not consider calibration, cosine, and temperature errors.
Federer, C. A., and C. B. Tanner, 1966. Sensors for measuring light available for photosynthesis. Ecology 47:654-
657.
McCree, K. J., 1972. The action spectrum, absorptance and quantum yield of photosynthesis in crop plants.
Agricultural Meteorology 9:191-216.
Radiation weighting factors for
PPFD (defined plant response to
radiation), YPFD (measured plant
response to radiation), and Apogee
quantum sensors (sensor sensitivity
to different wavelengths of
radiation).
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