Apogee SQ-300 Series Owner's Manual - Page 11

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Spectral Errors and Yield Photon Flux Measurements:

Apogee quantum sensors are calibrated to measure PPF for either sunlight or electric light. The

difference between the calibrations is 14 %. A sensor calibrated for electric lights (calibration source is T5

cool white fluorescent lamps) will read approximately 14 % low in sunlight.

In addition to PPF measurements, Apogee SQ series quantum sensors can also be used to measure yield

photon flux (YPF): photon flux weighted according to the plant photosynthetic action spectrum (McCree,

1972) and summed. YPF is also expressed in units of µmol m

more closely correlated to photosynthesis than PPF. PPF is usually measured and reported because the

PPF 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, PPF is widely

accepted. The calibration factor for YPF is 4.50 and 4.45 µmol m

measurements, respectively.

The weighting functions for PPF and YPF are shown in the graph below, along with the spectral response

of Apogee SQ series quantum sensors. The closer the spectral response matches the defined PPF or YPF

spectral weighting functions, the smaller spectral errors will be. The table below provides spectral error

estimates for PPF and YPF 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 PPF and YPF

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.

-2

-1

, and is similar to PPF, but is typically

-2

-1

per mV for sunlight and electric light

Radiation weighting factors for PPF

(defined plant response to radiation),

YPF (measured plant response to

radiation), and Apogee SQ Series

quantum sensors (sensor sensitivity to

different wavelengths of radiation).