User Manual

Spectral Errors and Yield Photon Flux Measurements
Apogee quantum sensors are calibrated to measure PPFD for either sunlight or electric light. The difference
between the calibrations is 12 %. A sensor calibrated for electric lights (calibration source is T5 cool white
fluorescent lamps) will read approximately 12 % low in sunlight.
In addition to PPFD measurements, Apogee SQ series quantum sensors can also be used to measure yield photon
flux density (YPFD): photon flux density weighted according to plant photosynthetic efficiency (McCree, 1972) and
summed. YPFD is also expressed in units of µmol m
-2
s
-1
, and is similar to PPFD, but has been reported to be more
closely correlated to photosynthesis than PPFD in some studies. 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
calibration factor for YPFD is 4.50 and 4.45 µmol m
-2
s
-1
per mV for sunlight and electric light measurements,
respectively.
The weighting functions for PPFD and YPFD 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 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 SQ Series quantum
sensors (sensor sensitivity to
different wavelengths of
radiation).