User Manual

Identify Measurement Parameter Command example:
1IC_001!
The Identify Measurement Parameter Command for
sensor address 1, C command, data value 1.
1,RW,Watts/meter squared,incoming solar radiation;<CR><LF>
The response from sensor address 1, SHEF code RW,
units of Watts/meter squared, and additional
information of incoming solar radiation.
Sunlight and Electric light Calibration
Apogee SQ-421 quantum sensors are calibrated to measure PPFD for both sunlight and electric light. The
difference between the calibrations is 12 %. The electric light calibration (calibration source is T5 cool white
fluorescent lamps) will read approximately 12 % low in sunlight. The aM!, aMC!, aC!, or aCC! commands return the
sensor PPFD measurement with electric light calibration. The aM2!, aMC2!, aC2!, or aCC2! commands return the
sensor PPFD measurement with sunlight calibration.
Spectral Errors and Yield Photon Flux Measurements
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.