User Manual

Sensor Calibration
All Apogee un-amplified quantum sensor models (SQ-100 and SQ-300 series) have a standard PPFD calibration
factor of exactly:
5.0 µmol m
-2
s
-1
per mV
Multiply this calibration factor by the measured mV signal to convert sensor output to PPFD in units of µmol m
-2
s
-1
:
Calibration Factor (5.0 µmol m
-2
s
-1
per mV) * Sensor Output Signal (mV) = PPFD (µmol m
-2
s
-1
)
5.0 * 400 = 2000
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,
Example of PPFD measurement with an Apogee
quantum sensor. Full sunlight yields a PPFD on
a horizontal plane at the Earth’s surface of
approximately 2000 µmol m
-2
s
-1
. This yields an
output signal of 400 mV. The signal is
converted to PPFD by multiplying by the
calibration factor of 5.00 µmol m
-2
s
-1
per mV.
Sensor Output
400 mV
Full Sunlight
(2000 µmol m
-2
s
-1
)