Apogee SQ-212 Owner's Manual - Page 13

Browse online or download pdf Owner's Manual for Accessories Apogee SQ-212. Apogee SQ-212 16 pages. Quantum sensor

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 weighted according to plant photosynthetic efficiency (McCree, 1972) and summed. YPFD is also

expressed in units of µmol m

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 0.9 µmol m

µmol m

-2

-1

per mV for models SQ-215 and SQ-225.

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.

, and is similar to PPFD, but is typically more closely correlated to photosynthesis than PPFD.

-2

-1

per mV for models SQ-212 and SQ-222, or 0.45

-2

-1

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).