Spectral Response and Spectral Error
Apogee DLI meters are calibrated to measure PPFD or ePPFD under sunlight and electric light. However, errors
occur in various light sources due to changes in spectral output. If the light source spectrum is known, then errors
can be estimated and used to adjust the measurements. The definitions, or weighting functions, of PPFD and
ePPFD are shown in the graph below, along with the spectral responses of the three DLI meter models. The closer
the spectral response matches the defined PPFD or ePPFD spectral weighting functions, the smaller the spectral
errors will be. The table below provides spectral error estimates for PPFD and ePPFD 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 ePPFD 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, or temperature errors.
Mean spectral response of replicate DLI meters compared to PPFD and ePPFD weighting function. Spectral
response measurements were made at 10 nm increments across a wavelength range of 300 to 800 nm in a
monochromator with an attached electric light source. Measured spectral data from each meter were
normalized by the measured spectral output of the monochromator/electric light combination, which was
measured with a spectroradiometer.
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