In this work we present Shifted Excitation Raman Difference Spectroscopy (SERDS) as a potential spectroscopic tool for outdoor measurements in precision agriculture. A dual-wavelength diode laser at 785 nm is used as an excitation light source which provides an optical power up to 100 mW in cw-operation. Both emission lines for SERDS show single mode operation with a spectral width of ≤ 11 pm and a spectral distance of about 10 cm-1 over the whole power range. Raman experiments on apples are carried out and show Raman signals from wax layer and β-carotene. Raman investigations under daylight conditions are performed to simulate outdoor measurements. Here, polystyrene (PS) is used as test sample. A broadband signal together with narrow absorption lines of water vapor and Fraunhofer lines of singly ionized calcium (Ca II) mask the Raman lines of PS. Only the strong Raman signal at 999 cm-1 is visible. SERDS efficiently separates the Raman signals of PS from the background signals and a 14-fold improvement of the signal-tobackground noise ratio is achieved.
In sweet cherry (Prunus avium), the red pigmentation is correlated with the fruit maturity stage and can be measured by
non-invasive spectroscopy. In the present study, the influence of varying fruit scattering coefficients on the fruit
remittance spectrum (cw) were corrected with the effective pathlength and refractive index in the fruit tissue obtained
with distribution of time-of-flight (DTOF) readings and total internal reflection fluorescence (TIRF) analysis,
respectively. The approach was validated on fruits providing variation in the scattering coefficient outside the calibration
sample set. In the validation, the measuring uncertainty when non-invasively analyzing fruits with cw method in
comparison with combined application of cw, DTOF, and TIRF measurements showed an increase in r2 up to 22.7 %
with, however, high errors in all approaches.
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