A time-resolved measurement, or time-domain system, could provide the TOF information of detected electromagnetic beams. The TOF photon technique enables determination of certain details of absorption and scattering phenomena in various samples. Many applications of time-resolved reflectance and transmittance have been widely investigated for use in biological tissues or agricultural products (Valero et al., 2004; Vanoli et al., 2009). Leonardi and Burns (1997, 1999a, 1999b) performed quantitative measurements in scattering media on the basis of TOF analysis and described the analytical possibilities of the technique. Nicolai et al. (2008) reported the soluble solids content and firmness of pear (Pyrus communis cv. Conference) by using a time-resolved technique, in which fruit or agricultural products were considered to be the scattering media. Furthermore, it was reported that the time-resolved profile was very efficient for estimating the absorption and scattering coefficients (Gributs and Burns, 2003a, 2003b; Pandozzi and Burns, 2007).
Recently, we proposed a technique combining TOF and NIRS, termed TOF-NIRS, capable of measuring the time-resolved profiles of NIR light with nanosecond resolution. The behavior of a NIR-pulsed laser beam into various fruit was compared with assess the absorption/scattering characteristics of NIR radiation (Kurata et al., 2008). Furthermore, the absorption/scattering conditions of NIR radiation in grapefruit were analyzed in detail using the cross-correlation function, which is used to calculate the similarity between reference and transmitted light (Kurata and Tsuchikawa, 2009).
In previous research, Tsuchikawa and Hamada (2004) applied TOF-NIRS for the detection of sugar and acidity in apple (Malus sylvestris var. domestica), in which attenuance of peak maxima, time delay of peak maxima, and variation of full width at half maximum of the time-resolved profile were used as explanatory variables for multiple linear regression, principle component regression, and partial least squares regression (PLSR) analysis. It was possible to predict both the sugar and acid contents in apple with high precision using TOF-NIRS.
In our previous paper (Kurata and Tsuchikawa, 2009), a Q-switched neodymium yttrium aluminum garnet (Nd:YAG) laser with high-output energy was used as the laser source. The principle of Q-switched Nd:YAG laser is briefly described below. The optical pumping continues as the light energy in the laser resonator is attenuated. During optical pumping, the number of atoms in an excited state in the laser medium increases. As a result, high-intensity output is obtained from the Q-switched Nd:YAG laser by raising the Q factor of the resonator. Here, the laser output of a time-resolved profile was composed of certain peaks because of the excitation characteristics of the Nd:YAG laser. To compensate for this fluctuation, the smoothed time-resolved profile was averaged over a specified time period. The multiple smoothing calculations were done not only for a single-shot pulse, but also for a period of several seconds. Such measurement is unsuitable for online measurement, although the pulse laser itself only requires nanosecond resolution. The time-resolved profile of a single-shot pulse is useful in investigating phenomena in a short time domain.
In this report, a diode-pumped solid-state laser with high-output energy and single-pulse operation was used as the laser source. Analysis of the variation in the single-pulsed laser in a short time domain was used to estimate SSC and acidity in grapefruit, in which the wavelength was varied to obtain the best prediction model. With respect to validation of the PLSR model, TOF-NIRS was compared with the conventional NIR procedure in interactance mode. These basic comparative measurements are essential for constructing a precise, nondestructive online measurement for fruit.
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