[Measurement of fruit maturity based on laser-induced photoluminescence spectrum]

Grounding on the concepts of biophotonics measurement, the authors first used a red semiconductor laser (655 nm) to irradiate fruits. Compared with other kinds of illuminating sources, the red semiconductor laser is less expensive and takes little space. The laser-induced photoluminescence spectrums could be detected by coupling fibre-optics probe when the fruits are illuminated by laser. And the spectrum has a distinct peak of relative intensity around the 685 nm wavelength that varies with the degree of fruit maturity. Sugar content measurement was used to prove the laser-induced photoluminescence measurement. The authors tested the sugar content of the fruit specimens, and found that the relative peak value of the fruits' laser-induced photoluminescence spectrum decreases with the increase in their sugar content. The authors used partial least-squares (PLS) regression to perform an analysis of the relationship between the laser-induced photoluminescence intensity and the sugar content, fitting a curve of the two parameters. The correlation coefficient r of the fitted value and the actual value is 98.92% for red-inside plum and 97.31% for nectarine. So the authors could generalize that there is an approximate linear relationship between the peak value of laser-induced photoluminescence intensity and the sugar content of fruits, and we could use the maturity measurement based on this concept to decide the fruit ripeness. The authors designed the analytic program for this laser-induced photoluminescence spectrum measurement system, which mainly realizes two functions: generating the standard ripe spectrum of a certain kind of fruit from a quantity of their spectra, and, according to this standard spectrum, determining the maturity degree of an unknown spectrum, and at the same time, displaying the unknown laser-induced photoluminescence spectrum. Incorporating this analytic program with the optical spectrometer, it becomes conceivable to test the fruit maturity very conveniently and quickly. The measurement system of fruit maturity based on laser-induced photoluminescence spectrum has also been used to test various fruits. This measurement is nondestructive and inexpensive, and does not require complicated equipment, a feature of great importance in real-time measurement of fruit maturity.