Near-infrared nondestrutive evaluation of food based on multi-frequency Lock-in detection

Infrared spectroscopy and imaging find widespread application in food analysis to guarantee quality, chemical integrity and to detect foreign bodies. The use of thermal or hyper-spectral cameras operating in the near or in the middle infrared range is continuously increasing aiming at implementing in-line inspection during the production line. Anyway such techniques, especially when working on the near-infrared range, still remain mainly limited at inspecting the surface or the early sub-surface layer of the samples. The need of bulk inspection techniques capable of working in-line is therefore still unfulfilled. To partially reply at this request, a through-transmission laser imaging system operating simultaneously with four different near-infrared wavelengths was developed. A multi-frequency Lock-In technique is exploited to counterbalance the huge attenuation due to the transmission and to efficiently extract the information at each wavelength from a single acquired signal. Experimental results obtained on several specimens with or without packaging are reported and the multispectral image processing is discussed.