The light penetration depth in intact potatoes was found to be approximately 20 mm. On the other hand, when employing a commercial on-line NIR interactance imaging system, lower prediction performance (R2¼0.83) was obtained [4]. The lack in performance for measurements of whole unpeeled potatoes, with the latter instrument, can most likely be ascribed to shallow light penetration (o10 mm) into the potato combined with the heterogeneous distribution of dry matter content within the potato tuber