Structured Illumination Reflectance Imaging for Enhanced Detection of Subsurface Tissue Bruising in Apples

Abstract

Abstract. In this research, a structured illumination reflectance imaging (SIRI) system was used as a novel method for detection of fresh bruises on apples. The SIRI system projects sinusoidal patterns of illumination onto samples, and image demodulation is then used to recover depth-specific information through varying the spatial frequency of the illumination pattern. The capability of SIRI was demonstrated through the detection of artificially induced bruises on ‘Golden Delicious’ and ‘Delicious’ apples with varying levels of bruising. It was hypothesized that by confining the light penetration depth near the surface of each fruit, subsurface defects such as bruising should be more apparent under SIRI than conventional planar illumination imaging. Three 120° phase-shifted reflectance images were acquired from 60 fruit each of ‘Golden Delicious’ and ‘Delicious’ varieties at 0 h, 4 to 6 h, and 24 h after impact bruising for each of the four spatial frequencies (i.e., 0, 0.10, 0.15, and 0.25 cycles mm-1). The reflectance images acquired by the system were then demodulated into an alternating component (AC) and direct component (DC), where the AC contained depth-specific information and the DC image represented the diffuse reflectance from the apple sample under uniform (or planar) illumination. Bruise detection algorithms were developed and applied to the demodulated AC and DC images. The SIRI system achieved 70% to 100% bruise detection rates, compared to 0% to 50% detection rates under conventional planar illumination. However, detection results were influenced by both severity of bruising and bruise development after impact; better bruise detection results were obtained when bruises had developed for 4 to 6 h after impact. SIRI has demonstrated a superior capability of detecting fresh bruises, and it is promising as a new imaging modality for quality detection of agricultural products. Keywords: Apples, Bruises, Defects, Fruit, Imaging, Nondestructive, Quality, Sorting, Structured illumination.