A cross-polarized freeform illumination design for glare reduction in fruit quality inspection

Janos C. Keresztes, Richard John Koshel, Russell A Chipman, John C. Stover, Wouter Saeys

Research output: Chapter in Book/Report/Conference proceedingConference contribution

2 Scopus citations


Common illumination systems in short wave infrared (SWIR) hyperspectral imaging (HSI) include direct or indirect tungsten halogen lights. While direct lights provide more radiation onto the samples than dome setups, thus being more energy efficient, the acquired images often suffer from specular reflections and gloss. Glare artifacts in images increase variability in the data limiting the accuracy of machine vision algorithms for defect detection and quality inspection, or even providing false positives. Although domes are known to provide a near Lambertian illumination and glare free images, glossy regions and heterogeneities may remain in the data in practice. More particularly, in the field of fruit and vegetable quality inspection, due to their waxy surface, it remains challenging to design an efficient realistic lighting system. This paper suggests a new approach to optimize the illumination of fruit and vegetables based on measurements of the bidirectional reflectance distribution function (BRDF), shape and Stokes parameters. From these measured values, a BRDF model is loaded into ray-Tracing software for realistic illumination engineering in order to determine the most suitable illumination scheme. This concept is applied to apples and a cross polarizer (CP) with freeform optics (FO) optical configuration is proposed, which allows the FO to be optimized to maximize uniformity in the field of view of the imager and removes the parallel polarized gloss on the apples. The performance of this CP illumination system was determined experimentally for a set of apples. This cross polarized (CP) illumination system provided a uniformity (U) of 92% and an efficiency (ν) of 90%, while U = 87% and ν = 14% for an ideal dome configuration when illuminating a rectangular target. The simulated imaged apples with assigned optical properties performed better with CP (U=80%) than when using a dome (U=73%) by 7%. Finally, the sensitivity of the design for the light positioning and spectral tolerance are investigated.

Original languageEnglish (US)
Title of host publicationProceedings of SPIE - The International Society for Optical Engineering
ISBN (Print)9781628418187
StatePublished - 2015
EventOptical Systems Design 2015: Illumination Optics IV - Jena, Germany
Duration: Sep 7 2015Sep 8 2015


OtherOptical Systems Design 2015: Illumination Optics IV


  • BRDF
  • crosspolarization
  • freeform optics
  • illumination design
  • non-imaging optics
  • polarization
  • SWIR hyperspectral imaging

ASJC Scopus subject areas

  • Applied Mathematics
  • Computer Science Applications
  • Electrical and Electronic Engineering
  • Electronic, Optical and Magnetic Materials
  • Condensed Matter Physics


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