Correlations between Refractive Index and Retroreflectance of Glass Beads for Use in Road-marking Applications under Wet Conditions

  • Shin, Sang Yeol (Department of Materials Science and Engineering, Korea Aerospace University) ;
  • Lee, Ji In (Department of Materials Science and Engineering, Korea Aerospace University) ;
  • Chung, Woon Jin (Division of Advanced Materials Engineering, Kongju National University) ;
  • Choi, Yong Gyu (Department of Materials Science and Engineering, Korea Aerospace University)
  • Received : 2019.06.18
  • Accepted : 2019.08.20
  • Published : 2019.10.25


Visibility of road-surface markings is one of the critical issues that should be secured for self-driving cars as well as human drivers. Glass beads are taking on the role of retroreflectors, and therefore are considered a necessity in modern pavements. In this context, retroreflectance is sensitively dependent not only on the refractive index of glass beads but also on that of the surrounding medium. This implies that the optimum refractive index of glass beads immersed in water, i.e. under wet conditions, is different from that of glass beads surrounded by air, i.e. under dry conditions. A refractive index of approximately 1.9, which is known to maximize retroreflectance under dry conditions, actually exhibits much poorer retroreflectance under wet conditions. This suggests that glass beads with optimal refractive index for wet conditions need to be installed together with those for dry conditions. We propose a facile but practical model capable of calculating retroreflectance of glass beads surrounded by an arbitrary medium, here water in particular, and experimentally verify its capability of assessing the refractive index of commercial glass beads. Changes in retroreflectance according to the mixing ratio of glass beads with different refractive indices are also discussed, in an effort to propose the proper use of glass beads produced for dry and wet conditions.


Glass beads;Road surface marking;Refractive index;Retroreflectance;Pavements


Supported by : Ministry of Trade, Industry and Energy of Korea


  1. L. A. Ivanov, D. V. Kiesewetter, N. N. Kiselev, V. I. Malyugin, and V. A. Slugin, "Measurement of retroreflection by glass beads for road marking," Proc. SPIE 6251, 62510U (2006).
  2. T. Grosges, "Retroreflection of glass beads for traffic road stripe paints," Opt. Mater. 30, 1549-1554 (2008).
  3. J. T. Lee, T. L. Maleck, and W. C. Taylor, "Pavement marking material evaluation study in Michigan," ITE J. Inst. Transp. Eng. 69, 44-51 (1999).
  4. T. Schnell, F. Aktan, and Y. C. Lee, "Nighttime visibility and retroreflectance of pavement markings in dry, wet, and rainy conditions," Transp. Res. Rec. 1824, 144-155 (2003).
  5. D. M. Burns, T. P. Hedblom, and T. W. Miller, "Modern pavement marking systems: Relationship between optics and nighttime visibility," Transp. Res. Rec. 2056, 43-51 (2008).
  6. Glass beads for traffic paint, KS L 2521, Korean Standard Association, Seoul (2017).
  7. H. Fuquan, L. Shangying, and W. Shaomin, "The refractive index measurement of high refractive index glass beads," Acta Photon. Sin. 30, 753-756 (2001).
  8. F. Sarcinelli, R. Pizzoferrato, and F. Scudieri, "Study of the refractive index of microscopic glass beads by light-refraction analysis," Appl. Opt. 36, 8999-9004 (1997).
  9. J. L. Hand and S. M. Kreidenweis, "A new method for retrieving particle refractive index and effective density from aerosol size distribution data," Aerosol. Sci. Technol. 36, 1012-1026 (2002).
  10. A. Leblance-Hotte, R. St-Gelais, and Y.-A. Peter, "Optofluidic device for high resolution volume refractive index measurement of single cell," in Proc. 16th International Conference on Miniaturized Systems for Chemistry and Life Sciences (Japan, Oct. 2012), pp. 1330-1332.
  11. T. Yamaguchi, "Refractive index measurement of high refractive index glass beads," Appl. Opt. 14, 1111-1115 (1975).
  12. R. W. Spinrad and J. F. Brown, "Relative real refractive index of marine microorganisms: a technique for flow cytometric estimation," Appl. Opt. 25, 1930-1934 (1986).
  13. S.-Y. Li, S. Qin, D.-H. Li, and Q.-H. Wang, "Using a laser source to measure the refractive index of glass beads and Debye theory analysis," Appl. Opt. 54, 9688-9694 (2015).
  14. S. Y. Shin, J. I. Lee, W. J. Chung, S.-H. Cho, and Y. G. Choi, "Assessing the refractive index of glass beads for use in road-marking applications via retroreflectance measurement," Curr. Opt. Photon. 3, 415-422 (2019).
  15. Standard test method for measurement of retroreflective signs using a portable retroreflectometer at a 0.2 degree observation angle, ASTM E1709-16e1, ASTM International, Pennsylvania (2016).
  16. Standard test method for measurement of retroreflective signs using a portable retroreflectometer at a 0.5 degree observation angle, ASTM E2540-16, ASTM International, Pennsylvania (2016).
  17. M. D. Stoudt and K. Vedam, "Retroreflection from spherical glass beads in highway pavement markings. 1: Specular reflection," Appl. Opt. 17, 1855-1858 (1978).
  18. O. Smadi, R. R. Souleyrette, D. J. Ormand, and N. Hawkins, "Pavement marking retroreflectivity analysis of safety effectiveness," Transp. Res. Rec. 2056, 17-24 (2008).