As the title of this paper suggests, there are new optical systems that may be used for security applications.
Several will be discussed here, along with a new selection procedure for consideration by those who search for
utilization of the most effective "security" system to meet their requirements.
Edwin A. Grant,
"Recent advances in retroreflective security materials", Proc. SPIE 1210, Optical Security and Anticounterfeiting Systems, (1 April 1990); doi: 10.1117/12.17918; https://doi.org/10.1117/12.17918