Study of the imaging properties of retro-reflective materials used in head-mounted projective displays (HMPDs)

The concept of head-mounted projective display (HMPD) has been recently proposed as an alternative to conventional eyepiece-type head-mounted displays (HMDs). An HMPD consists of a pair of miniature projection lenses and flat panel displays mounted on the head and retro-reflective sheeting material placed strategically in the environment. Recent efforts have been made to demonstrate the feasibility of the imaging concept and prototypes have been built. Our research indicates that the quality and properties of the retro-reflective material play critical roles in the overall imaging quality of HMPDs. The retro-reflective sheeting material is commonly used in traffic control and photonic lighting systems, rather than optimized for imaging purpose as in the HMPDs. The size and shape of the microstructures cause artifacts on images. In this paper, we will mainly focus on the evaluation of the various existing retro-reflective materials, and the examination of the impact of the material characteristics on imaging properties. The basic structures of the existing materials are briefly reviewed, the characteristic parameters used to quantify the material properties are defined, and a few samples are evaluated. The characteristics of interest include: the size and shape of the microstructure, the distribution pattern and density of the microstructure, retro-reflectivity, the profile of the reflected light, diffraction artifacts and ghost imaging. Finally, a comprehensive analysis are presented to examine how the material characteristics play their roles in an imaging system, such as the HMPD, and predict the imaging artifacts caused by these characteristics.