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Refractive Index:The refractive coefficient of glass bead to light. They are divided into series of 1.5Nd, 1.9Nd,2.2Nd. Nd is the unit of refractive index.（如下图1）
图1
Roundness Ratio:The percentage of round glass beads with total glass beads. The higher the value, the better the quality.
Opacity:The percentage of nontransparent glass bead with total glass bead. The lower value, the better quality.
Retro-reflection:When light illuminates surfaces and returns its source, it will be retro-reflection. When the objects have more than 1.9Nd refractive index, there will be formed retro-reflection.（如上图1）
“Encapsulated-Lens” type retro-reflective articles:A cover film disposed in front of the glass bead, and on the front of the glass bead while retaining their air-incident interface so as to maintain the necessary optical relationships for retro-reflective. High intensity grade reflective sheet belongs to this type. Said glass bead are 1.9nd glass bead. （如下图2）
图2
“Enclosed-Lens” type retro-reflective articles:A cover film disposed in front to the glass bead so that said bead do not exposed in air. Engineering Grade and Super Engineering Grade belong to this type. Said glass beads are 2.2nd glass bead （如下图3）
图3
“Exposed-Lens” type retro-reflective articles:Glass beads are on the surface of reflective article and exposed in the air. Reflective fabric, reflective leather, reflective ink and reflective paint belong to this category. Said beads are 1.9Nd glass bead（下图4）
图4
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