Sapphire and Ruby Ball Lenses
藍寶石及紅寶石球形透鏡皆由Al203製成。紅寶石或摻紅寶石的藍寶石顯示紅色，是因為其包含微量的氧化鉻 (紅寶石球形透鏡中的鉻含量通常超過0.5%)。 While their physical and chemical properties are basically the same, their optical properties are somewhat different. Sapphire has superior optical transmission qualities. Ruby Balls are easier to see and therefore easier to handle for physical applications.
For general information about ball lens, as well as how to calculate the NA and Focal Length, view Understanding Ball Lenses.