Most of our trial lens sets are using the K9 glasses(left side) instead of normal window glasses(right side), which is higher light transmission and reflect index.
K9 glass, sometimes referred to as K9 crystal, is a variety of optical borosilicate crown glass. The letter "K" is a reference to the German word for "crown" (Krone) and the number 9 refers to the lead content of the glass (9%). K9 glass has high optical clarity, and is used in many contexts requiring this characteristic.
Parameter of K9 Lenses used Reflex Index: 1.523, Abba Index 58, λ=546.07, Processing Standard: GB/T1050