This ranking system is a clip in which are mounted prescription lenses, and that attaches to the frame on its inner side. The lenses that adapt treatments are white and without sunscreen, as the sunglasses of the glasses are kept.
Prescription lenses are CR39.
The optical clip is usually the most economical way to graduate a sporting spectacle, however it is the least recommended:
Increased risk of fogging lenses
Reduced field of vision.
On the other hand it is advisable in cases where the user uses the same goggle for the use of contact lenses or have a high ranking (more than 6.00 diopters).