These lenses are manufactured by adding metal oxides, such as iron, cobalt, chromium, strontium, nickel, manganese, and some rare earth metal oxides like neodymium, to the general optical glass formula. These metal oxides allow the glass to selectively absorb certain wavelengths of electromagnetic waves in light; for example, cerium and iron oxides can absorb large amounts of ultraviolet light. Using these lenses reduces the amount of certain wavelengths passing through the lens, mitigating or preventing eye damage. Different colored protective lenses absorb different colors of light. These glasses fall into two main categories: absorptive and reflective, with the former being the most common.
Lens blanks are generally round, and their specifications are indicated by their diameter in mm, including φ48, φ50, φ52, φ56, φ60, φ65, and φ70. The most commonly used sizes are φ56, φ60, and φ65. Due to the popularity of large frames, the use of larger lenses such as φ70 has increased.
