• Most frequently used in higher powered minus lenses generally exceeding � 6.00 diopters.
• Since it is made with titanium or �titanate� instead of lead, it is relatively light in weight, and has a lower rate of chromatic dispersion while maintaining a 1.70 index of refraction.
• Can be chemically treated for impact resistance.

Photochromic Glass:

• Made with silver halide crystals which respond to ultraviolet light of wavelengths between 300 � 400 nm.
• Lenses automatically darken with increased illumina- tion and lighten as illumination decreases.
• Has �exposure memory.� Lenses will change more readily after initial exposures to light. However, if not used for some period of time, a new �breaking-in� period may be necessary.
• Replacing one lens only can be problematic since the new lens may not match the color of the original, even after a �breaking-in� period.

CR-39:

• Most popular lens material used today for a wide variety of single vision and multifocal designs.
• Inherently impact resistant: unlike glass, no special treatment of a CR-39 lens is necessary to pass federal standards of impact resistance.
• Fragmentation: When a plastic lens breaks the pieces are larger and fewer than with a glass lens. Also, the edges of the broken fragments are not as sharp as glass and there generally is an absence of fine particles.