How does E-SPF work?
The higher the E-SPF, the better the UV protection
The Eye-Sun Protection Factor is inspired by the protection index displayed prominently on daily moisturizers, sunscreens and sun-protective clothing, and used to rate effectiveness against ultraviolet radiations : the higher the E-SPF value, the greater the level of UV protection.
Wearing a lens with an E-SPF value of 25 means that your eye is 25 better protected that without any lens. (The E-SPF excludes direct eye exposure from around the lenses*)
E-SPF 25 means your eye is 25 times better protected than without any lens.
All lenses are not equal in terms of UV protection
UV protection is not linked to the color of your lenses. In fact, some transparent lenses can protect you even better than some sun lenses**. Whatever its tint, a lens’ protection is a result of a combination of a material and a coating, which varies from one lens to another. As a result, not all lenses are equal in terms of UV protection.
Make sure you get the right protection for your eyes
With the Eye-Sun Protection Factor, you can now compare the levels of UV protection provided by all kinds of eyewear, including clear, photochromic, and sun lenses (tinted or polarized). Ask your Eye-Care Professional for advice.
Up to 25 for corrective clear lenses
Up to 50+ for sun lenses
Based on measurements made up until now***, the E-SPF values vary from 2 to a maximum of 25 for daily eyeglass lenses and up to 50+ for sun lenses. 
25 is the best rate obtained today for a clear lens with perfect transparency.
For sun lenses, the E-SPF can reach a maximum of 50+
*Lens performance only: The E-SPF excludes direct eye exposure that depends on external factors (wearer’s morphology, frame shape, position of eyewear…)
**Your daily eyeglasses can’t replace your sunglasses in full sunlight: sunwear equipment offers better visual comfort while protecting a wider area around the eye because frames are usually larger.
** April 2012