What does “Eye Protection Factor” mean?
There are different Protection Factors (PF) ratings that have been developed in an attempt to quantify the UVR protection that products such as fabrics, sunscreen and eyewear can provide. The PF gives an indication of the amount of UVR that is blocked by the product.
There is a formula for determining PF that uses the effective UVR dose (ED) for unprotected skin and includes consideration of the spectral irradiance, erythemal spectral effectiveness, spectral transmission of the item, bandwidth and wavelength. The formula is shown below.
Depending on the product there are different rating scales eg fabric uses UPF (Ultraviolet Protection Factor), sunscreen uses SPF (Sun Protection Factor) and sunglasses use EPF (Eye Protection Factor). EPF has a numerical rating scale from 1 to 10 with a rating of 9 or 10 transmitting almost no UVR. It is important to remember that the PFs have been determined by looking at UVB only.
SPF is a ratio that tells you how much energy it takes to produce a minimal sunburn through a sunscreen product compared to how much energy it takes to produce the same sunburn without the sunscreen. That means that, if you normally get a minimal burn in 20 minutes, it would take you 15 times as long, or five hours, to get one using a sunscreen with an SPF of 15. Of course, these numbers are generalizations, and will vary with several conditions, including skin type.