B15 XLT Lens
Ray-Ban B15 XLT lenses are brownish in color, block 100% of all UV rays and allow only 15% of the visible light to pass through them (that's an 85% reduction!). These brown lenses also offer a bit more contrast by cutting down on the amount of blue light, as opposed to a G15 lens (which is described in the next paragraph). Blue light is very easily captured by the eye and has the effect of softening other colors that are harsher to our vision. In other words, by reducing the amount of blue light, the differences between colors and brightness of objects are more pronounced.
G15 XLT Lens
The G15 XLT lenses have the same properties as the B15 XLT lenses; however they're of grayish/greenish color. The G15 lens is the original aviator lens that the military requested Ray-Ban to develop. The challenge was to design a lens that was 'color neutral'; in other words, a lens that filters light like the human eye does. If you want a lens that offers a little less contrast between colors, then the G15 XLT lens is for you because it offers similar color-sensitivity to the human eye. With a lower contrast lens, the difference in color and brightness of objects will be dampened. This means that the colors you see through these lenses will be natural and soft on the eyes.
Both the B15 and G15 XLT lenses are made of impact resistant optical glass that is naturally scratch resistant. Quality and stability are remarkable and over time you will notice virtually no fading whatsoever in the color.
Polarized lenses essentially cut down the amount of glare produced by reflective surfaces like water or other semi-transparent objects, and even some metallic surfaces. Glare refers to the difficulty of seeing in bright light.
Sunlight, for example, bounces off surfaces in all directions (i.e. vertically, horizontally or diagonally). For people who are active in the 'horizontal plane', it means that the bright sunlight bounced horizontally enters the eye in a similar fashion. This causes the pupils to constrict and significantly reduce contrast. That's why drivers have difficulty seeing the road ahead of them if they're driving in very sunny conditions (if the sun is ahead of them).
Polarized lenses have a laminated surface with vertical stripes that create a filter, which effectively cuts out the horizontally-polarized light and allows only vertically-polarized light in. In other words, if you happen to be on a clear lake you can probably see under the surface or if you're driving, you can now see the road ahead.
Some limitations to polarized lenses are that LCD displays may become distorted or you may notice a 'rainbow' effect if your car's windows are tinted.
All 3 lenses are very useful, particularly the polarized ones, but expect to pay a premium on the sunglasses.
The lens base simply refers to the base curve or the front curve of the lens measured in 'diopters', which is a term that really only matter to opticians. You'll find that many Ray-Ban aviators have a 'Lens Base 6', which basically means that they're sufficiently curved and designed to function as sunglasses (i.e., they cover the eyes enough).
Lens size is measured in millimeters and refers to the length (or width) of one lens. Bear that in mind when making a purchase. Many Ray-Ban models come in several different sizes of lens. Ensure you get the size that fits your face or you may end up with a pair of aviator shades that don't fit properly.
Some Ray-Ban sunglasseshave lens sizes that are designated as extra small, small, etc. This is common for models that do not resemble traditional glasses with two lenses. These sunglasses have a 'uni-lens' design, where the lenses are connected seamlessly through the bridge. For this reason, traditional sizing is not applicable. These shades are for a high fashion look.