Transparency and Tint:
This what differentiates the suitability of a goggle lens for sunny or dark cloudy days. A darker lens with less transparency or light transmission reduces the level of light and is good for a bright sunny day.
The choice of the lens tint has a way of accentuating the contour of the snow on a cloudy flat light day.
A proper light color tint (light yellow, orange, pink, etc) will actually help you see better and thus ski better while your friends are struggling just to see what is in front of them.
Polarized:
This adds expense to a lens but is very valuable in reducing glare and peak light bursts due to glare which reflect off of the snow and ice. I notice the difference at the end of the day.
UV protection:
It is not only the visible light but also the Ultraviolet (UV) "A", "B" and "C" rays that you must be concerned with especially on a sunny day. A dark lens may reduce the visible light reaching your eye which will have the effect of dilating your pupil.
This will let in more UV than if you were not wearing goggles at all and were viewing with a contracted pupil adjusted for the bright light.
The bottom line is that a tint alone is worse than unprotected vision. You really need to combine a tint with a Polarized lens treated to block UV "A", "B" and "C" rays.
Photochromatic:
These lenses get darker when the sun gets brighter and lighter when the sun is reduced.
Ventilation:
Ventilation is necessary to avoid a development of moisture on the lens which will impede vision.
Too much ventilation and your eyes may tear at higher speeds.
Lens removal/change:
Most goggles will allow a lens change. The new Giro goggles shown below show their new mechanism for lens changes. It supports a more positive fit and replacement.
Prescription Lenses:
Ski goggles are available with prescription lenses either mounted directly to the view shield or held behind the shield with lenses mounted to a universal insert which will hold lenses in any ski gogggle.
See SportRx for more information.
Oakley Ski Goggles: Canopy
Goggles with Displays: GPS
Recon Instruments: Displays time, temperature, altitude and speed. Options to recall distance and your mountain stats. Can download information to your computer to plot your trail on Google Earth. Interfaces with Droid app for buddy tracking.
Zeal Optical Trancend: This solar powered backpack will recharge your GPS Goggles and any USB equipment.
