u factor rating

Hello. I am trying to find out if a window has a U-factor rating of .30, what percentage of light (UV rays) are being blocked? Can you assist? Thank you very much!!

U-Factor measures the rate of heat transfer and tells you how well the window insulates. U-factor values generally range from 0.25 to 1.25 and are measured in Btu/h·ft²·°F. The lower the U-factor, the better the window insulates.

What you are asking about is Low E which is a thin film applied to the glass which can block UV rays.
Call the manufacturer for that info.