M7 Labs uses a tool called a spectrometer to evaluate how lenses filter light across the ultraviolet, visible, and beginning near infrared portion of the light spectrum. They calculated the ratio of total UVA energy passing through the windscreen divided by the total UVA energy of the sunlight shining towards the windscreen. These measurements provide a calculated value for the percentage of UVA that passes through the windscreen.
Some windscreens do an excellent job of filtering out UVA light - but not all. UVA is a pronounced problem for pilots because the intensity of UVA radiation increases approximately 15% for every 3,000 feet of altitude above sea level. The data also bases on the service ceiling for each aircraft because altitude plays a key role in how much UVA a pilot might be exposed to.
Out of the 73 aircraft tested, 45 windscreens removed 100% of the UVA light, including the Stemme models. Seven aircraft allowed 30% or more UVA light to enter through the windscreen. Most cockpit windscreens do a good to reasonable job of filtering out UVA light.