I found this at
http://pinlockusa.net/FAQWhat does “Day Time Use Only” mean?
If you look at your helmet shield, it is marked with “VESC-8”. This refers to a United States Standard for light transmittance for a single pane visor/shield. All Visors/shields must meet certain standards, one of which is the transmittance of light.
The requirement for motorcyclists’ eye protection is found in Section 7 of the Vehicle Equipment Safety Commission’s regulation VESC-8 adopted in 1971 and amended in 1980:
Section 7. Light Transmittance of Eye Protection Devices (EPD)
7.1 CLEAR – A clear EPD is a device which transmits not less than 85% of the incident visible radiation.
7.2 TINTED – A tinted EPD is a device which transmits less than 85% of the incident visible radiation but not less than 20%.
7.2.1 A tinted EPD shall not impair the wearer’s ability to discern color.
7.2.2 A tinted EPD shall not be used at night.
A shield may have less than 85% transmittance as long as it is not less than 20% and does not impair the ability to discern color. Any shield that transmits less than 85% cannot be used at night and can only be used during the day time.
Most shields/visors have a 90 – 91% light transmittance. When you combine the shield/visor with another shield/visor or in our case with a Pinlock lens, the light transmittance will be reduced. The combination of the Pinlock lens and shield will have a light transmittance range from over 20 % to a little over 80%. Therefore, the requirement to place “Day Time Use Only” on the lens. Pinlock does not impede the wearer’s ability to discern color.