BS EN ISO 12312-1:2013¹¹³  attributes filters for sunglare use into five groups, according to their range of luminous transmittance (Tv): 
 

Filter category	Description	Range of luminous transmittance in the visible spectral range
0	clear or very light tint	from over 80% to 100%
1	light tint	from over 43% to 80%
2	medium tint	from over 18% to 43%
3	dark tint	from over 8% to 18%
4	very dark tint	from over 3% to 8%

Filters suitable for road use and driving shall be of categories 0, 1, 2 or 3 and in addition:

1. the spectral transmittance of filters suitable for road use shall be not less than 0.2 x Tv for wavelengths between 475 and 650 nm, and
2. the relative visual attenuation coefficient Q of filters of categories 0, 1, 2 and 3 suitable for driving and road use shall be not less than 0.8 for red and yellow signal lights, not less than 0.6 for yellow, green and blue signal lights.

Sunglass filters with a luminous transmittance of less than 75% are not suitable for road use in twilight or at night.