...
2.24.9 The entire apparent surface of direction indicator lamps must be visible 15° up and 5° down. Minimum horizontal visibility requirements (of the right direction indicator lamps) are shown in the following diagram.
Direction indicators for the front of the vehicle:
Category 1:For use at a distance not less than 40 mm from the dipped-beam headlamp and/or the front fog lamp;
Category 1a:For use at a distance greater than 20 mm but less than 40 mm from the dipped-beam headlamp and/or the front fog lamp;
Category 1b: For use at a distance less than or equal to 20 mm from the dipped-beam headlamp and/or the front fog lamp.
Categories 2a and 2b: Direction indicators for the rear of the vehicle.
Category 2a:Rear direction indicator lamps with steady luminous intensity.
Category 2b:Rear direction indicator lamps with variable luminous intensity.
Categories 5 and 6: Supplementary side direction indicators for use on a vehicle also equipped with categories 1, 1a or 1b and 2a or 2b direction indicators
https://www.unece.org/fileadmin/DAM/trans/main/wp29/wp29regs/2015/R006r6e.pdf
...