BRITISH STANDARDS
 

Impact resistance for mechanical strength F is tested using  a ballistic test -  firing a 6mm steel ball weighing 0.86g travelling at maximum 45 m/s at the time of impact.
 
mechanical strength S is tested by simply dropping a 43g 22mm steel ball onto the lens from a height of 15 inches.
 
Grade F is suited for tasks like drilling, sawing, grinding, hammering or risk from any process which could emit particles whose energy equates to the test specifications above. 
 
Grade S can be used in hazardous areas where there is risk of small flying particles from other workers or processes in the vicinity but where the wearer is not close to the source of the hazard

SAFETY GOGGLE

 
Housing has higher impact resistance with greater protection from hazardous liquids, dust and gases. Gasket on the rear forms a tight seal with the wearers skin.   
         
Goggles provide the minimum protection with mechanical strength B (6mm 0.86g steel ball at max 120m/s on impact)

Housing has even higher impact resistance but offering head protection and prevents damage to soft tissues of the face. Often safety spectacles are worn beneath in addition.

Face shields provide protection with mechanical strength A (6mm 0.86g steel ball at max 190m/s on impact)