Gravel /ËÉ¡rÃ¦vÉl/ is a loose aggregation of rock fragments. Gravel is classified by particle size range and includes size classes from granule- to boulder-sized fragments. In the Udden-Wentworth scale gravel is categorized into granular gravel (2 to 4 mm or 0.079 to 0.157 in) and pebble gravel (4 to 64 mm or 0.2 to 2.5 in).
OD - This is shorthand for oculus dextraous which means Right Eye. OS - This is shorthand for oculus sinister which really means Left Eye. OU - This is shorthand for oculus uterque which really means Both Eyes. If you see this, you'll want to enter the prescription the same on both eyes. Sphere or Power - SPH or PWR - This is the main strength of your eyeglass prescription, and is written in 0.25 increments. If the field has 'SPH', 'PL', 'PLANO', or '00', this means your there is a 0.00 power as should be entered as so.
Lastly, there can also be numbers denoting the amount of near reading strength needed (bifocal or progressive). They usually go from +0.75 to +3.00, depending on age and visual need. The letters OD and OS in front of a prescription let us know which eye each string of numbers is for. OD (Oculus Dexter) stands for right eye and OS (Oculus Sinister) for left eye, while OU (Oculus Uterque, Oculus Unitas or Oculus Uniter) means both eyes.