Specific gravity (SG) is defined as the ratio of the density of a given material to the density of water at a specific temperature and pressure, typically at 4 °C (39 °F) and 1 atm (760.00 mmHg).
The density of a material is defined as its mass per unit volume. The symbol of density is ρ (the Greek letter rho).
Where: ρ is the density,
m is the mass,
V is the volume.
We use specific gravity to compare between different lens materials in the same prescription and similar optical index in order to provide patient with as light as possible lenses.
Abbe Number/Values for the most commonly used lens materials