Optical Density, also referred to as OD, is the amount of energy transmitted through a lens. This uses a logarithmic scale, which is scaled by factors of ten. An increase of 1 point is a tenfold difference in measurement of OD. For example, an OD of Zero is 100% transmission. An OD of 1 is 10%, OD 2 = 1%, OD 3= 0.1%, OD4=0.01% and so on. On the other hand, the OD can also show how much light is blocked. For example an OD of 2 deters 99%, OD of 3 deters 99.9%, OD of 4 deters 99.99% and so on.