# Optical Density

Optical density is a measure used for filters to describe how much light of a blocked wavelength still passes the filter

Abbreviation: ODn

the optical density is given by the formula:
ODn = 10^-n = 1/10^n
OD0 = 10^-0 = 1/10^0 = 1 = 100% light passes
OD1 = 10^-1 = 1/10^1 = 0.1 = 10% light passes
OD2 = 10^-2 = 1/10^2 = 0.01 = 1% light passes
OD3 = 10^-3 = 1/10^3 = 0.001 = 0.1% light passes
OD4 = 10^-4 = 1/10^4 = 0.0001 = 0.01% light passes
The optical density is an input value, demanded by the Application.
OD2 equals 1 percent.
For a camera of 256 intensity levels this equals 2.6 intesity values. This is not much stronger than the sensor noise.

OD3 entspricht 0.1 Prozent.
For a camera of 256 intensity levels this equals 0.26 intesity values. This is less than the sensor noise.

The higher the number the better the blocking.
The higher the number the more expensive the filter.
Typical values for comemrcial filters are OD2,
higher quality filters have OD3
Filters ilters for outdoor use might need OD4 or higher to block too intensive sunlight.
For a + b = c we get ODa + ODb = ODc
so you can generater an OD6 filter by two OD3 filters. However, the optics should be prepared for the thicker glass then.