What is optical density (OD) and how does it relate to % transmission?
Optical Density (OD) is a measurement that tells us how well an optical filter or material blocks light. It's especially useful for filters that let very little light through—such as those designed to block certain colors or wavelengths.
Definition:
OD is calculated as:
OD = -log10(T)
Where T is the transmission value (from 0 for no light, to 1 for all light passing through).
- % Transmission (%T) is how much light passes through, as a percentage.
- Formula:
%T = T × 100% - As OD increases, % transmission drops quickly.
- OD 1 = 10% transmission (0.1)
- OD 2 = 1% transmission (0.01)
- OD 3 = 0.1% transmission (0.001)
- OD 4 = 0.01% transmission (0.0001)
- OD 5 = 0.001% transmission (0.00001)
| Optical Density (OD) | Transmission (Fraction) | Transmission (%) |
|---|---|---|
| 0 | 1 | 100% |
| 1 | 0.1 | 10% |
| 2 | 0.01 | 1% |
| 3 | 0.001 | 0.1% |
| 4 | 0.0001 | 0.01% |
| 5 | 0.00001 | 0.001% |
For filters that block almost all light, OD is easier to use than very tiny percentages. For example, instead of writing "0.0001%", you can simply say "OD 6".
Measurement Note:At very high OD levels, measurements can hit the instrument noise floor—the smallest value your instrument can detect.
Summary:- OD is a logarithmic measure of how much light is blocked.
- Higher OD = less light passes through.
- It's essential for comparing and specifying optical filters.
