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).
Relationship to % Transmission (%T):
- % Transmission (%T) is how much light passes through, as a percentage.
- Formula:
%T = T × 100% - As OD increases, % transmission drops quickly.
Examples:
- 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% |
Why use OD?
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.
Frequently Asked Questions
https://www.kupooptics.com/en/blogs/q-a/optical_density
What is optical density (OD) in optical filters?
Optical density (OD) is the base-10 logarithm of the ratio of incident to transmitted light intensity: OD = log₁₀(I₀/I). It quantifies how much a filter attenuates light. OD 1 = 90% blocked (10% transmitted), OD 2 = 99% blocked, OD 4 = 99.99% blocked, OD 6 = 99.9999% blocked.
How is OD different from transmission percentage?
Transmission percentage is linear: 50% T means half the light passes. OD is logarithmic: OD 2 means 1% T (99% blocked), OD 4 means 0.01% T. OD makes it easier to express very high blocking levels—"OD 6" is far clearer than "0.0001% transmission."
Why is OD used to specify filter blocking rather than percentage?
For high-blocking applications (laser protection, fluorescence rejection), the blocking levels are so extreme that percentages become impractical. OD 6 (one part in a million) is far easier to communicate than "0.0001% transmission." OD also adds directly when stacking filters: OD 4 + OD 4 = OD 8 combined blocking.
What OD level do I need for my application?
General rule: OD 3–4 for indoor/controlled light; OD 4–5 for outdoor/bright ambient; OD 5–6 for precision spectroscopy, fluorescence, or military laser protection. The required OD equals log₁₀(background power / acceptable background at detector). Match OD to your system's noise floor, not to a round number.
