How this calculator works
Optical density is the negative base-10 logarithm of transmittance. It adds when independent passive attenuation layers multiply their transmissions. The same ratio is often called absorbance in spectroscopy, but attenuation by reflection or scattering can make apparent absorbance differ from absorption alone. Incident and transmitted intensities must use matching measurement geometry and units.
Formula
T = I/I0; OD = -log10(T) = log10(I0/I); T = 10^(-OD); attenuation in dB = 10 OD.
Worked example
Incident intensity 100 W/m2 and transmitted intensity 1 W/m2 give T = 0.01, OD = 2 and 20 dB attenuation. OD = 1 transmits 10%.
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Frequently asked questions
Can optical density be negative?
An active gain system can give a negative log ratio. This tool is limited to passive samples with transmission at most one.
What does OD 3 mean?
It transmits 0.1% of the incident intensity under the stated measurement conditions.
Can I use this to certify laser eyewear?
No. Eyewear selection also depends on wavelength, exposure conditions and certified protection specifications.