Table of Contents
What Is Optical Density?
Optical density (OD), also called absorbance, is a logarithmic measure of the fraction of light absorbed by a material. An OD of 1 means 90% of light is absorbed (10% transmitted), OD of 2 means 99% absorbed, and OD of 3 means 99.9% absorbed. It is the primary measurement in UV-Vis spectrophotometry used in chemistry, biology, and materials science.
Optical density follows the Beer-Lambert law, which states that absorbance is proportional to both the concentration of the absorbing species and the path length through the sample. This linear relationship makes spectrophotometry an invaluable tool for quantitative analysis of solutions.
Beer-Lambert Law
Where A is absorbance (OD), T is transmittance (0-1), ε is molar absorptivity, c is concentration, and l is path length.
OD Conversion Table
| OD | Transmittance (%) | Light Blocked (%) |
|---|---|---|
| 0.1 | 79.4% | 20.6% |
| 0.5 | 31.6% | 68.4% |
| 1.0 | 10% | 90% |
| 2.0 | 1% | 99% |
| 3.0 | 0.1% | 99.9% |
Frequently Asked Questions
What is the practical measurement range?
Most spectrophotometers give reliable results between OD 0.1 and OD 2.0. Below 0.1, the signal is dominated by noise. Above 2.0, stray light in the instrument causes significant errors. For samples with high OD, dilute before measuring.
What is the difference between OD and absorbance?
They are the same quantity. "Optical density" is the older term, while "absorbance" is preferred in modern usage. Both equal -log10(T). Note that some fields use "optical density" to mean different things (e.g., in photography it refers to film density).