Optical Density Calculator
Calculate optical density from incident and transmitted light intensity for spectroscopy and lab absorbance measurements.
What Is Optical Density?
Optical density (OD), also called absorbance, measures how strongly a sample attenuates light. Spectrophotometers and lab filters use OD to compare incident and transmitted beam intensity without working directly with raw photon counts.
Formula
$$OD = \log_{10}\left(\frac{I_0}{I}\right)$$\(I_0\) is incident intensity and \(I\) is transmitted intensity. Transmittance is \(T = I / I_0\), so \(OD = -\log_{10}(T)\).
Practical Uses
OD appears in UV-Vis spectroscopy, laser safety filters, and microbiology plate readers. Each unit of OD (base 10) means transmitted light drops by a factor of 10.
Related tools: Negative Log Calculator and Beer-Lambert Law Calculator.
Frequently Asked Questions
What is the difference between OD and absorbance?
In spectroscopy they are usually the same quantity when base-10 logarithms are used.
Can transmitted intensity exceed incident intensity?
No for pure absorption. If measured values suggest otherwise, check calibration, stray light, or fluorescence effects.
What does OD = 1 mean?
Transmitted intensity is 10% of the incident beam because \(I_0 / I = 10\).
How do I recover transmitted intensity from OD?
Use \(I = I_0 / 10^{OD}\).
Why use logarithms instead of percent transmission?
Logarithmic scaling matches how detectors and many optical laws behave over wide intensity ranges.
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