Absorbance and Transmittance
How absorbance (A) and percent transmittance (%T) are related, why scientists prefer absorbance, and how to convert between them.
What Is Transmittance?
Transmittance (T) is the fraction of incident light that passes through a sample without being absorbed. If you shine a beam of light with intensity I₀ into a cuvette and measure intensity I coming out the other side, then:
T = I / I₀
T ranges from 0 (all light absorbed) to 1 (no light absorbed)
Percent transmittance (%T) is simply T multiplied by 100. A solution that transmits half the light has T = 0.50 or %T = 50%.
Absorbance and Transmittance: The Equations
Absorbance (A) and transmittance (T) are related logarithmically. This is why absorbance is sometimes called “optical density” — it expresses the same information on a logarithmic scale.
Transmittance to Absorbance
A = −log₁₀(T)
Absorbance to Transmittance
T = 10−A
Because of the negative logarithm, the relationship is non-linear: doubling the concentration doubles A, but it does not halve %T. This is why scientists prefer working with absorbance for quantitative analysis — it gives a direct, linear relationship with concentration via the Beer-Lambert law.
Why Use Absorbance Instead of Transmittance?
Linearity with concentration
A = εlc is linear. A plot of A vs. c gives a straight line through the origin. A plot of %T vs. c gives an exponential decay curve that is much harder to work with.
Additive for mixtures
Absorbances of individual components in a mixture add together: Aₜₒₜₐₗ = A₁ + A₂ + … Transmittances multiply (Tₜₒₜₐₗ = T₁ × T₂), which is less intuitive for quantitative work.
Direct proportionality to path length
Doubling the cuvette length doubles A. The same change compresses %T toward zero non-linearly, making it harder to compare across different setups.
Transmittance-Absorbance Reference Table
Memorize the first few rows — they come up constantly in exams and lab work.
| %T | A | What It Means |
|---|---|---|
| 100% | 0 | No absorption - blank/reference |
| 50% | 0.301 | Half the light absorbed |
| 25% | 0.602 | 75% of light absorbed |
| 10% | 1.000 | 90% absorbed - common upper limit for reliable readings |
| 1% | 2.000 | 99% absorbed - stray light becomes significant |
| 0.1% | 3.000 | 99.9% absorbed - beyond most instruments |
Quick Converter
Practical Tips for Lab Work
Reliable range
Most UV-Vis instruments give accurate results when A is between 0.1 and 1.0 (10–80% T). Outside this range, detector noise or stray light introduces error.
Dilute high-absorbance samples
If A > 1.0, dilute the sample and multiply the measured concentration by the dilution factor. A reading of A = 2.0 means only 1% of light reaches the detector.
Blank your instrument
Always set %T = 100 (A = 0) with a cuvette containing only the solvent. This zeros out absorbance from the cuvette walls and solvent itself.
%T is on your spectrophotometer
Many instruments display both %T and A. Older instruments may only show %T. You can always convert: A = −log₁₀(T/100) = 2 − log₁₀(%T).