Absorbance vs. Transmittance
Every UV-Vis measurement ultimately answers one question: how much light did the sample absorb? The answer can be expressed as transmittance or absorbance, and understanding both — and the relationship between them — is fundamental to spectroscopy.
Transmittance: the raw ratio
Transmittance (T) is defined as the fraction of incident light that passes through the sample: T = I / I0, where I0 is the intensity of the reference beam and I is the intensity after the sample. A perfectly transparent sample has T = 1 (100 %); a completely opaque sample has T = 0. Transmittance is what a detector actually measures — it is a linear ratio of two light intensities.
Absorbance: the logarithmic scale
Absorbance (A) is the negative base-10 logarithm of transmittance: A = -log10(T). Because of this logarithmic compression, absorbance values span a convenient range: T = 100 % gives A = 0.000, T = 10 % gives A = 1.000, and T = 1 % gives A = 2.000. Absorbance is dimensionless and strictly non-negative for a real absorbing sample.
Why absorbance is preferred for quantitation
The Beer-Lambert law states that absorbance is directly proportional to concentration (A = εlc), making it ideal for quantitative analysis. Transmittance, being a ratio, does not share this linear relationship with concentration. Plotting A against concentration yields a straight line whose slope encodes the molar absorptivity; plotting T would give an exponential curve that is harder to work with.
Practical measurement range
Most UV-Vis instruments report reliable data in the range A = 0.1 to 2.0 (T = 79 % to 1 %). Below A = 0.1 the signal difference between sample and reference is small and noise-dominated. Above A = 2.0, very little light reaches the detector, amplifying the effect of stray light and dark-current noise. High-quality double-beam instruments such as the K LAB Alpha — which uses a Czerny-Turner monochromator and a separate reference beam — extend linear dynamic range by continuously correcting for source fluctuations.
Converting between the two
Conversion is straightforward: A = -log10(T / 100) when T is expressed as a percentage, or A = -log10(T) when T is expressed as a decimal. Inversely, T(%) = 10(2 – A) × 100. Most software handles this automatically, but understanding the underlying math helps when interpreting raw detector output or troubleshooting unexpectedly high absorbance readings.