Absorbance, Transmittance & Reflectance Explained
Every spectroscopy measurement ultimately records how light interacts with a sample. The three fundamental quantities — transmittance, absorbance, and reflectance — describe this interaction from different perspectives. Understanding what each one means, how they relate to each other, and when to use each representation is essential for interpreting spectra correctly and making quantitative measurements.
Transmittance
Transmittance (T) is the fraction of incident light that passes through a sample without being absorbed or scattered. It is defined as:
T = I / I₀
where I₀ is the incident intensity and I is the transmitted intensity
Transmittance ranges from 0 (complete absorption) to 1 (no absorption). In practice, spectra are usually displayed as percent transmittance (%T), which is simply T × 100. A sample with 50%T absorbs half the light; a sample with 10%T absorbs 90%.
FTIR instruments traditionally display spectra in %T because this is the natural output of the measurement — the detector measures how much light reaches it. Peaks in a transmittance spectrum point downward (toward lower %T) at absorption frequencies.
Absorbance
Absorbance (A) is the base-10 logarithm of the reciprocal of transmittance:
A = −log₁₀(T) = log₁₀(1/T)
A = 2 − log₁₀(%T)
Absorbance is dimensionless and ranges from 0 (no absorption) upward with no theoretical maximum, though in practice values above 2.0 are unreliable.
The key advantage of absorbance is its linear relationship with concentration via the Beer-Lambert law (A = εlc). Doubling the concentration doubles the absorbance. This linearity makes absorbance the preferred representation for quantitative work, calibration curves, and any analysis where you need to relate spectral intensity to amount.
| %T | Absorbance | Light Absorbed |
|---|---|---|
| 100% | 0.000 | None |
| 50% | 0.301 | 50% |
| 10% | 1.000 | 90% |
| 1% | 2.000 | 99% |
| 0.1% | 3.000 | 99.9% — unreliable measurement range |
Absorbance ↔ Transmittance Converter
A = −log₁₀(T/100) | %T = 10⁻ᴬ × 100
Transmittance (fraction): T = 0.5000
Light absorbed: 50.00%
Reflectance
Reflectance (R) is the fraction of incident light that is reflected from a sample surface. Like transmittance, it ranges from 0 to 1 (or 0% to 100%). Reflectance spectroscopy is used when the sample is opaque, highly absorbing, or when non-destructive surface analysis is needed.
There are two main types of reflectance:
- Specular reflectance — mirror-like reflection from a smooth surface at the angle of incidence. The reflected spectrum is related to the refractive index and absorption coefficient of the material. Used for thin films and polished surfaces.
- Diffuse reflectance — scattered reflection from a rough or powdered surface. Light penetrates the sample, undergoes multiple scattering and absorption events, and exits at various angles. The most common reflectance technique for powders and rough solids. DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) is the standard FTIR implementation.
Kubelka-Munk Transform
Raw diffuse reflectance spectra cannot be directly used for quantitative analysis because reflectance is not linearly related to concentration. The Kubelka-Munk (K-M) function transforms reflectance into a quantity that is proportional to concentration:
f(R∞) = (1 − R∞)² / (2R∞) = K/S
where R∞ is the absolute reflectance, K is the absorption coefficient, and S is the scattering coefficient
The K-M function is the reflectance analog of the Beer-Lambert law. It is valid when the sample is optically thick (infinitely thick relative to penetration depth), the particle size is uniform, and scattering is isotropic. In practice, diluting the sample in a non-absorbing matrix like KBr improves linearity.
ATR and Pseudo-Absorbance
Attenuated Total Reflectance (ATR) is the most common sampling technique in modern FTIR. It is technically a reflectance method — the infrared beam reflects off the internal surface of a high-refractive-index crystal while an evanescent wave penetrates into the sample. However, ATR spectra are displayed in absorbance-like units and are routinely treated as absorption spectra.
ATR spectra differ from true transmission spectra in two ways: relative peak intensities change because the evanescent wave penetration depth increases at lower wavenumbers, and peak positions can shift slightly due to anomalous dispersion. ATR correction algorithms compensate for the wavelength-dependent penetration depth.
When to Use Each Representation
| Use Case | Representation | Why |
|---|---|---|
| Quantitative analysis | Absorbance | Linear with concentration (Beer-Lambert law) |
| Qualitative FTIR identification | %Transmittance | Traditional display; most reference libraries use %T |
| Spectral subtraction | Absorbance | Absorbances are additive; transmittances are multiplicative |
| Powders / rough solids (DRIFTS) | Kubelka-Munk | Linearizes the reflectance–concentration relationship |
| NIR chemometrics | Absorbance or log(1/R) | Linearity required for PLS/PCA models |
| ATR-FTIR routine analysis | Absorbance (ATR) | Instrument default; compatible with most search algorithms |
Related Resources
- Beer-Lambert Law Explained — the equation connecting absorbance to concentration
- Spectroscopy Unit Converter — convert between absorbance, transmittance, wavenumber, and wavelength
- Spectral File Viewer — view spectra in absorbance or transmittance mode
- FTIR Interpretation Guide — how to read and interpret FTIR absorption spectra
- Wavenumber vs Wavelength — spectroscopic units explained
Frequently Asked Questions
How do you convert transmittance to absorbance?
Absorbance = −log₁₀(T), where T is the transmittance as a fraction (0 to 1). If you have percent transmittance: A = −log₁₀(%T/100) = 2 − log₁₀(%T). For example, 50%T = 0.301 absorbance, 10%T = 1.000 absorbance, and 1%T = 2.000 absorbance.
What is the difference between absorbance and transmittance?
Transmittance (T) is the fraction of light that passes through a sample (T = I/I₀). Absorbance (A) is the negative logarithm of transmittance (A = −log₁₀T). The key difference for analytical work: absorbance is linearly proportional to concentration (Beer-Lambert law), while transmittance has an exponential relationship. This is why absorbance is preferred for quantitative measurements.
What is the Kubelka-Munk function?
The Kubelka-Munk function f(R) = (1−R)²/(2R) transforms diffuse reflectance data into a quantity proportional to the ratio of absorption coefficient (K) to scattering coefficient (S). It is the reflectance analog of the Beer-Lambert law, linearizing the relationship between reflectance and concentration for powdered or rough-surface samples measured by DRIFTS.