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The Beer–Lambert law, also known as Beer's law, the Lambert–Beer law, or the Beer–Lambert–Bouguer law relates the attenuation of light to the properties of the material through which the light is travelling. The law is commonly applied to chemical analysis measurements and used in understanding attenuation in physical optics, for photons, neutrons, or rarefied gases. In mathematical physics, this law arises as a solution of the BGK equation.
The law was discovered by Pierre Bouguer before 1729, while looking at red wine, during a brief vacation in Alentejo, Portugal. It is often attributed to Johann Heinrich Lambert, who cited Bouguer's Essai d'optique sur la gradation de la lumière (Claude Jombert, Paris, 1729) – and even quoted from it – in his Photometria in 1760. Lambert's law stated that the loss of light intensity when it propagates in a medium is directly proportional to intensity and path length. Much later, the German scientist August Beer discovered another attenuation relation in 1852. Beer's law stated that the transmittance of a solution remains constant if the product of concentration and path length stays constant. The modern derivation of the Beer–Lambert law combines the two laws and correlates the absorbance, which is the negative decadic logarithm of the transmittance, to both the concentrations of the attenuating species and the thickness of the material sample. The first modern formulation was given possibly by Robert Luther and Andreas Nikolopulos in 1913.
A common and practical expression of the Beer–Lambert law relates the optical attenuation of a physical material containing a single attenuating species of uniform concentration to the optical path length through the sample and absorptivity of the species. This expression is:
- is the absorbance
- is the molar attenuation coefficient or absorptivity of the attenuating species
- is the optical path length in cm
- is the concentration of the attenuating species
A more general form of the Beer–Lambert law states that, for attenuating species in the material sample,
- is the attenuation cross section of the attenuating species in the material sample;
- is the number density of the attenuating species in the material sample;
- is the molar attenuation coefficient or absorptivity of the attenuating species in the material sample;
- is the amount concentration of the attenuating species in the material sample;
- is the path length of the beam of light through the material sample.
- is the radiant flux transmitted by that material sample;
- is the radiant flux received by that material sample.
Attenuation cross section and molar attenuation coefficient are related by
where is the Avogadro constant.
In case of uniform attenuation, these relations become
The law tends to break down at very high concentrations, especially if the material is highly scattering. Absorbance within range of 0.2 to 0.5 is ideal to maintain linearity in the Beer–Lambart law. If the radiation is especially intense, nonlinear optical processes can also cause variances. The main reason, however, is that the concentration dependence is in general non-linear and Beer's law is valid only under certain conditions as shown by derivation below. For strong oscillators and at high concentrations the deviations are stronger. If the molecules are closer to each other interactions can set in. These interactions can be roughly divided into physical and chemical interactions. Physical interaction do not alter the polarizability of the molecules as long as the interaction is not so strong that light and molecular quantum state intermix (strong coupling), but cause the attenuation cross sections to be non-additive via electromagnetic coupling. Chemical interactions in contrast change the polarizability and thus absorption.
Expression with attenuation coefficient
The Beer–Lambert law can be expressed in terms of attenuation coefficient, but in this case is better called Lambert's law since amount concentration, from Beer's law, is hidden inside the attenuation coefficient. The (Napierian) attenuation coefficient and the decadic attenuation coefficient of a material sample are related to its number densities and amount concentrations as
In case of uniform attenuation, these relations become
In many cases, the attenuation coefficient does not vary with , in which case one does not have to perform an integral and can express the law as:
Assume that a beam of light enters a material sample. Define z as an axis parallel to the direction of the beam. Divide the material sample into thin slices, perpendicular to the beam of light, with thickness dz sufficiently small that one particle in a slice cannot obscure another particle in the same slice when viewed along the z direction. The radiant flux of the light that emerges from a slice is reduced, compared to that of the light that entered, by dΦe(z) = −μ(z)Φe(z) dz, where μ is the (Napierian) attenuation coefficient, which yields the following first-order linear ODE:
To describe the attenuation coefficient in a way independent of the number densities ni of the N attenuating species of the material sample, one introduces the attenuation cross section σi = μi(z)/ni(z). σi has the dimension of an area; it expresses the likelihood of interaction between the particles of the beam and the particles of the specie i in the material sample:
One can also use the molar attenuation coefficients εi = (NA/ln 10)σi, where NA is the Avogadro constant, to describe the attenuation coefficient in a way independent of the amount concentrations ci(z) = ni(z)/NA of the attenuating species of the material sample:
Under certain conditions the Beer–Lambert law fails to maintain a linear relationship between attenuation and concentration of analyte. These deviations are classified into three categories:
- Real—fundamental deviations due to the limitations of the law itself.
- Chemical—deviations observed due to specific chemical species of the sample which is being analyzed.
- Instrument—deviations which occur due to how the attenuation measurements are made.
There are at least six conditions that need to be fulfilled in order for the Beer–Lambert law to be valid. These are:
- The attenuators must act independently of each other.
- The attenuating medium must be homogeneous in the interaction volume.
- The attenuating medium must not scatter the radiation—no turbidity—unless this is accounted for as in DOAS.
- The incident radiation must consist of parallel rays, each traversing the same length in the absorbing medium.
- The incident radiation should preferably be monochromatic, or have at least a width that is narrower than that of the attenuating transition. Otherwise a spectrometer as detector for the power is needed instead of a photodiode which cannot discriminate between wavelengths.
- The incident flux must not influence the atoms or molecules; it should only act as a non-invasive probe of the species under study. In particular, this implies that the light should not cause optical saturation or optical pumping, since such effects will deplete the lower level and possibly give rise to stimulated emission.
If any of these conditions are not fulfilled, there will be deviations from the Beer–Lambert law.
Chemical analysis by spectrophotometry
The Beer–Lambert law can be applied to the analysis of a mixture by spectrophotometry, without the need for extensive pre-processing of the sample. An example is the determination of bilirubin in blood plasma samples. The spectrum of pure bilirubin is known, so the molar attenuation coefficient ε is known. Measurements of decadic attenuation coefficient μ10 are made at one wavelength λ that is nearly unique for bilirubin and at a second wavelength in order to correct for possible interferences. The amount concentration c is then given by
For a more complicated example, consider a mixture in solution containing two species at amount concentrations c1 and c2. The decadic attenuation coefficient at any wavelength λ is, given by
Therefore, measurements at two wavelengths yields two equations in two unknowns and will suffice to determine the amount concentrations c1 and c2 as long as the molar attenuation coefficient of the two components, ε1 and ε2 are known at both wavelengths. This two system equation can be solved using Cramer's rule. In practice it is better to use linear least squares to determine the two amount concentrations from measurements made at more than two wavelengths. Mixtures containing more than two components can be analyzed in the same way, using a minimum of N wavelengths for a mixture containing N components.
The law is used widely in infra-red spectroscopy and near-infrared spectroscopy for analysis of polymer degradation and oxidation (also in biological tissue) as well as to measure the concentration of various compounds in different food samples. The carbonyl group attenuation at about 6 micrometres can be detected quite easily, and degree of oxidation of the polymer calculated.
Application for the atmosphere
This law is also applied to describe the attenuation of solar or stellar radiation as it travels through the atmosphere. In this case, there is scattering of radiation as well as absorption. The optical depth for a slant path is τ′ = mτ, where τ refers to a vertical path, m is called the relative airmass, and for a plane-parallel atmosphere it is determined as m = sec θ where θ is the zenith angle corresponding to the given path. The Beer–Lambert law for the atmosphere is usually written
- a refers to aerosols (that absorb and scatter);
- g are uniformly mixed gases (mainly carbon dioxide (CO2) and molecular oxygen (O2) which only absorb);
- NO2 is nitrogen dioxide, mainly due to urban pollution (absorption only);
- RS are effects due to Raman scattering in the atmosphere;
- w is water vapour absorption;
- O3 is ozone (absorption only);
- r is Rayleigh scattering from molecular oxygen (O2) and nitrogen (N2) (responsible for the blue color of the sky);
- the selection of the attenuators which have to be considered depends on the wavelength range and can include various other compounds. This can include tetraoxygen, HONO, formaldehyde, glyoxal, a series of halogen radicals and others.
m is the optical mass or airmass factor, a term approximately equal (for small and moderate values of θ) to 1/cos θ, where θ is the observed object's zenith angle (the angle measured from the direction perpendicular to the Earth's surface at the observation site). This equation can be used to retrieve τa, the aerosol optical thickness, which is necessary for the correction of satellite images and also important in accounting for the role of aerosols in climate.
- Applied spectroscopy
- Atomic absorption spectroscopy
- Absorption spectroscopy
- Cavity ring-down spectroscopy
- Clausius-Mossotti relation
- Infra-red spectroscopy
- Job plot
- Laser absorption spectrometry
- Lorentz-Lorenz relation
- Polymer degradation
- Scientific laws named after people
- Quantification of nucleic acids
- Tunable diode laser absorption spectroscopy
- Bouguer, Pierre (1729). Essai d'optique sur la gradation de la lumière [Optics essay on the attenuation of light] (in French). Paris, France: Claude Jombert. pp. 16–22.
- Lambert, J.H. (1760). Photometria sive de mensura et gradibus luminis, colorum et umbrae [Photometry, or, On the measure and gradations of light intensity, colors, and shade] (in Latin). Augsburg, (Germany): Eberhardt Klett.
- Beer (1852). "Bestimmung der Absorption des rothen Lichts in farbigen Flüssigkeiten" [Determination of the absorption of red light in colored liquids]. Annalen der Physik und Chemie (in German). 162 (5): 78–88. Bibcode:1852AnP...162...78B. doi:10.1002/andp.18521620505.
- Ingle, J. D. J.; Crouch, S. R. (1988). Spectrochemical Analysis. New Jersey: Prentice Hall.
- Mayerhöfer, Thomas G.; Pahlow, Susanne; Popp, Jürgen (2020). "The Bouguer-Beer-Lambert Law: Shining Light on the Obscure". ChemPhysChem. 21: 2031. doi:10.1002/cphc.202000464.
- IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Beer–Lambert law". doi:10.1351/goldbook.B00626
- Fox, Mark (2010). Optical Properties of Solids (2 ed.). Oxford University Press. p. 3. ISBN 978-0199573370.
- Attard, Gary; Barnes, Colin (1998). Surfaces. Oxford Chemistry Primers. p. 26. ISBN 978-0198556862.