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in radiology, a measure of the ability of a medium to absorb radiation, expressed as the logarithm of the quotient of the intensity of the radiation entering the medium divided by that leaving it.

ab·sor·bance (A, A),

spectrophotometry log of the ratio of the radiant power of the incident radiation to the radiant power of the transmitted radiation.


Chemistry A logarithm of the percent transmission of a wavelength of light through a liquid. 
Microbiology A measure of the amount of light absorbed by a suspension of bacteria or an organic solution, measured by spectrophotometry. Absorbance values are used to plot the growth of bacteria in broth and gauge the purity and concentration of molecules in solution.


spectrophotometry 2 minus the log of the percentage transmittance of light.
Synonym(s): extinction (2) , optic density.


a spectrophotometric measurement of the light absorbed by a solution at a particular WAVELENGTH. The absorbance (A) derives from the percentage of light transmitted as follows:

where Io is the incident light intensity and I is the transmitted light intensity. The absorbance is related to the molar absorption coefficient (extinction coefficient) e (cm-1M-1), concentration c (M), and path length l (cm) as follows:

Absorbance can therefore be used to determine the concentration of a substance in solution, to follow conversion of a SUBSTRATE to a product in an enzymic reaction (see ENZYME), and so on.

Absorbance is sometimes referred to as OPTICAL DENSITY, although this term should be used for measurement of light scattering.


A measure of absorption equal to the logarithm to the base 10 of the reciprocal of the transmittance T, for a specified wavelength and expressed as A = −log10 T. Syn. optical density. See optical density; transmittance.
References in periodicals archive ?
Caption: Figure 7: Optical absorbance curves of (a) CA and (b) carmine.
For better visualization and understanding of the spectra, NanoUV-VIS displays a summary of the different variables determined in the analysis (maximum optical absorbance, SPR peak, and FWHM).
This process manifested itself in pronounced differences in the optical absorbance spectra of MDMOPPV films before and after their treatment with the solution of heteropolyacid in the chloroform-methanol mixture.
Results of HPLC characterization of the conjugates were highly consistent with the structures expected from the synthetic steps in their preparation, with elution times and optical absorbances appropriate for the calculated molecular weights and compositions.
An acceleration of charge recombination between [Q.sub.A.sup.*-] and [Y.sub.Z.sup.*] was measured in isolated PSII reaction centers with optical absorbance spectroscopy, a result consistent with [Y.sub.D.sup.*] raising the midpoint potential of [P.sub.680.sup.*+]/[P.sub.680].
To identify an appropriate wavelength responsive to the acidity developed in aged rice grains, the changes in optical absorbance of the dye solution following incubation with rice grains were measured.
The third feasibility test (FT3) has been aimed at investigating the effects of the competition between [beta]-lactam antibiotics and CENTA on the optical absorbance of milk samples.
Caption: FIGURE 5: Optical absorbance of [{[[[(Te[O.sub.2]).sub.0.7][([B.sub.2][O.sub.3]).sub.0.3]].sub.0.7][(ZnO).sub.0.3]}.sub.l-x] ([La.sub.2][O.sub.3] NPs), with different molarfraction of [La.sub.2][O.sub.3] NPs.
The disadvantages of 2P-water mixtures (material degradation with exposure to light or long-term storage, and slight optical absorbance) were not significant problems in the present measurements.
Perhaps it was related to the optical absorbance characteristics of this specific protein.
The method reported in [29] can get the lowest value of LOD reported so far for paraoxon detection using optical absorbance spectrometry or chemical colorimetry.