What is absorbance?

Absorbance converts transmittance into a logarithmic quantity, defined as A=-log₁₀(T). Less light transmitted through the sample usually means higher calculated absorbance. Because the conversion is logarithmic, equal increases in absorbance correspond to proportional decreases in transmittance, not equal absolute decreases in transmitted intensity.

This expression is useful because, where the Beer–Lambert relationship applies, absorbance has a linear relationship with absorbing-species concentration and effective path length. Nucleic acid measurements commonly read absorbance at a specified wavelength and estimate concentration using confirmed method coefficients and path length. The calculation is valid only if the sample, Blank, wavelength and conversion model all match the method.

If results corrected back to the original sample disagree across dilutions, or absorbance is no longer approximately proportional to concentration, a single reading should no longer be interpreted directly as concentration. Check for excessive concentration, scattering or bubbles, Blank mismatch, unstable effective path length and whether the instrument meets method requirements in that signal range.

Absorbance is not an absolute label independent of conditions, nor can it alone establish sample identity, purity or activity. Wavelength, temperature, pH, container, effective path length, stray light and baseline all affect the result. Negative or very high absorbance is first a reason to investigate reference and measurement conditions, not a problem solved simply by clipping or renaming the value.