Ultraviolet and visible absorption spectrophotometry is the measurement of the absorption of monochromatic radiation by solutions of chemical substances, in the range of 185 nm to 380 nm, and 380 nm to 780 nm of the spectrum, respectively.
The magnitude of the absorption of a solution is expressed in the terms of absorbance, A, defined as the logarithm to base 10 of the reciprocal of transmittance (T) for monochromatic radiation.
Where I₀ is the intensity of incident radiation,
And I is the intensity of transmitted radiation. The absorbance depends on the concentration of the absorbing substance in the solution and the thickness of the absorbing layer taken for measurement.