The basic law that is the foundation for all absorption photometry. It predicts the linear relationship between the monochromatic light absorbance (A) of a solution and its concentration (c). The law is given as A — εlc, where A = absorbance, ε = molar absorptivity, 1 = path distance, and c = concentration. It is also known as the Beer-Lamber or Bougher-Beer law.