…formulated Bouguer’s law (sometimes called Lambert’s law), regarding the attenuation of a light beam in a transparent medium. This law and his photometric work he published in his Essai d’optique sur la gradation de la lumière (1729; Optical Treatise on the Gradation of Light).

…which is also known as Lambert’s law, relates the amount of light absorbed and the distance it travels through an absorbing medium; and Beer’s law relates light absorption and the concentration of the absorbing substance. The two laws may be combined and expressed by the equation log I₀/I = kcd,…

According to Bouguer’s (or Lambert’s) law, each layer of equal thickness of the medium absorbs an equal fraction of the energy traversing it. According to Beer’s law, the absorptive capacity of a dissolved substance is directly proportional to its concentration in a solution.