Why is the linearity of the optical measurement lost at higher cell densities?

With higher cell density the probability increases that a cell (e.g., a bacteria, a solid particle) is in front or behind another cell. This cell is therefore "invisible" for an optical measurement method, and does not influence the result of the measurement. The linearity range of Beer-Lambert equation is limited from ~ 0-20 g/L CDW yeast. Sensors based on light transmission lose their linearity at higher cell density.