How is optical density used to measure cell growth?

Optical density (OD) measures light scattering by cells in suspension, providing rapid, non-destructive growth assessment. Measured at specific wavelengths (commonly 600 nm for bacteria), OD increases proportionally with cell concentration within a linear range. The technique involves: blanking spectrophotometer with sterile medium, measuring sample absorbance, and diluting dense cultures to maintain linear range (typically OD < 0.8). OD correlates with but does not directly measure cell number or biomass - calibration curves relating OD to CFU or dry cell weight are needed. Advantages include speed and simplicity; limitations include inability to distinguish viable from dead cells and sensitivity to medium components.