Optical density (OD) is a measure of how much light is absorbed by a material. It quantifies the ability of a substance to block or transmit light, with higher values indicating greater absorption. Optical density is commonly used in fields like photography, optics, and biology to assess the clarity or darkness of materials. In practical terms, optical density is calculated using the formula OD = log10(I0/I), where I0 is the intensity of the incident light and I is the intensity of the transmitted light. This concept is essential for understanding how materials interact with light, influencing applications such as spectrophotometry and lens design.