Light’s criteria is helpful in determining the fluid causing effusion in the lungs. The effusion can consist of exudate and transudate. The pleural effusion is exudate if one or more than one light’s criterion is met. Reference: https://emedicine.medscape.com/article/2172232-overview Image via: https://www.grepmed.com/images/1376/differential-pulmonary-diagnosis-effusion-criteria