Characterizing radiant sources like laser diodes accurately depends on the ability to measure their optical power output accurately. A number of vital device characteristics can be extrapolated from these optical power measurements, including maximum output power, threshold current, and kink test/slope efficiency. Measuring optical power accurately requires capturing a known fraction of the emitted energy from the source. Inconsistent radiation collection or inaccurate measurements can cause an acceptable device to be graded incorrectly or rejected, reducing the manufacturing yield. This application note offers an overview on using an integrating sphere to measure the optical power of radiant sources in a production environment.