The level linearity of an optical spectrum analyzer (OSA) refers to the ability of the device to provide accurate and consistent measurements of the optical power across different input signal levels. In other words, it assesses how well the OSA maintains a linear response in terms of power measurements over a range of input signal strengths and is measured in Decibels (dB).

Ideally, an OSA should exhibit excellent level linearity, i.e., when the input power increases or decreases, the OSA's measurements should follow a linear trend, and the output power displayed by the instrument should be proportional to the input power.