An important type of Noise in a Light beam is noise of its intensity. (Strictly speaking, one usually considers noise of the optical power, rather than of the intensity, but the common term is Intensity noise rather than power noise.) Intensity noise can be measured e.g. by recording the measured intensity as a function of time (e.g. with a photodiode) and properly processing the data. In many cases, an electronic Spectrum analyzer is used.
Intensity noise is usually quantified as Relative Intensity Noise (RIN), i.e., as noise of the power divided by the average power.