Methods and systems for determining one or more characteristics of light in an optical system are provided. One system includes a detector positioned in a path of light between a light source in the optical system and a specimen for which the optical system performs a process. The detector includes a center cut-out configured to allow only a first portion of the light from the light source to pass therethrough and four or more detector segments positioned around the center cut-out and in a path of only a second portion of the light from the light source. The four or more detector segments are configured to separately generate output responsive to the light incident thereon. The system also includes a control subsystem configured for determining one or more characteristics of the light from the light source based on the output separately generated by the four or more detector segments.