Systems and methods for determining a property of a specimen are provided. The specimen may be a product wafer. The method may include biasing a focused spot on the specimen. The method may also include measuring a parameter of a measurement spot on the specimen. The measurement spot may overlap the focused spot. In addition, the method may include determining the property of the specimen from the measured parameter. Systems and methods for varying the performance of a corona source are also provided. The method may include altering a property of the environment within the corona source. The property may include, but is not limited to, temperature, pressure, humidity, and/or partial pressure of a gas within the corona source.