The effective focal length of an optical system can be electronically controlled using switchable wave plates in conjunction with polarized light. After the battery is discharged, the test device measures the battery to obtain second terminal voltages and second electronic currents. The operator can use this information to design a custom prosthetic to fit the patient. An item storage cabinet and a method of tracking items are also disclosed. A tab depends from at least one of the first and second arms. Responsive to the detection of the first pending event, a second thread being processed within the multithreaded processor is monitored to detect an event handling point for the second thread.