This article reports a test setup designed to study the prebreakdown and breakdown characteristics of spacers relevant to field emission displays (FEDs). The electrical characteristics of different spacer structures, bridging vacuum gaps formed by two thin-film electrodes, were studied in an attempt to determine the maximum field to which such gaps can be stressed. An extra low light detection three dimensional imaging system using an intensified charge coupled device camera was developed to diagnose and record the different prebreakdown phases and the breakdown phenomenon. This system enabled us to obtain spatially resolved images of light activities emanating from the electrically stressed surfaces. The experimental results showed that the investigated insulation materials did not introduce significant leakage current under high field stress. Breakdown voltages of fiber, disk-shaped and thick-wall spacers (750 mu m high) were 12-14, 13-16, and 15-20 kV, respectively. The insulation capability of the thick-wall structure was limited by the edge effect of the thin-film electrodes and the quality of the thin-film electrodes rather than processes intrinsic to the spacer material or its geometry. This work provides some guidance to the design of spacers for FEDs.