A low pressure, low energy measuring device for indirectly and non-invasiy measuring the pressure of a gaseous medium in a sealed system, such as gas discharge lamp, is disclosed. The measuring device and method measures the luminance of an externally-induced, high-frequency glow discharge of a gas in the lamp by non-invasive means. Comparison of the measured luminance with calibrated luminance vs. pressure data provides the pressure for the gas. In one embodiment, the components of the measuring device form a stand-alone unit useful for testing multiple sealed systems. In another embodiment, certain components of the device are integrated into the lamp's discharge system to facilitate measurement in the field. A separate peripheral unit is required with this latter embodiment.