Image intensifiers (Gen II) have been quoted in the past of having On/Off, or rejection ratios of 107. In this paper we investigate the relationship of the On/Off ratio with incident wavelength. The data shows that the On/Off ratio can vary by as much as 102 with values of only 104 at ~200nm for a given image intensifier. This low On/Off ratio can limit the sensitivity of an intensified CCD camera under certain conditions where there is an abundance of UV, such as combustion. This is particularly true in situations where the CCD exposure and readout times, typically milliseconds, takes longer than the required 'On' time of the photocathode, which is typically nanoseconds. Test data also shows the advantage of switching the MCP on and off simultaneously with the photocathode, which brings the rejection ratios up to >108.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.