High-speed infrared video system for detection of thermal aliasing.

Abstract

The acquisition of images in video recording is inherently a sampling process. In most current systems, a frame is recorded once every 1/30 second (30 frames per second). Scene events that occur faster than the Nyquist rate (one-half the sampling frequency, or 15 Hz, in this case) result in an effect called aliasing, which gives misleading information about the scene's true temporal content. Decreasing the camera's integration time (analogous to shutter speed) reduces blurring in the image, but the presence of aliasing is solely determined by the camera's frame rate. In visible systems, video aliasing is often accompanied by visual (spatial) cues in the resulting imagery; e.g. wagon wheel rotating slowly and/or backward. But in the infrared, where the scene is a temperature/emissivity map, drawing conclusions about what is really happening is much more difficult. The term thermal aliasing is used to describe the effect of temporal subsampling in infrared video. Fluctuations in an object's temperature occurring at greater than the Nyquist rate give no visual cues, so there is no way to tell by just looking at the scene whether its frequency distribution is real or aliased. For this reason, some method of determining whether the IR camera's frame rate is sufficiently high to prevent thermal aliasing is essential to provide valid scene information. Using a triggerable infrared video camera with variable integration time, like the one described in this thesis, allows the aliasing-status of an infrared image sequence to be ascertained. Changing the integration time has the effect of shaping the recorded scene's frequency spectrum; how these spectra relate to each other is different depending on whether aliasing is occurring or not. This thesis describes the theory of video sampling with its application to infrared video systems, including thermal aliasing. Also, the design and use of a triggerable, short integration time platinum silicide infrared video camera system is described.