Comparison of two frame noise calculations for infrared linescanners

Averaging line noise standard deviations and averaging line noise variances are two commonly used techniques for determining frame noise or noise equivalent temperature difference (NETD). The two techniques are equivalent if all lines in the frame have the same noise statistics, that is, if the noise process is ergodic. However, if a few lines contain a larger amount of noise, the process is not ergodic and the two techniques yield different results. A simple theoretical relationship for the difference between the two is developed and compared to a computer simulation under a variety of conditions. Results of the simulation closely matched the theoretical relationship. We recommend the use of the variance average as a description for noise in an NETD calculation, since it is more sensitive to lines that exhibit excess noise.