Analysis of infrared radiation from optical window with graded temperature distribution

When the outside temperature rapidly changes, there is graded temperature distribution at optical window in infrared detection systems and the detection sensitivity will be affected. It is necessary to analyze the response of detector according to the temperature distribution. In this paper the graded temperature distribution is described in optical system parameters as environment data, the radial graded temperature function using orthogonal polynomial method is created, and the temperature of any position at optical window can be calculated easily. Based on the principles of blackbody radiation, the sampling plans of wavelength and thermal radiation source surface are presented. Having imaged each surface at object space or image space, the common entrance for infrared radiation from the window is found and named as thermal pupil, then sampling at the entrance to create millions of rays. These rays are traced through the system with energy and the power spectral density is calculated. Taken an infrared optical system as an example, the radiation of its optical window with temperature of 30°C and 55°C±5°C is analyzed. As shown by the example that the radial temperature gradient will lead to nonhomogeneous noise at the image.