Stray light, the unwanted light within an imaging system, can severely degrade the image quality of LWIR (Long-Wave Infrared) cameras, especially when it originates from external sources such as the sun or thermal emissions from internal components. While the Point Source Transmittance (PST) index measures an optical system's ability to mitigate stray light, it does not fully capture the extent of image degradation that results. In our study, we quantified image quality degradation by converting the distribution of stray light at the detector into digital grayscale levels. Based on this analysis, we conducted an optimal design that comprehensively accounts for both external and internal scattered light.