Evaluation and Correction of Narcissus in Infrared Systems with Cooled Focal Plane Arrays

Abstract

Infrared imaging systems with cooled focal plane arrays can achieve high thermal sensitivity performance. However, cooled infrared cameras are susceptible to additional stray light artifacts such as narcissus. Narcissus arises from the internal reflection of thermal radiation from a cooled detector and warm surrounding structures off refractive optical surfaces with imperfect anti-reflective coatings. Narcissus results in a spatially varying radiometric artifact, often manifesting as a cold spot across the focal plane array, which can severely degrade image uniformity and thermal sensitivity. This thesis presents an investigation into the manifestation of narcissus and how it can be simulated using ray tracing to quantify its impact on an infrared system. A discussion of different design strategies for correcting narcissus is also covered. The physical basis of narcissus is examined through paraxial optical analysis, employing Lagrange invariants to characterize the relationship between lens surface geometry and the formation of narcissus artifacts. A key outcome of this analysis is the derivation of the Narcissus Induced Temperature Difference (NITD), a quantitative figure of merit that represents the apparent temperature shift observed by the detector from the narcissus artifact. The NITD figure of merit enables the evaluation of narcissus across the image plane and guides the lens designer toward possible corrective actions. To reinforce the theoretical analysis, a case study is conducted on a MWIR camera system. Reverse raytracing simulations model the thermal self-emission background across the focal plane and determine surface-level contributions to NITD. The simulation workflow enables the generation of spatially resolved thermal irradiance and NITD maps, which inform the distribution and magnitude of narcissus across the focal plane. Various mitigation strategies are then explored, including optical system cooling, optimization of lens curvature and spacing, improvements to anti-reflective coatings, and tilting plano optical elements to redirect reflected thermal radiation away from the detector. This thesis demonstrates that integrating NITD analysis into a design workflow while employing different corrective measures can significantly reduce narcissus to enhance image quality and thermal sensitivity performance. The results highlight the importance of incorporating narcissus modeling and optimization early in the design cycle to ensure system performance requirements are met without high cost and scheduling risk if discovered later in the project timeline. The methods developed in this work provide a structured and practical approach for addressing narcissus in cooled infrared systems and can be readily integrated into existing optical design and analysis workflows.