Reducing narcissus with a GRIN

Abstract

Narcissus is caused by the reflection of the cold stop off a lens surface back to the image plane of a cooled infrared system and can be very difficult to remove from a lens design perspective. New infrared GRIN materials show the ability to reduce the amount of narcissus in an optical system without the reduction in performance or addition of optical elements. BACKGROUNDIn a typical workflow for an infrared optical design a significant amount of effort is spent defining the requirements and developing a design that meets those requirements followed by refining the design concept with realistic tolerances. At this point the design is at state where the designer can examine the effects of narcissus which typically reveal one or more surfaces that contribute an unacceptable amount of narcissus to the system. In a scanned system, this can be disastrous because the narcissus may not be removable. The impact on a system with a staring focal plane array (FPA) is usually less severe 1 . The narcissus can temporarily be mitigated by non-uniformity correction (NUC), but this is highly dependent on the location of the element that causes the problem. Narcissus is caused by the reflection of the image of the cold stop back to the image plane from individual lens surfaces 2 . All refractive thermal systems have some amount of narcissus as each optical surface provides a reflection path back to the image plane. Each surface provides a reflection back to the image plane. The level of narcissus from each surface is dependent on the strength of the image that is formed of the cold space back to the FPA. This is a function of both the optical design and the coatings that are applied to the individual surfaces.Narcissus manifests itself in two different methods. The first form is uniformly distributed across the FPA and affects the background level of the sensor. This does degrade the performance of the system by introducing an additional noise source, but after processing corrections and adjusting gain, level, and contrast, it is general imperceptible to the user. It can impact the dynamic range of the system, so it cannot be completely ignored. The second form of narcissus, and more impactful, is non-uniformly distributed across the FPA and limits the performance of the sensor. This non-uniform narcissus is observed in the image from Figure 1 where there appears to be a dark circle forming in the middle of the image. This dark circle is the image of the cold stop as well as the warm space that surrounds it. The large contrast in signal is due to the large delta in temperature from the cold space to the warm space (delta T = 220K for a system at 300K and the detector at 80K). In both cases, changes to the system such as temperature, FPA drift, focus, etc. can cause the narcissus to reappear as conditions deviate from the state that the NUC was performed. Figure 1: Thermal image with strong narcissus contribution 3 .

Focused Narcissus evaluation in a MWIR zoom system

“…Narcissus appears when the detector cold shield is imaged on to the detector from reflections at some of the optical surfaces of the optical system. There are several publications about techniques to reduce Narcissus through lens design [4] , baffles, lenses [5], [6] and coatings specifications [7] . Depending on the application, some of these approaches have shown good performance in reducing or even eliminating the Narcissus effect [8], [9] .…”