The Narcissus effect occurs when a cooled detector inside an infrared (IR) system sees a reflection of itself, resulting in a dark spot in the center of the image. Indeed, every object emits far infrared radiation based on their property and temperature. In addition, the radiation reflected by each surface of the optical system also returns to the detector. The Narcissus effect is a recurring issue with IR systems, which affects the system's image quality. Therefore, studying this effect using Ansys software is of interest, as it enables us to visualize and quantify the effect. To meet this demand the two following workflows were created, in Ansys Zemax OpticStudio and Ansys Speos respectively. The first one in Ansys Zemax OpticStudio with the Narcissus Induced Temperature Difference for the lens system and the second one in Ansys Speos to visualize the narcissus effect with thermal sources.
Overview
Step 1: Analysis of the Narcissus Induced Temperature Difference (NITD) number with Ansys Zemax OpticStudio
This first narcissus analysis method is based on the concept of Narcissus Induced Temperature Difference (NITD) number, in Ansys Zemax OpticStudio. The NITD is calculated with the blackbody spectral radiance of the different parts of the optical system (housing, detector, ambient air…). It can be understood as an equivalent of the unit for contrast in the visible range. This NITD number is calculated and used in a .ZPL Macro in Ansys Zemax OpticStudio. This macro relies on the prior creation of the system's Ghost File using the ghost focus generator (more information about ghost focus generator are available here: Stray Analysis with Ghost Focus Generator )”. The macro also needs the user to input housing, detector, and ambient air temperatures. This finally produces six graphic representations using the NITD number. Those six graphs provide a visual representation of the NITD and also the contribution of each lens. This method allows you to quickly see whether narcissus will cause a problem on your infrared system. However, users should be aware there is no method to correlate NITD to radiometric metrics such as Irradiance and Luminous flux. Also, the optical system needs to have several requirements: it must be designed from detector to ambient, the stop surface must be surface 1, it must not have any coordinate breaks or dummy surfaces, and design must be rotationally symmetric.
More information can be found here to perform this analysis: OpticStudio narcissus analysis macro.
The macro used in this method including comments and additional explanation can be found here.
Step2 : Visualization of Narcissus Effect with Ansys Speos
In the second step, a workflow with Ansys Speos enables the visualization of the narcissus effect in the resulting image and the contribution of each optical component to the image quality. The optomechanical component and the sensor geometry are defined as thermal sources. The thermal source function allows to create blackbody far infrared radiation by defining the temperature of the component. In addition to this, we create a source positioned in front of the device, which can be considered as the object source. This method has the advantages of creating an irradiance map (*.XMP) in W/m², which provides a visual representation of the narcissus effect.
More information can be found here to perform this analysis: Stray Light Analysis for A&D - Narcissus effect on IR camera.