Narcissus Macro: Lens Setup
“Narcissus”. is the composite image of multiple images of the detector formed by reflections from the surfaces of the objective lens located in front of it. These “ghost” images are generally out of focus in varying degrees spanning the detector in corresponding measures .The composite image therefore is a summation of the multiple distributions of radiation on the detector. .These vary in both Intensity and location from the center of the detector. The very complex set of reflected ray intercepts on the detector is best analyzed by computer ray trace analysis.
Ghost Image Ray Trace Analysis
Analysis of these ghost images is performed by tracing. rays from the detector which is set up as an object emitting rays toward the optics in front of it. Rays are traced from the detector outward to the lens and back to the detector after reflection from a lens surface for each ghost lens configuration. Each surface of the lens is treated in sequence as a mirror generating a collection of ghost lens configurations.
For each ghost lens configuration, rays from many object points on the detector and points within the pupil are traced by employing a custom designed computer program, using the Zemax software ZPL macro programming language.. A description of the macro and its operation follow.
Set Up and Use of the Narcissus Macro
After the Narcissus macro is inserted into the Zemax Macros folder, it is accessed through the Zemax drop down menu, “Macros”, .located on the Zemax options bar.
Instructions for use are presented as “remarks” in the code which can be accessed through the edit option in the Zemax dialog box which initially appears. The setup steps for execution of the macro are presented in these instructions. The steps are summarized here..
Setup begins with reversal of the lens so that the detector becomes the object surface for ray tracing the ghost images created by the lens surfaces which, sequentially, are treated as mirrors for the various ghost lens configurations. Object heights and system numerical aperture are entered; lens semi-diameters are as established from the original lens form. Aperture stop location and size also are as in the original design..
An auxiliary plano surface is added to the lens system after reversal. This surface is treated as a reference surface which generates a ghost image of maximum strength at the detector. All other ghost images are compared to this master ghost image in order to determine their relative strengths in the combined ghost image tally at the focal plane.
Ghost lens configurations for each lens surface are generated by executing the Zemax option “Ghost Image Generation”. The “single bounce” and “save” selections are made. Prior to running the ghost generator option, all older ghost images on file must be deleted to ensure that the macro is addressing only the currently generated ghost lens configurations.
Narcissus Macro: Operation Flow
The flow chart below describes the operations flow of the macro.
Input data is entered by the operator via the macro input window:
- Lens analysis title
- Surface umber of the first ghost lens surface
- Surface number of the last ghost surface
- Detector length
- Detector width
- Object space f-number
- NARC_ΔT, the noise equivalent temperature difference of the maximum strength ghost image (calculated separately…. typically, 0.02 -.0.05°K)
The program creates ghost lens configuration names from the input data compatible with the Zemax file name protocol.
Ray tracing ghost lens configurations
Each ghost lens configuration is then called up automatically in sequence for ray tracing. Rays are traced within a series of program loops with the ghost lens configuration being the outermost loop. For each ghost lens configuration rays are traced for a 5 x 5 array of field points, and a 40 x 40 pupil grid.
Rays are traced from the detector through the subsequent lens surfaces and out to the lens surface which is being treated as a mirror for the ghost lens configuration being traced. Ray passage is normal up to this point, being identical to that of the lens in its original form. After reflection from the ghost lens surface, however, a quite different situation exists.
Rays traveling back to the detector can encounter a number of different situations which prevent them fro reaching the detector. If a ray height exceeds the lens semi-diameter, it is aborted; if the ray height exceeds the lens radius of curvature it is also aborted; and finally if the ray undergoes internal reflection at the lens surface, it is again aborted. Thus, of the large number of rays launched from multiple points on the detector and multiple rays within the pupil grid, only a relatively few make their way back to the detector.
The ray intercept coordinates of the rays which are successful in getting back to the detector are recorded in a file.. After every ray from every field point and every pupil point has been traced and ray intercepts on the detector are recorded for that ghost lens configuration, the entire process is repeated for the next ghost lens configuration until the last ghost lens configuration has been traced.
Ghost ray detector intercepts processing and program output
The ray intercepts on the detector of all rays traced and filed are read out of the file and sorted into annular zones centered on the detector. The number of rays contained within each of the ten annular zones is weighted by dividing the number of rays by the area of the respective zone.. The resultant quantity is proportional to irradiance, or flux per unit area, and is the quantity which translates to an electrical signal via the responsivity characteristic of the detector. The electrical signal is then amplified and used to drive a monitor to the displayed scene image. The ghost image summation then appears as narcissus radiance, a circular hot spot in the center of the displayed image, shown as either “hot”: or “cold”, depending on system settings.
The program output comprises three tables. The first table below presents the weighted ray counts per zone for each of the ghost configurations. It further provides the sums of the weighted ray counts per zone, yielding the desired bottom line result.
Also, in the first table, the first two columns provide paraxial axial marginal ray height and angle data at the detector surface. This data indicates the degree of defocus of the ghost image at the detector.
The second table below presents the weighted ray count data of the first table normalized to the maximum value, usually the weighted ray sum for the first zone.
The third table blow expresses the same data in terms of the system noise equivalent temperature difference.
The bottom line sums presents the end results of the analysis, the narcissus irradiance expressed in terms of the noise equivalent temperature difference of the system. The data is plotted in the graph below..
This infrared objective lens exhibits a relatively low maximum narcissus irradiance of .022 degrees Kelvin at the center of the displayed image.. Change in irradiance across the format is relatively gradual, as well, making for a displayed image which is reasonably free of the narcissus problem.
The absence of a purely a monotonic function as evidenced by the “bumps” in the curve can possibly be due to the relatively small number of ghost images and with differences in peak irradiance and distribution across the detector..
The extensive number of rays and field points used in the ray trace analysis suggests that the discontinuities seen in the graph are not due to approximations made in the calculation process, although extensive analysis would be required to prove this point.