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The analysis and control of stray light, composed of ghost images and flare, is an important but complex task for the design of imaging systems. Ghost images arise from multiple reflections off of surfaces in the primary optical path. Ghost images that impinge on the image plane at or near a focus is of specific concern—flare can arise from light reflecting off of lens mounts, non-optical surfaces of the lenses (such as flats and edges), and as a reflection off of the detector itself re-imaged back onto the detector. Modeling light reflected off of the detector can be complicated by diffraction from the microstructure of the detector. In this application note, we will discuss various computational approaches to simulating stray light in an imaging lens. All computation was done with Synopsys software, specifically CODE V, RSoft Photonic Device Tools, and LightTools.
Ghost images arise from reflections off of two surfaces in the primary optical path and can include a reflection from the detector itself. Light is then re-imaged back onto the image plane. If that light is near focus, then the resulting ghost image can be substantial. Ghost images can typically be analyzed by sequential ray tracing software since the surfaces involved are from the main imaging path.
"Flare" is stray light that can arise from reflection or scattering off of non-optical path surfaces or simply by passing through surfaces which are not part of the designed optical path.
Examples of surfaces that may give rise to flare include:
The detector itself will reflect a portion of the incident light back into the lens. This reflected light can, in turn, be scattered or reflected back onto the detector, causing ghost images or flare. For solid-state detectors such as CMOS chips, the microstructure of the detector can lead to significant diffraction patterns in the reflected light which may have an effect on the returning stray light pattern.
Shown below is a typical workflow for the analysis of stray light in a camera system when using Synopsys software.
Download "Ghost Image Reduction in Lens Systems with Nano-Structured AR-Coatings Using CODE V, RSoft and LightTools" PDF
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