Stray Light

What is stray light? 

Stray light is any electro-magnetic radiation that is unwanted and interferes with the performance of an optical system’s intended functions. Unwanted stray light can occur in either imaging or projection systems, although it is usually more critical to control the former. Stray light can originate from the object the optical system is capturing or from unintended external emitters, or, in the case of infra-red sensitive systems, it can originate from elements of the system itself emitting light due to their own heat.

Examples of stray light include:

  • Light reflections off mechanical mounting surfaces inside the optical system
  • Light leaking through a gap in the system enclosure
  • Light scattering off dust and other imperfections on the system’s optical surfaces
  • For ground-based astronomy, sky glow, caused by the reflection of municipal lights from the atmosphere, can be a major source of stray light
  • The Sun, Earth and Moon are common unwanted external sources for an orbiting telescope

Two types of stray light

Stray light can be characterized in two distinct types: ghosts and flare or veiling glare.

Ghosts occur in imaging systems when light from a source in the image field undergoes two or more unwanted reflections and then falls on the imaging device, creating an unwanted ghost image. For digital cameras, one of the most common sources of ghost images is light that reflects off the imaging device back into the optical system and then reflects off a lens surface back to form a secondary image.
 
Flare, or veiling glare, generally occurs when light scatters inside the optical system either off imperfections in the optical surfaces, or off mechanical elements in the system. Veiling glare can also be caused by atmospheric reflection of light such as with haze or sky glow.
 
  • Ghosts
    • Typically caused by unintended reflections between imaging surfaces
    • Higher or unblocked diffractive orders from gratings
    • Secondary imaging of bright scattering surfaces
       
  • Flare or veiling glare
    • Light incident on the image from outside of the optical system’s field
    • Bright sources within the field of view, thermal radiation emanating from warm surfaces
    • Often the result of light scattering within the optical system
Ghost example: Picture Photo taken with a cell phone camera that clearly shows three sharply focused ghost images of the candle flames. There is also a fourth, extended ghost image centered on the middle sharp ghost image. | Synopsys

Ghost example: Photo taken with a cell phone camera that clearly shows three sharply focused ghost images of the candle flames. There is also a fourth, extended ghost image centered on the middle sharp ghost image.

Why is it important to find stray light in a design?

Stray light can reduce the contrast of the image by adding unwanted light to the image. For detection systems, stray light can reduce their sensitivity. For commercial imaging systems, this can create unappealing images. For light projection systems, stray light can cause unwanted bright spots in the beam pattern.

How does software help find stray light?

Ghost images can usually be analyzed with sequential ray tracing software. Since the reflectivity of the surfaces is typically quite low, only ghosts that involve two reflections are typically of interest. This limits the problem sufficiently so that each possible ghost path can be examined individually and assessed for potential impact.

Ghost example: A sequential ray trace of a single ghost image path. The light from an object in the field of view passes through the lens and forms an image on the detector at right. Some of the light is then reflected by the detector back into the lens. One of the lens surfaces then reflects the light back to the detector at a different location. This ghost image is of interest because the ghost light is nearly focused on the detector, which will lead to a much brighter ghost image than if the light were spread out over a larger area.  | Synopsys

Ghost example: A sequential ray trace of a single ghost image path. The light from an object in the field of view passes through the lens and forms an image on the detector at right. Some of the light is then reflected by the detector back into the lens. One of the lens surfaces then reflects the light back to the detector at a different location. This ghost image is of interest because the ghost light is nearly focused on the detector, which will lead to a much brighter ghost image than if the light were spread out over a larger area.

Flare can enter the optical system through many and often unexpected optical paths. Monte-Carlo software is used to investigate the contributions. Brute-force methods will randomly generate many rays and analyze how the energy is distributed through the model. Variance Reduction methods are used to improve the efficiency of finding contributions coming from paths that include low probability events such as high- angle scattering.

Unintended light example: Light from an object in the field of view scatters off the lens mount and then is reflected by a lens surface to the detector.  | Synopsys

Unintended light example: Light from an object in the field of view scatters off the lens mount and then is reflected by a lens surface to the detector.

Computational approaches to simulating stray light

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 from surfaces in the primary optical path. Ghost images that impinge on the image plane at or near a focus are of specific concern⁠—flare can arise from light reflecting off 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 from the detector can be complicated by diffraction from the microstructure of the detector. Read this application note that covers that covers various computational approaches to simulating stray light in an imaging lens. All computations were done with Synopsys software: CODE V, RSoft Photonic Device Tools, and LightTools.

Using software to model stray light

Synopsys offers several software options for modeling and analyzing stray light. Choosing the right software depends on your application, and our products allow for co-simulations.

  • For modeling the design and real-time simulation of automotive forward, rear, and signal lighting, LucidShape software provides a complete set of design, simulation and analysis tools.
Ray History Sensor Example – Edge light design ray restoration from a surface sensor.  | Synopsys

Ray History Sensor Example – Edge light design ray restoration from a surface sensor

  • For software to model imaging or free-space telecommunication systems, CODE V is a computer- aided design software used to model, analyze, optimize, and provide fabrication support for the development of optical systems for applications such as aerospace, cameras, information display, microlithography, and photonics.

Stray light workflow

Shown below is a typical workflow for the analysis of stray light in a camera system when using Synopsys software.

Typical workflow for the analysis of stray light in a camera system when using Synopsys software.  | Synopsys

 

 

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