Identify Stray Light in Imaging Systems with LightTools

Stray light is undesired light radiation that interacts with the components of a system and degrades its performance by generating noise, that significantly reduces image quality and affects accuracy. These unintended reflections or scattering of light can be either from the object that the optical system is capturing or external emitters such as the sun or moon in case of cameras and telescopes.

Stray light could distort colours, prevent detection, affect contrast and data readability that tend to severely impact safety and efficiency. For example, the ADAS system in vehicles rely on multiple cameras for detection but the headlamps of on-coming traffic could affect the visual information being processed. Similarly, in medical imaging systems poor contrast and distortion could affect accurate diagnosis and treatment.

Fig.1. Examples of flare and ghost images in photos taken with a camera

To limit undesirable light reflections to a minimum, baffles, stop surfaces, or surface treatment/anti-reflective coatings can be used. However, the first step is to identify the source of stray light and examine the optical paths that create it.

Engineers can leverage LightTools optical design engineering software to accurately model the real-world performance of such systems and address stray light effects at the design stage itself, thus reducing multiple iterations and increasing efficiency.

LightTools Workflow For Stray Light Analysis

The aim is to examine the impact of reflections from lens surface and identify its optical path.

• Open or import the lens model into LightTools. Align the geometry according to the global axis system and ensure all lens surfaces are grouped together and named sequentially.
• Assign material properties to all components of the system, including lens, mirrors, baffles, mechanical mounts from the material library.
• Choose optical property for the lens surfaces and the ray trace method. This defines if the surfaces will be reflecting, transmitting or has probabilistic ray split with Fresnel loss

Fig.2. geometry of lens system in LightTools and its wireframe section view

• Multiple configurations with varying optical /material properties or any independent input variable can be created at once. This helps to evaluate more than one design concept simultaneously without needing to create the model from scratch.
• Define an object source, this could simply be a rectangular plane source the size of the image or the actual image itself can be used as a source with the image processor utility.

Fig.3. define multiple configurations simultaneously(left) and a plane light source creation (right)

• Multiple configurations with varying optical /material properties or any independent input variable can be created at once. This helps to evaluate more than one design concept simultaneously without needing to create the model from scratch.
• Define an object source, this could simply be a rectangular plane source the size of the image or the actual image itself can be used as a source with the image processor utility.
• Add receiver filter to the exit surface and input the ray trace parameters to run the simulation. Check the ray path collection is enabled for the receiver as this will record the complete data for each optical path traced through the system. Run the Monte-Carlo ray trace.
• Analyse the Illuminance patterns for each configuration interactively or using the parameter analyser. The ray paths traversed through the system and its corresponding illumination pattern could be visualized individually using the Ray Path tool.

Fig.4. Visualizing rays traced through the system using Ray Path tool and its corresponding illuminance pattern

Fig.5. Ray path causing double reflections, path details and the corresponding stray light illuminance pattern produced

The tool provides complete data about the sequence of surfaces the rays are traced through and the total power, along with the illuminance pattern.

It assists engineers in detecting direct and single-bounce reflections from optical and mechanical surfaces, as well as where the ghost images will land, their size, and brightness, dependent on the geometry and coatings of each optical surface. Identifying the worst reflection pairs will help tweak the surface to reduce impact on final image.

By integrating Keysight LightTools into your early-stage workflow, you can proactively eliminate critical stray light and ghosting issues before they reach production