Exploring The Facets Of Stray Light With Simulation

Seems like everywhere you look, there’s someone snapping a memorable group photo or perfect selfie with their phones. As the line between traditional and cellphone cameras continues to blur, manufacturers of these handheld devices are pressed to find the best combination of software and hardware to achieve image quality that was previously unthinkable.

Of course, mobile photography has come a long way — so much so that the point-and-shoot cameras we used to use on vacation have pretty much been relegated to the past. Today’s top-rated smartphones can deliver near-digital single reflex lens (DSLR) camera performance, leading to sharper image quality.

Smartphone camera systems are essentially a collection of camera components, technologies, and systems. Many have multiple lenses with different focal lengths and characteristics. Common types include wide-angle, ultrawide-angle, telephoto, and macro lenses. Each lens serves a specific purpose, enabling users to capture a variety of shots.

The challenge with these handheld camera applications is that they must be highly performant and subject to many lighting conditions. Whether indoors or out — subject to sunlight, shade, and shadows; or at night with interference from streetlights, signage, and the light emitted from the headlamps of passing traffic — consumers expect consistent performance.

As light strays, image quality suffers

One of the biggest considerations of optical system design is stray light. By definition, stray light is unwanted scattered or specular light at the camera sensor, which is unintended in the optical design and degrades the optical performance of the camera system. It’s also the common cause of reduced image contrast, blurring, and discolored images snapped with smartphone cameras.

There are two types of stray light: ghosts and glare, which is also known as veiling.

• Ghosts are reflections that appear as bright spots in an image when light from a source in and close to the edge of the camera’s field of view experiences two or more unwanted specular reflections before falling on the sensor.
• Glare happens when light scatters inside a camera’s optical system. Scattering can be caused by many factors, including optical surface imperfections or as a function of flawed system design.

It’s a big issue for manufacturers of handheld devices, who are tasked to identify and eliminate the possibility of stray light early in the design phase. The trick is to eliminate, through analysis and control, the impedance of stray light on the optical system of a given device. But because the cameras can be used in virtually any environment, their design must account for stray light in many different lighting conditions.

To solve this problem, it’s important to seek ways to identify extraneous critical light source positions, which can be inside or outside the camera’s field of view. The next step is to design methods for remediating the stray light’s influence, typically by reconfiguring the lenses in the camera or swapping in new materials that have different optical properties.

A different perspective in a single workflow

Ansys Optical simulation and design software can simulate light behavior and propagation through the digital modeling of optically enabled products like cellphones, which leads to more accurate, robust camera designs. Through the convergence of multiple Ansys products, it’s possible to see the propagation of stray light in an entirely new camera system.

There are four steps in the analysis of stray light in a camera system using Ansys tools, namely:

1. Import Zemax OpticStudio lens design to Speos using the Zemax Import tool within Zemax OpticStudio. This step involves using a compact and efficient lens system for mobile phone cameras designed with OpticStudio. Using the OpticStudio API, you can read OpticStudio lens data parameters and automatically recreate each lens as a native computer-aided design (CAD) geometry based on their mathematical representation. Data from OpticStudio can also be imported into the Speos projection lens feature, giving you access to all lens parameters.

The tool can then import OpticStudio materials into a Speos material format and apply optical properties on the lenses, and the imager is then converted into an irradiance sensor. The reference origin for all the geometries and irradiance sensor corresponds to the position of the image plane. In the final step (pictured below), the lens system is added to the optomechanical parts (grey) and lens edges (yellow), which are already predefined in Speos.

The lens system is added to the optomechanical parts (grey) and lens edges (yellow), which are both already predefined in Speos.

2. Detect all possible critical sun positions and light leakage for the complete system. In this step, all possible critical sun positions are studied in one simulation using a reverse ray-tracing simulation approach. It is a powerful method that sends rays from the imager through the camera system to the sky. With this approach, light leakage can also be detected in the mechanical system, and the Speos ray tracing algorithm takes all material behaviors of all geometries into account. These areas can then be categorized by criticality and ray paths inside and outside of the camera’s field of view (FOV).

Light sources within the camera’s FOV can undergo multiple secondary reflections at lens surfaces, which leads to ghost reflections and lens flare on the imager. As light sources outside of the FOV could cause stray light scattering on mechanical and optical parts, Speos Light Expert (LXP) feature capabilities can also be used to visualize and export these ray paths for a specific area on the intensity result. This feature can be used to identify and study the stray light conditions that will impact your design, including light sources within the camera’s FOV and potential light leakage in your system’s mechanical housing from beyond the FOV.

3. Simulate stray light from four sun positions within the camera FOV (optional). In this step, a full-system stray light simulation is run for four different sun positions (from 0° to 15°) within the camera FOV using Speos.

A full-system stray light simulation for four different sun positions within the camera field of view (FOV).

4. Analyze the stray light path sequences and mitigate ghost stray light for one sun position. This step involves identifying the most critical ray path sequences (in terms of irradiance hitting the sensor) and object interactions leading to the stray light on the imager for the 5° sun position by utilizing LXP and sequence detection features.

Illustration of the 20th most energetic stray light paths sorted by energy hitting the sensor. The sequences are ordered by energy hitting the sensor.

Read the “Stray Light Analysis – Smartphone Camera” study for a more comprehensive step-by-step guide through the workflow on how to analyze stray light for a smartphone camera system using Ansys Speos capabilities.