The Role of Stray Light Measurement in Display Design and Testing
For engineers working on the design and testing of displays, understanding and measuring stray light is essential. It’s not just about identifying visual defects; it is about ensuring that the display performs reliably under a wide range of conditions and viewing angles. This narrative explores the technical significance of stray light measurement and how tools from Radiant Vision Systems can support this critical aspect of display development
Understanding the Nature of Stray Light
Stray light can manifest in several ways: as a faint glow around bright objects (halo), as ghost images caused by internal reflections, or as elevated black levels that reduce contrast. These effects are particularly problematic in high-dynamic-range (HDR) displays, where deep blacks and bright highlights are essential for image fidelity. In AR/VR systems, stray light can break immersion or cause visual discomfort. In automotive applications, it can even pose safety risks by obscuring critical information.
From a design perspective, stray light is often a byproduct of complex optical paths, imperfect materials, or insufficient shielding. It’s not always visible under standard testing conditions, which is why specialized measurement techniques are required to accurately detect and quantify it.
Imaging-Based Measurement: Capturing the Full Picture
One of the most effective ways to evaluate stray light is through imaging-based measurement. Radiant Vision Systems designed their ProMetric® Imaging Colorimeters and Photometers to capture high-resolution, spatially resolved images of a display’s luminance and chromaticity. This capability is crucial for identifying localized stray light artifacts that might otherwise go unnoticed.
For example, in an OLED display, light leakage from adjacent pixels can create a halo effect around bright objects. An imaging colorimeter can visualize this effect across the entire display, allowing engineers to assess its severity and trace its origin. Designers and engineers find this information beneficial during the prototyping phase, as it will enable them to evaluate iterative design changes quickly and quantitatively.
Moreover, imaging systems enable repeatable and objective measurements, which are essential for quality control in mass production. By comparing captured images to a reference sample, engineers can ensure that each unit meets the required visual standards.
Quantifying Impact: Contrast and Uniformity
Stray light not only affects the appearance of a display but also impacts key performance metrics, such as contrast ratio and luminance uniformity. These parameters are critical in applications where visual clarity is paramount, such as medical imaging displays or cockpit instrumentation.
Radiant’s TrueTest™ Software integrates with their imaging systems to automate the analysis of these metrics. For instance, the software can measure the luminance of black and white regions to calculate the contrast ratio or evaluate brightness uniformity across the screen. Stray light can elevate the black level or cause uneven illumination, and the test results will reflect these effects.
This kind of quantitative analysis is invaluable for engineers who need to validate design changes or troubleshoot performance issues. It also supports compliance with industry standards, such as those set by the International Committee for Display Metrology (ICDM).
Near-Eye Displays: Simulating the Human Eye
In AR/VR systems, the proximity of the display to the user’s eye introduces unique challenges. Stray light can be particularly disruptive in these environments, where even minor artifacts can affect immersion and usability. Traditional measurement tools may not accurately capture what the user sees, especially at off-axis angles.
To address this, Radiant offers the XRE Lens, a specialized optical system that simulates the human eye’s view of a near-eye display. Engineers use the lens to capture images from the same perspective as the end user, revealing stray light effects that they might otherwise miss.
For example, internal reflections within a waveguide can produce ghost images that are only visible when viewed from a specific angle. The XRE Lens enables engineers to detect and analyze these artifacts, providing the necessary data to refine optical designs and enhance the user experience.
Angular Analysis: Beyond the Front View
Displays are rarely viewed straight-on in real-world use. Whether it is a passenger glancing at a dashboard display or a gamer turning their head in a VR headset, off-axis viewing is a common phenomenon. Stray light can behave very differently at different angles, which is why angular measurement is so important.
Radiant designed the Near-Field Measurement Systems (PM-NFMS™) to capture the angular distribution of light emitted by a display or optical component. This data helps engineers understand how light is scattered or reflected at various angles, which is essential for diagnosing and mitigating stray light issues.
For instance, a diffuser might perform well at normal incidence but scatter light excessively at oblique angles. By measuring the angular light distribution, engineers can make informed decisions about material selection and optical design.
Component-Level Evaluation: Lenses and Diffusers
Stray light often originates not from the display itself but from the optical components used in the system, such as lenses, diffusers, or light guides. These components can introduce scattering or reflections that contribute to stray light, especially if their surfaces are not adequately treated or aligned.
Engineers can use Radiant’s measurement systems to evaluate these components individually, allowing them to isolate each one’s contribution to stray light. For example, by measuring the output of a lens under controlled conditions, it’s possible to determine whether it’s introducing unwanted reflections or scattering. This data supports better component selection and integration, ultimately leading to cleaner optical systems.
Stray Light Correction: Enhancing Measurement Accuracy
While measuring stray light is essential, correcting for it is equally critical, especially when requirements call for high accuracy in the UV and IR spectral regions. Instrument Systems, a Konica Minolta company, a leader in precision optical measurement, addresses this challenge using a sophisticated stray light correction matrix based on a method developed by NIST.
This process involves illuminating the spectroradiometer with monochromatic light from a tunable laser. Ideally, this light should only affect a narrow band of pixels on the detector. However, it will identify any signal detected outside this band as stray light. You can generate a device-specific correction matrix by scanning across the entire spectral range. This matrix is then applied to future measurements, effectively removing the influence of stray light and restoring spectral fidelity.
The benefits of this correction are substantial:
· Improved accuracy in UV and IR measurements.
· More reliable color coordinate determination in the visible range.
· Enhanced safety assessments, including the evaluation of blue light hazards in compliance with photobiological safety standards.
This correction technique is particularly valuable in applications like UV-LED testing, automotive lighting, and display characterization, where even minor spectral inaccuracies can lead to significant performance or safety issues.
Supporting Simulation and Design Optimization
Measurement is only one part of the equation. To effectively mitigate stray light, engineers also rely on optical simulations to model how light behaves within a system. However, the accuracy of simulations depends on the quality of the data they use.
Radiant’s systems provide the real-world data needed to validate and refine these simulations. By feeding accurate luminance and angular distribution data into tools like Zemax or LightTools, engineers can ensure that their models reflect actual performance. Enabling more effective design optimization and reducing the need for costly physical prototypes.
Conclusion: A Critical Tool for Display Engineers
Stray light measurement is not just a quality control step; it is a fundamental part of display engineering. It affects everything from image clarity and contrast to user comfort and safety. For engineers working on the cutting edge of display technology, having the right tools to measure and analyze stray light is essential.
The Konica Minolta Sensing family of companies offer a suite of instruments and software that support this work at every stage of the development process. From imaging colorimeters that capture full-field luminance data to near-eye lenses that simulate the user’s perspective to angular measurement systems that reveal off-axis behavior, these tools provide the insights engineers need to design better displays.
By integrating stray light measurement into the design and testing workflow, engineers can ensure that their displays not only meet technical specifications but also deliver a superior visual experience in the real world.
