Catalog of Regulatory Science Tools to Help Assess New Medical Devices
This regulatory science tool is a method for measuring the line spread function (LSF) and calculating the modulation transfer function (MTF) for head mounted displays (HMDs).
Technical Description
Spatial resolution is an important image quality parameter for many applications of medical extended reality (MXR). The spatial resolution is frequently measured using a grille pattern or a sinusoidal pattern at different spatial periodicities to measure the contrast modulation function (CTF) or the MTF, respectively. For HMDs, grille patterns are more commonly used since it allows for rendering patterns at spatial frequency as high as 1-on-1-off pixels. However, this approach requires repeated measurements with test patterns at different spatial frequencies and the grille pattern provides the CTF opposed to the MTF. It is appropriate for augmented reality (AR) and virtual reality (VR) devices. The tool specifies the following information:
- Test pattern: Render a vertical or horizontal line that is one-pixel wide or as narrow as possible through the rendering pipeline. This pattern allows for the spatial resolution or angular resolution to be measured at a single location in the field of view (FOV) from a single measurement. The details are described in a peer-reviewed publication [1] and in the Instructions on Implementing the Line Spread Function Measurements. The test pattern will be rendered on the headset and centered with respect to the optical axis of the HMD. To assist with this, the FDA developed a WebXR tool [2] that can be used to render the line pattern on an HMD with a compatible web browser.
- Experimental setup: This method uses a high-resolution monochromatic array LMD with photopic response mounted on a 5-axis goniometer or robot system to align the center of the entrance pupil of the HMD. Prior work demonstrated that eye-rotation methods, opposed to pupil rotation methods, are necessary to capture the impact of eye gaze due to the pupil shift as the eye rotates [1]. Therefore, the rotation axis should be placed approximately 12 mm behind the entrance pupil of the HMD for the eye-rotation method. Technical requirements on the LMD used are specified in IEC 63145-20-10 standard [3] and Sec. 19.2 of the Information Display Measurements Standard (IDMS) [4]. It is recommended to have at least 20:1 for the LMD to HMD pixels ratio to minimize the impact of the LMD modulation transfer function and ensure adequate sampling of the HMD pixels.
- Image acquisition and processing procedures: Images of the rendered line should be acquired using the LMD. The captured images can be used to determine the spatial resolution and MTF of the HMD using the procedure described in detail in [1] and in the Instructions on Implementing the Line Spread Function Measurements. The measurement may be repeated with the line rendered at different angular locations in the FOV and in the vertical and horizontal directions to determine the spatial resolution across the FOV and in orthogonal directions. The measurements can also be repeated for red, green, and blue lines to measure the chromatic dependence of the spatial resolution.
Detailed step-by-step instructions on implementing this tool are described in the Instructions on Implementing the Line Spread Function Measurements.
Intended Purpose
This method is intended to characterize the spatial or angular resolution and MTF of an HMD. The method is applicable to AR and VR HMDs. The method can be implemented by medical device developers and testing houses for evaluating spatial resolution of MXR devices.
Testing
The method has been extensively tested and validated on multiple HMDs: HTC VIVE, HTC VIVE Pro [1] and the Microsoft HoloLens 2. The tests involve repeating the measurements at different locations in the FOV of the HMD and repeating on a conventional monitor to characterize the performance of the LMD. The method is also related to the method in ISO 12233-2023 for measuring the slanted-edge response function for cameras [5].
Limitations
The LSF method requires a high-resolution array LMD, such as a camera, and a 5-axis goniometer for alignment. The relative minimum linewidths in the horizontal and vertical directions may depend on the subpixel design of the display. This method is also intended for only the horizontal and vertical directions on the HMD and not for off-axis line orientations.
Supporting Documentation
The method with detailed instructions and validation are described in the following publication:
R. Beams, B. Collins, A. S. Kim and A. Badano, "Angular Dependence of the Spatial Resolution in Virtual Reality Displays," 2020 IEEE Conference on Virtual Reality and 3D User Interfaces (VR), Atlanta, GA, USA, 2020, pp. 836-841, doi: 10.1109/VR46266.2020.00108. https://didsr.github.io/WebXR-tools/Custom/ IEC 63145-20-10:2019 Eyewear display - Part 20-10: Fundamental measurement methods - Optical properties. Information Display Measurements Standard, SID, 2023. ISO 12233:2023—Photography Resolution & Spatial Frequency - Instructions on Implementing the Line Spread Function Measurements
Contact
Tool Reference
- In addition to citing relevant publications please reference the use of this tool using RST24MX02.01