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Radially Variant Contrast Measurement Method on VR Head-mounted Displays

Catalog of Regulatory Science Tools to Help Assess New Medical Devices 

This regulatory science tool presents a method to measure the radially variant contrast on virtual reality head-mounted displays (VR HMDs) using circular concentric ring patterns, which can be implemented to characterize image quality, including Michelson contrast and effective resolution, on VR HMDs equipped with optical lenses.

 

Technical Description

This tool describes a method to measure the radially variant contrast on virtual reality head-mounted displays (VR HMDs) using circular concentric ring patterns. The method can be implemented to characterize the image quality, such as Michelson contrast and effective resolution, on VR HMDs with optical lenses. It resolves the limitation of conventional grille method, which does not consider the spatial variation of image quality on VR HMDs due to optical aberration.

The input to this tool is a set of circular concentric ring patterns that should be rendered on a VR HMD. This tool specifies the experimental setup, image acquisition, and processing procedures to measure the Michelson contrast on the evaluated device. The output is a spatial distribution of the Michelson contrast or a 2D contrast map. The list below offers a technical summary of the RST:

  • Circular concentric ring pattern: The 2D concentric ring pattern consists of alternating black and white rings with detail described in a peer-reviewed publication [1] and in the Instructions on Implementing the Radial Contrast Measurement Method [6]. 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 concentric ring patterns on an HMD with a compatible web browser.
  • Experimental setup: This method uses a high-resolution array light measuring device (LMD) with photopic response mounted on a three-axis stage that provides a wide-view spatial luminance measurement. Technical requirements on the LMD used for the contrast measurement on HMDs are specified in IEC 63145-20-10 standard [3] and Sec. 19.2 of the Information Display Measurements Standard (IDMS) [4]. The optical axes of the HMD and LMD should be aligned following procedures described in Sec. 19.3 of the IDMS [4].
  • Image acquisition and processing procedures: 2D luminance profiles of the concentric ring patterns with various spatial frequencies (for example, low, medium, and high spatial frequencies) should be acquired using the LMD. On a captured concentric ring image, a spatial (radial-angular) distribution of the Michelson contrast across the display field of view (FOV) is computed following the procedure described in [1] and in the Instructions on Implementing the Radial Contrast Measurement Method [6]. The measurement may be repeated at different interpapillary distance (IPD) settings following the eye position of the intended user by moving the LMD’s entrance pupil location to the corresponding eye position.

Detailed step-by-step instructions on implementing this tool are described in the Instructions on Implementing the Radial Contrast Measurement Method [6].

Intended Purposes

The method described in this tool can be implemented by both medical device developers and testing labs to complete bench testing for evaluation of the image quality on MXR devices.

The MXR device can be potentially intended for but not limited to visualization, pre-surgical planning, intraoperative procedures, diagnostics, and patient therapy. The proposed contrast measurement using this tool is to ensure adequate image quality on the MXR device for the intended use.

Testing

The method has been extensively tested and validated on two VR HMDs – the HTC VIVE Pro and Meta Quest 2 [1]. The tests involve repeating the measurement using 1) different spatial frequencies of the patterns, 2) various IPD settings, and 3) different entrance pupil location. The results demonstrate that the method is reproducible on multiple HMDs with different hardware and software [1]. The contrast measured using this concentric ring method is also equivalent to the standardized grille method as described in IEC 63145-20-20 [5] using the appropriate grille pattern orientation.

Limitations

The radial contrast measurement takes a wide-view image that is limited by the FOV and resolution of the LMD. The experimental setup emulates a pupil rotation scheme using a static LMD. To cover a wider display FOV, an eye rotation setup as described in IEC 63145-20-10 [3] can be implemented by rotating the LMD’s optical axis to the target position. The radial contrast measurement method can be applied to an eye rotation setup. However, it can be burdensome to repeat the contrast measurement at multiple locations (for example, a 9-point-measurement as recommended in IEC 63145-20-20 [5]).

This tool can be potentially applied to waveguide-based augmented reality (AR) devices with optical see-through content. The result should be comparable to the conventional grille method.

Supporting Documentation

  1. Zhao, C, Beams, R, Badano, A. Radially variant contrast measurement in virtual reality headsets using circular concentric ring patterns. J Soc Inf Display. 2023; 31(5): 387–397. https://doi.org/10.1002/jsid.1208
  2. Github WebXR Tool
  3. IEC 63145-20-10:2019 Eyewear display - Part 20-10: Fundamental measurement methods - Optical properties.
  4. Information Display Measurements Standard, SID, 2023.
  5. IEC 63145-20-20:2019 Eyewear display - Part 20-20: Fundamental measurement methods - Image quality.
  6. Instructions on Implementing the Radial Contrast Measurement Method

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