A recent webinar by Radiant Vision Systems’ Optics Development Manager highlighted new advancements in image quality assessment, which is required when integrating prescription lenses into augmented, mixed, and virtual reality (AR/MR/VR) devices. Such techniques are becoming increasingly important in a market where the use of virtual technologies is becoming ever more widespread, and most customers require prescription lenses.
With the rapid growth in the use of AR, MR, and VR (collectively XR) devices in many industries, current trends indicate that AR/MR devices will become a much larger proportion of the current market.
With such considerable growth in AR/MR users, numerous aspects must be considered for user experience, one of which is that a greater proportion of potential users require prescription correction for their eyesight.1 More than 4 billion people worldwide wear prescription lenses, equating to around two-thirds of the global population. Projections suggest that by 2050, half of the people on the planet will require prescription lenses of some sort.2
Approximately 84 % of adults over the age of 45 use vision correction lenses, and the need for glasses among children has also increased significantly—around 40 % now require them. This marks a doubling in the number of children needing prescription lenses since the 1970s.
Thus, there is a clear need for manufacturers to consider the development of customizable optical configurations for XR devices that are worn in place of regular eyeglasses and for lenses that can be inserted into certain devices to mimic the prescription of a person’s prescription glasses. These lenses may be created through 3D printing and cold casting.3
Assessing the Imagining Quality of AR/MR Devices
The “Novel Testing Methods for AR/MR Devices and Smart Glasses with Vision Correction” webinar by Eric Eisenberg at the 2023 Photonics Spectra Optical Metrology Summit discussed the challenges associated with incorporating prescription lenses into AR/MR devices. He then offered novel solutions provided by Radiant Vision Systems to address such challenges.
Eric Eisenberg has a deep understanding of the technical considerations necessary to successfully implement optical technology within imaging systems, thanks to his extensive firsthand experience with optical technology and imaging. This article will provide a summary of the important takeaways from this talk, which is freely available online from Photonics Spectra.
Challenges Associated with Incorporating Prescription Lenses into VR/AR/MR Devices
Ophthalmologists prescribe lenses based on spherical error, which can be either positive or negative, and cylindrical error to correct astigmatism, a condition where the eye focuses more effectively in one direction than another.
For spherical error, prescriptions can provide a correction range of up to + 6 / -6 diopters of optical power scale for each measurement, while two or more diopters can be prescribed for astigmatism.
Image Credit: A typical vision prescription with a range of +6/-6 diopters for each measurement, out to 2 decimal places. Radiant Vision Systems
Prescriptions are written in 0.25 diopter increments and, for astigmatism, in 1/5 degree angular increments. Since prescribed lenses are tailored to an individual's eye physiology, each lens is uniquely crafted to correct their specific vision error. This huge number of variations poses a significant challenge when measuring display system quality when prescription lenses are incorporated into XR devices.3
The key to effective quality inspection of AR/MR devices is measuring the display precisely as a human wearer will see it. Device makers must ensure that the images shown appear bright enough and are not blurred or distorted, among other visual quality criteria.
Factors influencing image quality include focal distance—the virtual distance from the wearer’s eyes where images are most sharply focused. Typically, a device’s optical system produces images at a focal distance suited for individuals with “normal” vision who do not require prescription lenses. However, issues arise when the optimal focus distance for a user with prescription glasses diverges from the device’s default setting.
For example, if images are designed to be sharp at a virtual focus distance of two meters, they will appear blurry to a user whose prescription lenses adjust focus differently. This discrepancy highlights the need to accommodate various focal distances to accurately assess a device’s image quality.3 So, how can a device be tested at all of the possible focal distances, among all of the other variables of prescription vision?
A traditional approach for prescription lens compensation is the “Brute Force” method, which mechanically inserts reversed compensation optics to account for the user’s corrective optics. This method, while direct, is complex, requiring numerous lenses and moving parts. Thus, it proves highly impractical when it comes to evaluating the image quality of XR devices for several reasons.
The first reason is insufficient space to incorporate additional optical lenses between the human eye’s entrance pupil (its position within a headset) and the device. Moreover, the manual adjustment of the measurement system is time-consuming, making it unfeasible to test hundreds of thousands, or even millions, of devices through mechanical lens replacement.
A more efficient approach would be to utilize existing optics and mechanics inside the imaging system to automatically adjust to correct for any present prescription. Radiant Vision Systems has developed a novel XR device testing solution that incorporates electronic lenses with this capability.3
A Novel Approach to AR/MR Image Quality Testing
Diopter (Spherical) Compensation
Combining the electronic focus adjustment capability of Radiant’s XRE Lens with TrueTest™ TT-ARVR Software for automated focus control removes the need for multiple, mechanically moving optical lenses. Electronic focusing with this software means substantial changes in the focus of the overall system can be achieved through small electrical changes.
Spherical correction, aimed at addressing near-sightedness or far-sightedness, is achieved by adjusting the focal distance of the imaging system “beyond infinity” through the use of negative diopters.
In an AR/MR system, the relationship between the optical power and the movement of lens motors can be represented linearly. This allows for the internal microlens to be adjusted to a negative imaging power. This adjustment is crucial because while the device being tested might display the image at infinity, the addition of a prescription alters the focal point, effectively projecting the image “past infinity.” Hence, the spherical correction needed by the prescribed lenses can be compensated for by simply altering the focus of the XRE Lens’s microlens.
Unlike normal lenses, the XRE Lens is capable of “focusing beyond infinity,” which allows diopter compensation to be accomplished through the electronic adjustment of the XRE Lens. The focal point of this lens automatically changes as a function of the prescription power, and different focus positions can be checked and iterated accordingly. Software control ensures no degradation in modulation transfer function (MTF) measurement capability.
Image Credit: Compensating for near-sighted optics by “focusing beyond infinity”, diverging the light rays to correct for near-sightenedss (negative diopters). Radiant Vision Systems
For example, measurement results after the insertion of a 4-diopter lens into the DUT showed practically identical values for the contrast of the MFT but at different motor counts. The position of where the contrast is with regard to motor counts is irrelevant if the peak is the same value. Therefore, Eric Eisenberg is highly confident that this method will account for the +6 to -6 diopter range for spherical corrective lenses.
Astigmatism Compensation
With astigmatism, the amount of optical compensation required differs from one axis to the other, and a cylindrical component must be added to adjust for this and focus the eyes. The astigmatism angle can range from 0 to 90 degrees, and the pattern used to measure the quality of the focus of the device needs to be oriented at the same angle as the astigmatic element of the prescribed lenses.
Adjusting the pattern generator of the software allows for random angles to be entered for contrast characterization and calculation of the MTF as a function of the orientation of these lines. If the prescription is known, this system can be programmed specifically to it, and if it is not known, the system will span through a range of values to find the prescription value. Once lines are orientated at the same angle as the astigmatic aspect of the prescription, practically identical MTF contrast that would be seen without prescription lenses is achieved.
Check Out the Webinar to Discover More About These Solutions
Radiant Vision Systems has provided a new and effective solution for the imaging assessment of AR/MR devices incorporated with prescription lenses, allowing for fast and precise assessment.
Automation is key to these novel advancements, and Radiant Vision Systems can offer state-of-the-art capabilities for manufacturing and testing XR devices. For a more detailed explanation of this method, you can watch this webinar online for free.
References and Further Reading
- Güzel, A.H. et al. (2023) ‘ChromaCorrect: Prescription correction in virtual reality headsets through Perceptual guidance’, Biomedical Optics Express, 14(5), p. 2166. Available at: https://www.researchgate.net/publication/366136297_ChromaCorrect_Prescription_Correction_in_Virtual_Reality_Headsets_through_Perceptual_Guidance.
- Eyewear Industry statistics and facts 2023, by Gidon Sadovsky. 2023. https://www.overnightglasses.com/eyewear-industry-statistics/. Accessed 18th of December 2023.
- Novel Method of Prescription Vision Compensation to Measure AR/MR Devices and Smart Glasses webinar by Eric Eisenberg, November 15, 2023. https://www.radiantvisionsystems.com/learn/webinars/novel-method-prescription-vision-compensation-measure-ar/mr-devices-and-smart. Accessed 18th of December 2023.
This information has been sourced, reviewed, and adapted from materials provided by Radiant Vision Systems.
For more information on this source, please visit Radiant Vision Systems.