An ideal imitation of the human eye enables the precise measurement of near-eye and head-mounted display quality

The all-new 2021 version of the Almalence Human Eye Simulator. Optically clear. Eye Tracking ready.

To assess the quality of head-mounted displays, it is necessary to capture images which exactly match with what a human eye would perceive. Indeed, a capturing device has to be capable of accurately replicating the human eye’s optical properties. If they are not, then this could lead to some drastic irregularities – a mismatch of entrance pupil diameter, for instance, would lead to quite different blurs and aberrations, or even sometimes visible Fresnel rings, which are not apparent to the human eye. Once you do have a capturing device in place that can match the optical properties of the human eye, however, then now comes the real challenge: the device has to be recognized as an “eye” by eye trackers – otherwise, there will simply be no chance of capturing a correct picture, as a wrong picture would be displayed in the first place, in case the HMD uses eye position-dependent rendering techniques like foveated rendering or dynamic aberrations correction – which have recently been becoming standard for high-quality near-eye displays.

Almalence, a pioneer in designing the eye-imitating cameras, has now begun to roll out an all-new and updated version of its powerful eye simulator, better than ever and ideally suited for near-eye display picture capturing and quality measurement tasks. It features made-to-order, optically clear eye corneas, flawlessly creating a perfect match to the form of a real, human eye that is indistinguishable for eye trackers – unlike other, off-the-shelf parts and solutions, which commonly result in deviations from the proper shape. A clear aperture for up to a 120° FOV enables the seamless capture of up to the entire field of view in one shot, without ever compromising the contrast and MTF of the true visible picture.

The ideal profile of both visible and IR light absorption and reflection is painstakingly implemented, in order to make the simulator’s iris look exactly like a natural iris to an eye tracker. An additional IR-cut filter also prevents unwanted reflections from the camera lens, which may spoof eye tracker readings.

Auto-focus capability, to avoid having to manually adjust the focal point when moving the “eye” inside head-mounted displays that exhibit a significant field curvature.

The platform encapsulates multiple capturing camera designs, including a 100° field of view camera which enables the user to capture the entire visible FOV in one shot. This feature is also quite useful for geometry distortion measurements. Specially designed narrow 78- and 34-degree FOV cameras are also included, engineered for high-precision optical measurements, including apparent resolution, chromatic aberrations, and more.

A monochromatic camera can also optionally be used with the eye simulator, in order to resolve ambiguity in color channel mixing between the HMD display and CFA filters inside the camera.

Almalence has also developed a powerful software for the processing and transforming of captured images, so that they can readily be used for correct measurements of geometry, MTF, channel crossing, and other quality characteristics, with industry-standard tools such as ImaTest. Together with a 6-DOF robo-arm and its controlling software, all of the above-mentioned features seamlessly combine to present a complete, easy-to-implement tool for head-mounted displays, picture quality assessment, and the profiling of geometry and aberrations correction.

Almalence Digital Lens to harness the full potential of Varjo’s human-eye resolution head-mounted display

From its beginning Varjo positioned itself as the leader of VR head-mounted displays megapixel race. Their “human-eye resolution” VR-1 truly shows more detail than any other existing HMD.

However, just offering a high pixel count does not mean the user will be able to see a crisp and clean picture through the HMD optics, and in fact, it is the optics which present a display quality bottleneck. In a head-mounted display there are severe design constraints especially in making the optics light weight and fit in a tight space. Those constraints lead to a compromised optical performance, resulting in color fringing and blur. Moreover, any movement of the eye pupil, which itself constitutes a lens element, makes the entire optical system quite different from the original optical design, so the blur and color fringing get even stronger as the eye looks off the optical axis.

Almalence Digital Lens is a computational lens aberrations correction solution which overcomes these limitations by compensating the aberrations of an HMD optics. It does the job of a corrective lens element which dynamically adjusts its properties depending on the eye pupil position. We were eager to check how that technology can improve picture clarity of the highest resolution HMD.

Note 1: This testing was performed by Almalence independently from Varjo. The Digital Lens test was implemented as a Unity application using public API.

Note 2: This is the very first testing, definitely showing sub-optimal results. We see a clear way to further improve the image clarity with the given headset.

We used a construction drawing as a test picture as it clearly demonstrates how the insufficient apparent resolution and clarity limit the VR usability.

The test picture

To take the images within the HMD, we used our camera system with our eye imitator, allowing to capture what a human eye would see.

In the first example the eye looks about 10 degrees off the center. The left part of the gaze area falls onto the high resolution “focus display”, the right part falls onto the lower resolution “context display”

Move the slider left/right to see the difference. Left: Varjo VR-1 as is; Right: VR-1 with Digital Lens. Despite the high display pixel count, the picture does not look very clear. One can even start feeling sick when trying to read the numbers. The very same display with the Digital Lens delivers much clearer and readable picture.

In the next example the eye looks straight at the center, along the optical axis – the ideal case in which the HMD delivers its highest possible picture quality. The gaze area is completely over the focus display. Even in that case the Digital Lens shows a noticeable improvement:

Left: Varjo VR-1 as is; Right: VR-1 with the Digital Lens. Same display, but more legible text and crisper lines.

The beauty of the Digital Lens solution is that it is a pure computational technique, adding no extra size, weight or mechanical complexity to the device.

As mentioned above, those are very first tests, more to follow. However the tests already prove that the Digital Lens is an indispensable technology for high-end VR headsets, allowing to harness the full potential of high display pixel count and density.

Accurate Eye Imitation for VR HMD Testing

When it comes to VR/AR HMD objective testing, you need an “eye” which is much more than simply a digital camera. Besides having the entrance pupil, focal length and aperture similar to those of a human eye to see like a human eye, the imitation must be properly perceived by the eye tracking modules.

Meet the First in the World accurate eye imitation created by Almalence.

Not only sees like an eye, but also looks like an eye

Using a special compound developed at Almalence lab, we managed to create an imitation of the iris that looks correctly in both visible and infrared light, which makes the eye imitation correctly detected by eye tracking devices. To make the imitation totally indistinguishable from an eye of a human being, the imitation is made to give proper reflections and glint locations.

Whist having all the above properties, the imitation has a cover glass made of an optical-quality transparent material, which does not distort the light passing to the camera. The camera, in its turn, has the entrance pupil properly positioned within the “eye” using special micro-mount.

With the above features together, we have achieved a perfect “two-way immersion” — the eye imitation not only sees exactly what a human eye would see, it also looks exactly as a human eye to the eye tracker.

The eye imitation will be used for better optical profiling of VR HMDs to achieve the highest possible quality with Almalence Digital Lens technology and for accurate objective testing of VR/AR HMD picture quality.

This eye does not lie