If you’re responsible for validating XR displays, you already know the challenge: standard test lenses often introduce artifacts, fail to maintain focus across virtual image distances, or simply can’t handle today’s ultra-wide fields of view. We work with clients to design modified conoscopes that deliver high resolution, artifact-free results across a wide range of VIDs (Virtual Image Distances), without compromising simplicity or accuracy. Whether you’re building out a new test station or refining your current setup, we can help you avoid the common pitfalls and contribute to a system you’ll never have to second-guess.
This page further expands on these and other key points beginning after figure 1. The approach described here is also the subject of a peer-reviewed paper presented at SPIE Photonics West 2025 by Steve Eckhardt and Matt Johantgen. The full paper is available as “Modified Conoscopes for XR Display Inspection” in the SPIE Digital Library.
Modified conoscopes for XR display testing offer higher resolution and better focus control across a wide range of virtual image distances (VIDs), improving test accuracy for AR/VR/MR headsets.
Traditional conoscope designs struggle with elliptical pupils, MTF degradation, and complex aberration correction, especially at wide field angles.
Adding an optical element before the stop enhances system symmetry, resulting in better MTF performance, reduced illumination falloff, and simpler optical correction.
Fewer components and smaller optics mean that modified conoscopes can reduce size, weight, and cost by up to 40–50%, while maintaining compatibility with standard image sensors.
Figure 1: Traditional Conoscope with Stop in Front for XR Display Inspection
Many different varifocal conoscopes for AR/VR testing can successfully accommodate up to ±4D (VIDs of ±250mm) for acceptance angles of ±40°. The ability of these lenses to focus through this range allows the XR device to be tested for its performance for users who are near-sighted or far-sighted. However, most test systems only require a focus range of infinity to 4D.
Although traditional conoscopes have been used successfully for XR display inspection, there are a few drawbacks, especially as the acceptance angles increase to accommodate wide field of view displays.
For this application, the conoscope is designed with a physical stop at the front pupil plane to prevent pupil wander issues common in designs that use an internal stop. A conoscope with a stop in front provides a circular pupil at the eye relief of the XR display, simulating the pupil of the human eye when the device is worn. The problem with this is that the front pupil becomes elliptical at wider field angles (see Figure 2A).
Figure 2A: Pupil Foreshortening of Traditional Conoscope
In addition to the obvious illumination falloff, this foreshortening also reduces the diffraction-limited MTF (Modulation Transfer Function) with increasing angle, as shown in Figure 2B.
Figure 2B: Diffraction Limited MTF vs. Field
Another challenge with traditional conoscope designs is that placing the pupil plane at the front forces the system to be optically asymmetric about the stop. This asymmetry introduces aberrations that scale with odd powers of field angle, particularly lateral chromatic aberration, coma, and distortion. Correcting these aberrations often requires a complex relay group with multiple lens elements.
Wide field of view requirements also drive up the size of the glass elements, which adds weight, increases the overall assembly size, and raises costs. And let’s be honest, nobody’s asking for a heavier, more expensive lens unless they absolutely need it.
Modified Conoscopes for XR Display Testing
So what happens when we add an optical element in front of the stop to increase symmetry? It turns out this simple modification addresses nearly all of the issues outlined above.
Figure 3: A Comparison of Traditional and Modified Conoscope Front Ends
This deceptively simple approach significantly decreases the angle at which the rays go through the stop (see Figure 3), resulting in a more circular pupil at wide angles. Because the pupil shape is less elliptical, the MTF diffraction limit increases to near the on-axis diffraction limit. This provides near uniform MTF across the entire FOV. To understand Figure 4, it helps to remember that the MTF of the human eye is limited to 30 cycles/degree. This modified conoscope is much better than the human eye. The illumination falloff is also improved because the pupil shape remains nearly circular at wide angles (see Figures 5).
Figure 4: Diffraction Limited MTF vs Field for Modified Conoscope
Figure 5: Relative Illumination Comparison
You may ask: if there is a lens in front of the stop, how can the entrance pupil be placed at the proper eye relief of the AR/VR device?
The answer is that the optical component in front of the stop is designed much like the cornea in the human eye. Its design keeps the distance from the front of the lens to the entrance pupil to be less than the minimum eye relief of the XR display (see Figure 6).
Figure 6: AR/VR Testing with Modified Conoscope
In summary, adding an optical element before the stop provides two benefits. First, by improving symmetry, the modified conoscope naturally decreases image defects, enabling a simpler optical design. In cost-sensitive applications, this can be taken a step further: image quality at the first image plane is often sufficient, which eliminates the need for a separate relay group.
Second, adding an optical element before the stop also reduces the required element diameters and internal image plane size. This keeps both the lens dimensions compact while producing an image compatible with standard industrial image sensors.
Together, these changes mean fewer elements, reduced diameters, and simplified mechanics, which can reduce the overall system cost by 40–50% compared to a traditional wide-FOV conoscope designed for AR/VR display testing.
To understand what that range looks like in practice, see our conoscope lens cost guide.
Need a modified conoscope or not sure where to start?
Connect with one of our optical engineers, we’re happy to help you evaluate your test setup or talk through design options.
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