# AR Displays: Next-Generation Technologies to Solve the Vergence-Accommodation Conflict
## 5 Multifocal Approach by LightSpace Technologies
Whereas, on the other hand we are limited by a maximum image refresh rate supported by the available SLM technology, thus limiting the number of available focal planes, raising concerns on the maximum allowable interplane spacing. Previous research suggests, that effects of discomfort in association tovergence-accommodation conflict vary based on the accommodation distance [3; 33]. Generally, it is assumed that \(\pm\)0.3 D mismatch between accommodation and vergence distances defines the comfort region; nevertheless, this value can be substantially larger at closer focusing distances reaching even up to \(\pm\)0.6 D [3]. From a different perspective, research by [PERSON] suggests, that accommodation response for depth-filtered stimuli with various spatial frequencies depends on the interplane spacing [15; 17]. Thus, in the light of currently and in the near future available spatial light modulators, an interplane spacing of 0.6-0.8 D appears to result in substantially linear accommodation response, even for objects with smallest physically representable details. Thus, a logical possible interplane spacing is 0.6 D, which even if depth blending is disregarded should not cause an onset of visual distress. In Figures 4 and 5 we provide an example showing what depth range can be covered with four and six focal planes, respectively. For simplicity, in these examples we consider the comfort region to be fixed at \(\pm\)0.3 D irrespective of the accommodative distance. The plots show a modulus of vergence-accommodation mismatch for fixed focal planes, the interplane spacing of which is around 0.6 D. Thus, even without interplane antialiasing-driven accommodative response, the vergence-accommodation mismatch does not exceed the comfort region. The example of Figure 4 demonstrates that a depth from 40 cm to infinity can be adequately represented with only four focal planes. It must be noted, that the vergence-accommodation mismatch at the actual infinity slightly exceeds the said comfort region--nevertheless, in practice it would not pose a problem. Moreover, the effective depth range can be even slightly extended, if larger interplane separations are allowed at close distances. Overall, this example shows that with four depth planes a 2.5 D accommodation range can be easily supported, making such a multifocal-display system well-suited for near-work heavy tasks. The example of Figure 5 adds two more focal planes, allowing  
Figure 3: Expansion of virtual image focal planes, when a projection volume is viewed through a magnifying eyepiece. even further expansion of the effective working-depth range to around 3.4 D. It must be noted that with a readily-available mass-produced digital micromirror device (DMD) spatial light modulators, in case of four depth planes, the volumetric full-color image refresh rate of 90 Hz can easily be attained. To verify the concept in practice, we developed a proof-of-concept virtual reality demonstrator from readily-available parts and custom-made stacks of optical diffuser elements. We chose Texas Instruments DLP\({}^{\text{\tiny{$\textregistered$}}}\)-based pico-projectors for the image source, as currently micromirror spatial light modulators are leading in respect of highest supported binary pattern rates. The assembled unit (Figure 6) had six image depth planes (-0.58 D spacing), 72\({}^{\circ}\) horizontal field-of-view, and around 6 cpd image resolution. The limited available control over pico-projectors yielded only 60 Hz volumetric refresh rate; nevertheless, in testing it did not cause a noticeable flicker. The ability to comfortably change accommodation was fully verified using both eyes, as well as monocularly, confirming presence of realistic monocular focus cues. An example of this is shown in Figure 7, where a view through ocular was captured by a camera with wide-aperture (F 2.8) lens. As can be noticed from magnified insets, the focus can be changed within the scene.