# AR Displays: Next-Generation Technologies to Solve the Vergence-Accommodation Conflict
## 5 Multifocal Approach by LightSpace Technologies
### Comparison to Alternative Multifocal Approaches  
In general, multifocal displays can be categorized by means, of how the focal planes are generated. In this regard screen-based or lens-based systems can be distinguished. In screen-based systems, there are either a reciprocating screen or multiple screens which optically are located at different distances. On the contrary, lens-based systems typically have a fixed image source--either a single screen or spatial light modulator--and the focal length is varied by active optical elements, thus these types of displays are also inclusive of systems utilizing alternative optical components for the focal-length modulation. A variant of screen-based system in the laboratory environment has been used by [PERSON] [14]. In this work a three-focal plane stereoscopic display prototype was constructed by segmenting a high-resolution LCD display into three portions. Angled beam splitters were used to align all screens on a single optical path. Nevertheless, from a practical point of view, this is an inefficient way of  
Figure 9: An actual appearance through the eyepiece of 3D content rendered: (**a**) On a single focal plane; (**b**) on all focal planes without interplane antialiasing; (**c**) on all focal planes with interplane antialiasing enabled. Photos taken with an APS-C sensor, using a 35 mm lens with aperture setting of F2.8. Prototype display operates six focal planes. synthesizing focal planes, as beam-splitters attenuate a large portion of light, which is decreased even further when considering inefficiency of optical image combiners required by the AR design. Alternatively, transparent displays have been anticipated to be a viable solution--in particular transparent OLED displays, which would allow dense and compact stacking avoiding utilization of beam-splitters [34]. Nevertheless, this has not resulted in a practical design, as ultra-high transparency was not ensured, and typically these display panels exhibited haze that is not practical for multi-layer stacking. The distinguishing characteristic of these two examples is that they do not require time multiplexing and all image focal planes could be shown simultaneously, thus improving image brightness. The solution by LightSpace also falls into the same display category with ultra-transparent low-haze optical diffuser elements (screens) overcoming stacking issue and light loss attributed to utilizing beam-splitters. In contrast this approach requires time multiplexing; nevertheless, switching characteristics of diffuser elements allow for obtaining image refresh rates that surpass the flicker perception threshold when a reasonable number of focal planes are used (3-16). Thus, the image source (spatial light modulator) becomes the limiting link in the reproduction of the image. In lens-based systems it is possible to distinguish optical elements that change their optical strength continuously and systems with discrete states. In case of optical elements with discrete states, which can include optical length modulators, birefringent lenses, and liquid-crystal lenses, the number of maximum image planes is limited. On one hand, to obtain multiple focal lengths these elements have to be stacked, which attenuates the overall light intensity, while on the other hand, on separate occasions it is limited by the switching time, which can be in the order of milliseconds. In contrast, continuously-tunable liquid lenses and electrostatically-deformable mirrors are optically more efficient and allow for an arbitrary number of focal planes. Nevertheless, often the settling time for a liquid focus-tunable lens is substantially high (5-10 ms), which is inhibitive of high image refresh rates. The alternative proposed by [PERSON] utilizes a liquid-tunable lens in conjunction with a real-time focus tracking system, meaning that the focal length is scanned continuously and there is no need to wait for the lens to settle [19].