Lastly, we introduce a criterion to evaluate the depth of field where high-resolution images can be reconstructed with a certain level of speckle reduction. For the experimental validation, we compare various light sources with different angle and wavelength diversity. Secondly, we verify the theoretical model used for light source optimization by simulation and experiment. We also demonstrate a prototype of partially coherent holographic near-eye displays using an optimal light source. The optimization finds a light source that reduces the speckle contrast while minimizing the sacrifice of resolution and depth of field. Using the theoretical models, we optimize the light source characteristics using a gradient decent method. Here, we introduce theoretical models that depict how the light source’s characteristics affect speckle contrast and spatial resolution. Despite numerous researches on speckle reduction of coherent illumination, there has not been a thorough and quantitative discussion about the trade-off between speckle and spatial resolution in holographic near-eye displays. Speckle patterns are updated by refreshing computer-generated holograms (CGHs) 23, 35, 36 or exploiting mechanical movement of optical elements (e.g. The temporal superposition is to update speckle patterns at a fast frame rate. The partially coherent light source includes a bundle of mutually incoherent light sources 26, 27, an optical fiber 28, 29, and spectrally-broadband light source (e.g. The optical superposition corresponds to applying a partially coherent light source for speckle reduction. Speckle reduction methods generally exploit the optical or temporal superposition of independent speckle patterns. Although various approaches 20, 23, 24, 25 have been introduced for speckle reduction, most speckle reduction methods accompany the sacrifice of spatial resolution or frame rate. Unfortunately, speckle is inevitably generated at the observer’s retina in holographic near-eye displays. As the speckle intensity is randomly determined, speckle might distort the original signal and reach a hazardous intensity level for the human visual system. Speckle can be interpreted as a randomly generated pattern via constructive or destructive interference. The major drawback is coherent interference (i.e., speckle), which degrades the signal-to-noise ratio and could be hazardous for the human visual system 20, 22. Despite the remarkable advantages of holographic near-eye displays, it is not negligible that there is a drawback of utilizing the coherent light source for the wavefront modulation. In the near-eye display field, a lightweight glasses-like optical design was introduced 3, 4, 21, which supports a large field of view, high resolution, and accurate focus cues. Holographic displays are versatile, which have been studied from several perspectives: display applications including near-eye displays 2, 3, 4, tabletop displays 15, 16, and projection-type displays 17 spatial-bandwidth product improvement using a scattering 18 or non-periodic 19 medium and speckle analysis of holographic displays according to light sources 20. The combination of the coherent illumination and the SLM enables the displays to reconstruct volumetric objects, and corrects the optical aberration accompanied by optical elements 3. Holographic displays modulate a coherent illumination’s complex amplitude to reconstruct arbitrary wavefront using a spatial light modulator (SLM). The intrinsic optical principle of holographic displays, wavefront modulation using a coherent light source, carries several advantages for near-eye displays compared to other approaches including multi-plane displays 5, 6, 7, 8, light field displays 9, 10, volumetric displays 11, 12, 13, and gaze-contingent displays 14. Recently, various researches related to holographic near-eye displays 2, 3, 4 have been introduced to show a new possibility of realizing an ultimate near-eye display. Besides, they prefer a more comfortable display device that is lightweight, wearable, and free from vergence-accommodation conflict. Customers expect a more immersive display device with a larger field of view, higher resolution, and faster frame rate. However, it is challenging to realize a near-eye display system that satisfies all commercial demands. With increasing commercial and economic demands in augmented and virtual reality 1, it has become an important topic to develop a daily usable near-eye display system. Near-eye display technologies have been widely advertised and recognized as a next-generation display platform that delivers an unprecedented immersive experience.
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