HoloChrome: Polychromatic Illumination for Speckle Reduction in Holographic Near-Eye Displays

Abstract

Holographic displays hold the promise of providing authentic depth cues, resulting in enhanced immersive visual experiences for near-eye applications. However, current holographic displays are hindered by speckle noise, which limits accurate reproduction of color and texture in displayed images. We present HoloChrome, a polychromatic holographic display framework designed to mitigate these limitations. HoloChrome utilizes an ultrafast, wavelength-adjustable laser and a dual-Spatial Light Modulator (SLM) architecture, enabling the multiplexing of a large set of discrete wavelengths across the visible spectrum. By leveraging spatial separation in our dual-SLM setup, we independently manipulate speckle patterns across multiple wavelengths. This novel approach effectively reduces speckle noise through incoherent averaging achieved by wavelength multiplexing. Our method is complementary to existing speckle reduction techniques, offering a new pathway to address this challenge. Furthermore, the use of polychromatic illumination broadens the achievable color gamut compared to traditional three-color primary holographic displays. Our simulations and tabletop experiments validate that HoloChrome significantly reduces speckle noise and expands the color gamut. These advancements enhance the performance of holographic near-eye displays, moving us closer to practical, immersive next-generation visual experiences.

Figures (29)

• Figure 1: Hyperspectral propagation model and perceptual optimization framework. This figure illustrates the three key components of the proposed HoloChrome framework: A hyperspectral propagation model (left) is used to generate polychromatic image data cubes that are converted to 3-channel RGB images using perceptual eye response curves, and compared to targets using a perceptual color loss (right). The hyperspectral propagation model begins with a polychromatic source spectrum that is used to sample a hyperspectral source aberration model with N-discrete wavelengths, generating a polychromatic field with wavelength-dependent amplitude and phase. These are processed through two spatial light modulators (SLM$_1$ and SLM$_2$) with hyperspectral lookup tables (LUT) that are sampled to create a complex polychromatic aperture to represent frequency domain aberrations. The polychromatic output field is then measured on a detector after applying wavefront propagation to simulate focal stack capture with a translation stage. The perceptual response incorporates spectral weighting based on the physical eye response and performs a differentiable color transformation (e.g., XYZ to sRGB) before MSE loss is computed.
• Figure 2: Comparison of HoloChrome with conventional holography methods (simulation). HoloChrome shows significant performance gains in speckle reduction compared to conventional methods. The figure compares the results of HoloChrome against three common holographic methods: Simultaneous Color, Time-Sequential Color, and Multiplexed Color. The PSNR values indicate that HoloChrome, both in single-frame and three-frame configurations, achieves higher image quality, with the three-frame HoloChrome providing over a 6dB improvement compared to conventional time-sequential color. Despeckled results are evident in the insets. Illumination spectra are shown in the top right of each column. Note: while simultaneous color markley2023simultaneous, time-sequential color lee2020widecurtis2021dcghchoi2022time, and multiplexed color aksit2023holohdr techniques have been demonstrated with single SLM architectures, here we use a dual-SLM architecture ($N_s=2$) for fair comparison against HoloChrome.
• Figure 3: Single frame ($N_t=1$) HoloChrome reduces speckle noise (simulation). Comparison of image quality with varying wavelengths for a single SLM frame. The first column shows the target focal stack. The second column shows a simultaneous color markley2023simultaneous result with 3 wavelengths optimized in a single frame, producing a PSNR of 21.7dB. The third column shows a HoloChrome result with 8 wavelengths, resulting in a PSNR of 26.0dB. The results demonstrate that HoloChrome effectively reduces speckle noise in single frame, with more wavelengths yielding better image quality and more accurate color. Note: while simultaneous color markley2023simultaneous, was demonstrated with a single SLM architecture, here we use a dual-SLM architecture ($N_s=2$) for fair comparison against HoloChrome.
• Figure 4: Comparison of 1 vs 2 SLMS (simulation, both phase-only). Introducing a dual-SLM configuration effectively mitigates the wavelength-dependent memory effect. The figure compares single-SLM and dual-SLM setups in both single-frame and three-frame configurations. The dual-SLM approach shows a clear reduction in speckle noise, with PSNR values indicating significant performance gains. In the three-frame configuration, the dual-SLM setup achieves near-perfect image reconstruction.
• Figure 5: Multisource vs HoloChrome (simulation). The figure compares the performance of Color Sequential, HoloChrome, Multisource, and a combination of HoloChrome and Multisource methods for speckle reduction, all using 3 SLM frames to form the full image to ensure a fair comparison. We also report the mean PSNR for each experiment over the full Focal Stack. The images demonstrate that both HoloChrome and Multisource methods work reasonably well in reducing speckle. However, they achieve speckle reduction through independent methods. The combination of both methods further improves the PSNR, indicating a synergistic effect.