![]() However, how temporal and spatial coherence affect the quality of the holographic reconstructed images especially quantitatively remains to be investigated. We could see that the coherence properties of the light source directly influence the quality of holographic reconstructed images on both image sharpness and speckle. As a result, the spatial filter will decrease the energy efficiency significantly. ![]() ![]() The low temporal coherence of LEDs could reduce the speckle, but the low spatial coherence of LEDs requires additional spatial filter such as pinholes or microscopic objectives to be used to select the emitted light from a local area of an LED in order to increase the spatial coherence and obtain reconstructed images with acceptable sharpness. In the past, holographic displays based on partially coherent light sources, such as the light emitting diodes (LEDs), has been reported 20, 21, 22. However, all these techniques either increase the complexity of the system or increase the computation costs and decrease the bandwidth of the reconstructed images. Several techniques have been reported 15, 16, 17, 18, 19 to tackle the speckle issue, such as time-averaged superposition of the same reconstructed image with uncorrelated initial random phases or different sub reconstructed images consisting of selected points of the same target image and applying phase grating or diffusers. However, the high degree of coherence also brings in significant speckle 12, 13, 14 in the reconstructed images, which affects the image quality greatly. Lasers are normally used in holographic displays because they have high spatial and temporal coherence. Light source plays a critical role in holographic displays, and the conventional requirement is a high degree of coherence for achieving sharp reconstructed images. Holographic displays can reconstruct three-dimensional (3D) images with full wavefront information 1, 2, 3, 4, 5, 6, which is free from issues such as lack of accommodation depth cue, discontinuous motion parallax and crosstalk 7, 8, 9, 10, 11. This will provide a quantitative way not only to optimize the image quality between uniformity and sharpness but also to determine the safety power level for different light sources when viewing the produced images by human eyes directly. Although the image sharpness and speckle are related to both coherence parameters, our results and subsequent analysis show that the spatial coherence can be linked directly to the image sharpness and the temporal coherence to the speckle. These two coherence properties are measured for various light sources of diode-pumped solid-state (DPSS) laser, laser diode (LD), light emitting diode (LED), super luminescent light emitting diode (sLED) and micro light emitting diode (mLED) in different settings, together with the quality of the holographic reconstructed images. Temporal coherence is related to the intrinsic spectrum bandwidth of the light source, while spatial coherence can be affected by the size of the light source and propagation distance in use. ![]() Coherence properties of different light sources and how they affect the image quality of holographic display are investigated.
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