consider yellow and green laser are travelling through a medium with the velocity 2.96 × 108 m/s and 2.98 × 108 m/s respectively. the change in frquency for both light is same. find that which laser light has more change in wave length?
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Coherence properties of different light sources and how they affect the image quality of holographic display are investigated. 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.
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. 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. 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.
Holographic displays can reconstruct three-dimensional (3D) images with full wavefront information1–6, which is free from issues such as lack of accommodation depth cue, discontinuous motion parallax and crosstalk7–11. Light source plays a critical role in holographic displays, and the conventional requirement is a high degree of coherence for achieving sharp reconstructed images. Lasers are normally used in holographic displays because they have high spatial and temporal coherence. However, the high degree of coherence also brings in significant speckle12–14 in the reconstructed images, which affects the image quality greatly.
Several techniques have been reported15–19 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, all these techniques either increase the complexity of the system or increase the computation costs and decrease the bandwidth of the reconstructed images.
In the past, holographic displays based on partially coherent light sources, such as the light emitting diodes (LEDs), has been reported20–22. 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. As a result, the spatial filter will decrease the energy efficiency significantly. 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. However, how temporal and spatial coherence affect the quality of the holographic reconstructed images especially quantitatively remains to be investigated.
In this work, we study the properties of spatial coherence and temporal coherence of different light sources and the resultant qualities of holographic reconstructed images. Two ways to increase spatial coherence for a partially coherent source have been introduced and the experimental results are in good agreement with the theoretical values. Subsequently we analyze quantitatively the influence of the two coherence properties on the sharpness of the reconstructed images and speckle. Finally, we conclude with some general requirements for optimized holographic image quality when using different coherence light sources, which can be applied to both digital and optical holographic displays including those viewed by eyes directly.
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