Study On Color Holographic Image Based On LED

Holographic display becomes more practical with the development of computer technology and the digital components such as digital optical modulator (acousto-optic modulator, electrically or optically addressed liquid crystal devices, etc.). Color holographic display uses three primary colors to achieve true color images, which have bright color, high contrast and three-dimensional information, makes it a unique advantage in three-dimensional image display technology. In the past two decades, color hologram has been developed rapidly. But the speckle and the crosstalk in the reconstructed image seriously degrade the image quality. Due to the strong coherence of laser, light interference due to the multiply reflection makes the surface of the reconstructed image looked as dirty. Many researchers researched on speckle problem, and the weak coherent light was proposed in holographic image. Light Emitting Diode (LED) has lower coherence and high color saturation compared to laser. It has been proved to be suitable for holographic imaging as light source. LED light source can solve the problem of speckle and improve the quality of holographic image.The algorithm of phase-only color holography based on the digital spatial light modulator (Liquid Crystal On Silicon, LCOS) was studied in this article. Firstly, we studied the fractional Fourier transform between parallel planes, and analyzed the impact of sampling rate on image size. We proposed a modified convolution calculation process. Based on this, the holographic image with variable image size was achieved without the limitation by sampling rate. Secondly, we presented a method using spatial shifting iterative GS method to achieve color holographic image. In this method, hologram was calculated for each separate RGB components, and then scaled and shifted on spatial to get a synthetic color hologram. Thirdly, we divided the spatial light modulator into three parts. Each part was loaded with different color holograms. The proposed method can also control the size of the image to ensure the spatial coincidence of the reconstructed image according to the RGB components. In the article, the author also experimentally compared several methods to generate color holographic display, such as the spatial-overlay, time-division and space-division, and both the results of simulation and experiment were presented. It showed that the spatial-overlay method can easily overcome the image crosstalk problems without increasing the complexity of the experimental apparatus. A research on the spectrum of white LED was presented and methods to increase its coherence were proposed. Experiments of color holographic imaging based on LED were performed in which the color holographic reconstruction with lower speckle and better spatial coincidence was obtained.Finally, we proposed an approach to reduce space coherence of laser with optical devices. Comparisons between our method and other methods (LED and laser based method) in holographic image were presented. The results proved that LED had an advantage over laser on the speckle problem. When the coherence of laser was decreased, speckle had been significantly decreased, thus furtherly confirmed the feasibility of holographic imaging using weak coherent light.