使用互補式雙相位時間和空間多工降低 AR-HUD之光斑

Abstract

在探討基於LCoS（液晶矽）技術及全像多色投影技術實現的雙焦平面車載抬頭顯示器（Dual-focal-plane Multi-color Head Up Display with Holographic Imaging）的研究過程中，我們發現利用雷射作為光源會引起嚴重的光斑（speckle）現象，這一現象顯著降低了成像質量。光斑的存在不利於駕駛者迅速清晰地識別路況或查看儀表板資訊，進而對整體的駕駛體驗產生負面影響，。為了評估全像投影成像中光斑問題的程度，本篇碩士論文采用了光斑對比度（Speckle Contrast, SC）作為衡量標準，通過對光斑對比度數值的測量，我們能夠判斷全像影像的質量優劣。 在我們的系統中，全像圖像是以MGSA(modified Gerchberg–Saxton algorithm)生成的。生成的全相片為隨機相位，而我們進一步的修改了此演算法，使生成的全相片具有似棋盤式的互補雙相位(Complementary Dual-Phase)，並結合上一個實驗所使用的時間多工和空間多工方法使我們的光斑對比度更有效降低，會在本文中做詳細的說明。 在本研究的光路系統設計中，我們採用紅綠藍雷射裝置及單面板LCoS來投影全像圖像至擴散器上。通過將LCoS與電腦連接，我們能夠控制雷射光與全像圖像的切換頻率，以實現同步。在光路中間位置設置了一個截波器（chopper），這樣做是為了在相對應的波長雷射光下，同時播放特定波長的全像圖像，從而達到時間多工的效果。為了捕捉這些投影圖像，我們使用了一台具有特定光圈大小的固定焦點相機。此外，我們設計了全像片，使得生成的全像影像被分割為左右兩側，分別對應紅色和綠色，經過LCoS投影後，一側展示綠色全像圖像，而另一側則展示紅色全像圖像。在光路的中心位置放置了一個分光鏡，用於重疊兩種不同顏色的全像圖像，實現空間多工效果。而最終達成以“互補式雙相位時間和空間多工”方法有效降低光斑對比度，並實現全彩效果。 經實驗量測後，我們最終可以得知，單色全像片的光斑對比度分別為：藍色為28.53%、綠色為22.80%、紅色為15.29%。先前實驗[1]中一般隨機相位的時間和空間多工光斑對比度為4.55%，而本實驗中所使用的“互補式雙相位時間和空間多工”光斑對比度為3.39%，能更有效地減少光斑現象。

In the exploration of Dual-focal-plane Multi-color Head-Up Displays with Holographic Imaging based on LCoS (Liquid Crystal on Silicon) and holographic multi-color projection technologies, we discovered that using lasers as the light source induces a significant speckle phenomenon, which considerably degrades image quality. The presence of speckles adversely affects the driver's ability to quickly and clearly discern road conditions or dashboard information, thereby negatively impacting the overall driving experience. To assess the degree of the speckle problem in holographic projection imaging, this thesis adopts Speckle Contrast (SC) as a metric for evaluation, allowing us to determine the quality of holographic images by measuring the speckle contrast values. In our system, holographic images are generated using the Modified Gerchberg–Saxton Algorithm (MGSA). Initially, the holograms produced had random phases. We have further refined this algorithm to create holograms with a checkerboard-like complementary dual-phase (Complementary Dual-Phase) pattern. This adaptation, combined with the time multiplexing and spatial multiplexing methods utilized in a prior experiment , has proven to reduce speckle contrast more effectively. This paper will provide a detailed explanation of this process. In the optical system design of our study, we employed RGB laser devices and a single-panel LCoS to project holographic images onto a diffuser. By connecting the LCoS to a computer, we could control the switching frequency of the laser light and the holographic images to achieve synchronization. A chopper was placed in the middle of the optical path to simultaneously play holographic images of specific wavelengths with the corresponding wavelength laser light, achieving time multiplexing. A fixed-focus camera with a specific aperture size was used to capture these projected images. Additionally, we designed the holograms so that the generated holographic images were split into left and right sides, corresponding to red and green colors. After projection through the LCoS, one side displayed the green holographic image, and the other side displayed the red holographic image. A dichroic mirror was positioned in the center of the optical path to overlap the two different colored holographic images, achieving spatial multiplexing. Ultimately, the "Complementary Dual-Phase Time and Spatial Multiplexing" method was successfully employed to significantly reduce speckle contrast and achieve a full-color effect. Experimental measurements revealed that the speckle contrast for monochromatic holograms was 28.53% for blue, 22.80% for green, and 15.29% for red. Compared to the speckle contrast of 4.55% achieved using the conventional method of random-phase time and spatial multiplexing in a previous experiment , the "Complementary Dual-Phase Time and Spatial Multiplexing" approach used in this experiment resulted in a speckle contrast of 3.39%, demonstrating a more effective reduction in speckle phenomena.