Introduction
Film-based media for the storage and projection of holographic images have existed for many years and have recently demonstrated very high quality color volumetric projections [1, 2]. There also exist several technologies that can project time-dependent non-holographic 3-D images [3] or give the illusion of 3-D through stereoscopic effects [4-6]. True holographic projection that is dynamic and digitally driven could form the basis of future scientific and commercial visualization systems. To accomplish this, a high-resolution computer-controlled real-time phase modulator is required. The Texas Instrumentsrsquo; Digital Light Processing (DLP) micro-mirror chip was developed as the core amplitude modulation element for computer projectors and movie projection [7]. We have now conceived of and constructed a device wherein the DLP is used to control the phase rather than the amplitude of the light directed upon it. This enables a new class of possibilities, from mega-channel phase controlled communications switches to 3-D color holographic television. We describe the prototype monochrome holographic display, the optical and computational process on which it is based, and show sample 3-D images obtained on the instrument.
2. Digital micro-mirror device
Spatial light modulators to alter the intensity and phase of light which have been studied include transparent films [1, 2, 8], liquid crystal displays (LCD) [2, 9] and digital micromirror devices (DMD) [10]. Our DLP systems are constructed using the Texas Instrumentsrsquo; 1024 x 768 (XGA) micro-mirror chip and associated electronics which are driven by a standard computer video driver card (Inside Technology LCD555/PCI Video Card). The high reflectance aluminum micro-mirrors are 16 x 16 microns with a one-micron gap, which gives a 17-micron row or column pitch. The Texas Instruments DLP DMD has current applications in TV, video and movie projectors. Most white light applications produce excellent quality 2-D images via the direct reflection or the zero diffraction order of light from the individual canting DMD mirrors in order to construct an image [7]. Coherent light applications ranging from interferometric deformation and depth measurement [10, 11] to holographic storage [12] have been demonstrated. Volumetric images with incoherent light have also been produced with the DMD by projecting a series of 2-D constructs to give a 3-D appearance [3, 13]. We have constructed a system that projects true dynamic 3-D holographic images from computer-generated holograms utilizing the lowest orders of diffracted light from a laser illuminated DMD. The canting mirror structure of the DMD does not preclude its use as a reflective holographic medium for phase and amplitude control. Its highly reflective properties provide a much higher throughput than a transmissive LCD [14]. One disadvantage of the DMD as a phase modulator is the multiple Fraunhofer diffraction orders that occur with coherent light due to the DMD grating pattern. However, it has been measured that over 88% of the diffraction energy can be coupled into a single Fraunhofer diffraction order [15]. By using the reconstructed image of the brightest diffraction order, we
estimate our upper limit in intensity can, likewise, be 88% of that delivered to the DMD. We have demonstrated the utility of the DMD as a 3-D image holographic medium by producing virtual and real 3-D images at finite distances, an essential condition for reconstruction with image depth. Our aim is to create a real-time, multi-color projection system for all digital holograms. This includes computer-generated holograms that have been studied for some time [16, 17]. Thus, our proof-of-principle demonstration is based on transcribing an existing digital hologram to the DMD and viewing the results.
3. Optical system
The optical system is composed of a 15 mW HeNe Laser, spatial filter, 10 cm focal length collimating lens, DMD, 40 cm focal length converging lens and an image reconstructor for real image viewing [see Fig. 1(a)]. The real image reconstructor may be a frosted glass plate, fiber optic magnifier, or CCD/digital camera for visualization of a planar cross-section of a 3-D image; or it may be a translucent block such as a thick Agarose gel to create a suspension of micro-scatter bodies to simultaneously view the whole 3-D real image. The “original image” depicted in the upper left in Fig. 1(a) is a bitmap of a 2-D irregular perspective object. Its computed interferogram is represented on the computer monitor [18]. The picture at the “image reconstructor” is a CCD camera photo of the actual image reconstructed on a frosted glass plate. This is an illustration of the DMDrsquo;s capability to reconstruct 2-D irregular perspective objects as well as full 3-D holographic scenes. The 3-D holographic virtual image can be observed by looking directly into the DMD [see Fig. 1(b)]. The convergent lens and image reconstructor are removed from the optical system and the laser intensity is substantially reduced (by 80%) with neutral density filters for viewing directly by eye. The DMD functions as a reflective holographic medium in either projection mode.
System refinements
To be of true utility, a system must be high-resolution, real-time,dynamic, have multiple colors, and attain this with simple hardware and reasonable computational resources. One of the first stated needs for such 3-D displays is in aircraft cockpits [20]. As an initial application of our DMD approach, we are constructing a prototype virtual 3-D heads-up display in which the display scenes are pre-computed interferograms to be superimposed to form the final hologram. We are developing algorithms to move elements of a scene without recomputing the entire volume, thus reducing computing requirements. Already, we can compute and redisplay a random 3-D position in the scene volume every few seconds. We are also
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用于存储和投影全息图像的基于胶片的介质已经存在多年，并且最近已经证明了非常高质量的颜色体积投影[1,2]。还有一些技术可以投射出与时间相关的非全息三维图像[3]或通过立体效应给出三维幻觉[4-6]。真实的全息投影是动态和数字驱动的，可以构成未来科学和商业可视化系统的基础。为此，需要一种高分辨率的计算机控制的实时相位调制器。德州仪器的数字光处理（DLP）微镜芯片被开发为计算机投影仪和电影投影的核心调幅元件[7]。我们现在已经构思并构造了一种装置，其中DLP用于控制相位而不是指向其上的光的幅度。从大型通道相控通信交换机到3D彩色全息电视，这实现了一种新的可能性。我们描述了原型单色全息显示器，它所基于的光学和计算过程，并显示了在仪器上获得的样品3-D图像。
2.数字微镜装置
用于改变已经研究的光的强度和相位的空间光调制器包括透明膜[1,2,8]，液晶显示器（LCD）[2,9]和数字微镜器件（DMD）[10]。我们的DLP系统采用德州仪器（TI）的1024 x 768（XGA）微镜芯片和相关电子设备构建，这些电子设备由标准计算机视频驱动卡（Inside Technology LCD555 / PCI视频卡）驱动。高反射铝微镜为16 x 16微米，间隙为1微米，可提供17微米的行间距或柱间距。德州仪器DLP DMD目前在电视，视频和电影放映机中有应用。大多数白光应用通过来自各个倾斜DMD镜的光的直接反射或零衍射顺序产生优质的2-D图像，以构建图像[7]。从干涉变形和深度测量[10,11]到全息存储[12]的相干光应用已被证明。使用DMD通过投射一系列2-D构造产生具有非相干光的体积图像以产生3-D外观[3,13]。我们已经构建了一个系统，利用来自激光照射DMD的最低阶衍射光，从计算机生成的全息图中投射真实的动态三维全息图像。 DMD的倾斜镜结构不排除其用作用于相位和幅度控制的反射全息介质。其高反射特性提供了比透射式LCD更高的吞吐量[14]。 DMD作为相位调制器的一个缺点是由于DMD光栅图案而在相干光下发生的多个夫琅和费衍射级。然而，已经测量到超过88％的衍射能量可以耦合到单个夫琅和费衍射级[15]。通过使用最亮衍射级的重建图像，我们
估计我们的强度上限同样可以达到DMD的88％。我们通过在有限距离处产生虚拟和真实三维图像，证明了DMD作为三维图像全息介质的实用性，这是用图像深度重建的必要条件。我们的目标是为所有数字全息图创建一个实时的多色投影系统。这包括已经研究了一段时间的计算机生成的全息图[16,17]。因此，我们的原理验证演示基于将现有的数字全息图转录到DMD并查看结果。
3.光学系统
该光学系统由15 mW HeNe激光器，空间滤波器，10 cm焦距准直透镜，DMD，40 cm焦距会聚透镜和用于实际图像观察的图像重建器组成[见图1（a）]。真实图像重建器可以是磨砂玻璃板，光纤放大镜或CCD /数码相机，用于可视化3-D图像的平面横截面;或者它可以是半透明的块，例如厚的琼脂糖凝胶，以产生微散射体的悬浮液，以同时观察整个3-D真实图像。图1（a）左上方所示的“原始图像”是2-D不规则透视对象的位图。其计算的干涉图在计算机监视器上表示[18]。 “图像重建器”处的图片是在毛玻璃板上重建的实际图像的CCD相机照片。这说明了DMD重建二维不规则透视物体以及全三维全息场景的能力。通过直接观察DMD可以观察到3-D全息虚像[见图1（b）]。会聚透镜和图像重建器从中移除光学系统和激光强度大幅降低（80％），中性密度滤光片可直接用眼睛观察。 DMD在任一投影模式下都用作反射全息介质。
系统改进
为了真正实用，系统必须是高分辨率，实时，动态，具有多种颜色，并且通过简单的硬件和合理的计算资源实现这一点。之一
这种3-D显示器的首要需求是在飞机驾驶舱中[20]。作为我们的DMD方法的初始应用，我们正在构建原型虚拟3-D抬头显示器，其中显示场景是预先计算的干涉图，以叠加以形成最终的全息图。我们正在开发算法来移动场景的元素而无需重新计算整个卷，从而降低了计算要求。我们已经可以每隔几秒钟在场景中计算并重新显示随机三维位置。我们还在探索算法和滤波器，以尽量减少不需要的衍射效应并提高图像质量。全息图像的分辨率直接取决于DMD规格，随着镜子数量的增加和/或它们各自的尺寸减小而变得更精细。全息图中的每个点对3-D对象上的每个可观察点进行采样，因此，全息图的一部分的丢失不等于重建图像的一部分的丢失。
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