Gradation representation method using a binary-weighted computer-generated hologram based on pulse-width modulation
The reconstructed 3D images using the gradation representation method for digital micromirror device (DMD). The gradation representation method can easily control the light intensities of the reconstructed 3D images with gradation without controlling the brightness of the reference light.
Holography can directly record and faithfully reconstruct a three-dimensional (3D) image. A computer-generated hologram (CGH) is a holographic interference pattern obtained by calculating the light propagated from a 3D object using a computer. Electroholography based on CGH has the potential to realize advanced 3D televisions.
To satisfy the customer requirements for vivid colors of images, a gradation representation of the reconstructed 3D images which enables the reconstructed 3D images to express in multi-colors is very useful for color electroholography. By employing the time-multiplexing technique based on multiple bit planes, the gradation of the reconstructed 3D images can be represented by controlling either the intensity of the reference light or the display time. As digital micromirror device (DMD) can be used as high-speed spatial light modulator, and achieve high speed playback of 3D movie, the study of the gradation method for the electroholography using DMD is very important to realize the ultimate 3D television.
A joint research team comprising Prof. Naoki Takada's group from Kochi University and Prof. Tomoyoshi Ito's group from Chiba University has proposed a simple gradation representation method using a binary-weighted CGH based on binary pulse-width modulation (PWM) in the electroholography for DMD. The proposed method can easily control the light intensity of the reconstructed 3D images with gradation without controlling the brightness of the reference light. The corresponding research results are reported in Chinese Optics Letters, Volume 15, No. 6, 2017 (M. Fujiwara, et al., Gradation representation method using a binary-weighted computer-generated hologram based on pulse-width modulation).
The proposed method uses multiple bit planes comprising binary-weighted CGHs based on binary PWM. In this scheme, the binary-weighted CGH is generated by changing the white in the conventional binary CGH, which is drawn in black and white, to gray. The light intensity of the reconstructed object point can be controlled by changing the gray level of a binary-weighted CGH. The object points of a 3D object are assigned to multiple bit planes according to their gray levels. The bit planes are sequentially displayed in a time-division-multiplexed manner. Consequently, the proposed method realizes a gradation representation of a reconstructed 3D object.
"The present work will be indispensable in color electroholography for realizing the ultimate 3D television", said Prof. Naoki Takada, a research member from Kochi University and a corresponding author of the related paper.
Further work will be focused on expressing the reconstructed 3D object in multiple colors using the proposed method, and applying the proposed method to real-time color electroholography.
基于二值脉冲宽度调制的灰度显示，让三维重建图像更逼真
图片说明: 利用灰度表示方法重建的三维图像。该方法可以在不调整参考光强度的情况下实现对于具有灰度特征的三维重建图像光强的轻松控制。
通过计算机处理从三维物体出射的光信号得到全息干涉图样，计算全息技术能够直接记录并忠实地重建三维图像。基于计算全息图的电子全息技术有望实现高端的3D电视。
当下，用户对影像质量的要求越来越严苛。为满足其对色彩生动的需求，发展灰度表示方法实现三维重建图像的多彩显示对于彩色电子全息技术是至关重要的。目前，通过采用基于多位平面的时分复用技术，三维重建图像的灰度可以通过控制参考光的强度或显示时间来实现。鉴于数字微镜器件可被用作高速空间光调制器，并可实现3D电影的高速回放，开展使用数字微镜器件来构建电子全息技术灰度方法的研究，对于实现高端3D电视非常重要。
来自日本高知大学的Naoki Takada教授与来自千叶大学的Tomoyoshi Ito教授组成的合作团队提出了一种使用二进制加权计算全息图的灰度表示方法，这一方法是通过二进制的脉宽调制技术控制图像显示在数字微镜器件上。该方法可以实现对于具有灰度特征的三维重建图像的光强的轻松控制，而不改变参考光的强度。相关研究成果发表在Chinese Optics Letters 2017年第6期上(Masato Fujiwara, et al., Gradation representation method using a binary-weighted computer-generated hologram based on pulse-width modulation）。
该方法采用包含二进制加权电子计算全息图（基于二进制脉宽调制）的多位平面。在这一方案中，二进制加权电子计算全息图是通过将传统使用黑白二色的二进制电子计算全息图中的白色调整成灰色来产生的。重建物点的光强可以通过改变二进制加权电子计算全息图的灰度来加以控制。根据三维物体上物点的灰度程度，可将其分配给多位平面。这些位平面以时分复用方式依次显示。藉此，便实现了三维重建物体的灰度表示。
Naoki Takada教授表示，这一成果对于利用彩色电子全息技术来实现高端3D电视是不可或缺的。下一步工作将集中于利用该方法以多色彩方式呈现3维重建物体，并将之应用到实时的有色电子全息技术中去。