COL Cover Story: Gradation representation method using a binary-weighted computer-generated hologram based on pulse-width modulation
At: 2017/6/9 9:10:00 by admin

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 color using the proposed method, and applying the proposed method to real-time color electroholography.

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