5. Experimental demonstration of 2π-PDM

We have demonstrated 2π-PDM experimentally to show color 3D imaging ability [38]. Figure 10 shows a completed model of the optical system illustrated in Figure 4(a). Four wavelength-multiplexed phase-shifted holograms were recorded sequentially by using a mirror with a piezo actuator. Before/after that, two intensity images of two reference waves were sequentially recorded only once. The wavelengths of the lasers were λ<sup>1</sup> = 640 and λ<sup>2</sup> = 473 nm. A monochromatic CMOS image sensor was used to record the holograms and reference intensities. The sensor has 12-bit resolution, 2592 × 1944 pixels, and the pixel pitch of 2.2 μm. The mirror with a piezo actuator moved Z = 0, 237, and ±320 nm sequentially to generate phase shifts that were required for 2π-PDM. Phase shifts (α1,α2) at (λ1,λ2) were (0,0), (2π(λ2/λ1), 2π), (2π, 2π(λ1/λ2)), and (−2π,−2π(λ1/λ2)). To investigate the phase shifts at their respective wavelengths, interference fringe patterns at the wavelengths were observed before the experimental demonstration, and details were explained in Ref. [38]. Two transparency sheets were set as a color 3D object. The logo of the International Year of Light (IYL) and the characters "2015" were printed on the sheets, and blue and red color films were attached to the logo and characters, respectively. The red "2015" sheet and blue logo sheet were set on the depths of 250 and 320 mm from the image sensor plane, respectively. Opaque sheets were also attached on blue and red color sheets to scatter the object illumination light. Therefore, the 3D color object had a rough surface and scattered object waves illuminated the image sensor. The object wave at the wavelength λ = 473 nm was extracted from three holograms and the object wave at λ = 640 nm was obtained by the procedures of Eqs. (7)–(15). For comparison, a colored object image was also reconstructed from a wavelength-multiplexed hologram.

Figure 10. Photograph of the constructed dual-wavelength optical system of 2π-PDM.

Figure 11 shows the experimental results. Wavelength-multiplexed monochromatic images such as Figure 11(a) were captured, and wavelength information was superimposed on space and spatial frequency domains as seen in Figure 11(a) and (b). Figure 11(c) and (d) were the images focused digitally at a distance of 320 mm from the image sensor plane and reconstructed by diffraction integral alone and 2π-PDM, respectively. Blue and red color films attached to the sheets absorbed red and blue light, respectively. However, Figure 11(c), which was obtained from a wavelengthmultiplexed hologram, indicated the superimpositions of not only the 0th-order diffraction wave and the conjugate image but also image components given by the crosstalk between Iλ1(x,y:α1) and Iλ2(x,y:α2). As a result, color information was not retrieved adequately. In contrast, Figure 11(d) showed the removal of the unwanted images, the crosstalk components, and the successful experimental demonstration of clear color imaging by 2π-PDM. Figure 11(e) and (f) were the object images focused on 250 and 320 mm depths from the sensor plane, which were obtained by an image-reconstruction procedure of 2π-PDM. Thus, we validated 2π-PDM in the imaging of wavelength dependency of absorption for a 3D object and high-quality color 3D imaging ability.

5. Experimental demonstration of 2π-PDM

216 Holographic Materials and Optical Systems

image was also reconstructed from a wavelength-multiplexed hologram.

Figure 10. Photograph of the constructed dual-wavelength optical system of 2π-PDM.

We have demonstrated 2π-PDM experimentally to show color 3D imaging ability [38]. Figure 10 shows a completed model of the optical system illustrated in Figure 4(a). Four wavelength-multiplexed phase-shifted holograms were recorded sequentially by using a mirror with a piezo actuator. Before/after that, two intensity images of two reference waves were sequentially recorded only once. The wavelengths of the lasers were λ<sup>1</sup> = 640 and λ<sup>2</sup> = 473 nm. A monochromatic CMOS image sensor was used to record the holograms and reference intensities. The sensor has 12-bit resolution, 2592 × 1944 pixels, and the pixel pitch of 2.2 μm. The mirror with a piezo actuator moved Z = 0, 237, and ±320 nm sequentially to generate phase shifts that were required for 2π-PDM. Phase shifts (α1,α2) at (λ1,λ2) were (0,0), (2π(λ2/λ1), 2π), (2π, 2π(λ1/λ2)), and (−2π,−2π(λ1/λ2)). To investigate the phase shifts at their respective wavelengths, interference fringe patterns at the wavelengths were observed before the experimental demonstration, and details were explained in Ref. [38]. Two transparency sheets were set as a color 3D object. The logo of the International Year of Light (IYL) and the characters "2015" were printed on the sheets, and blue and red color films were attached to the logo and characters, respectively. The red "2015" sheet and blue logo sheet were set on the depths of 250 and 320 mm from the image sensor plane, respectively. Opaque sheets were also attached on blue and red color sheets to scatter the object illumination light. Therefore, the 3D color object had a rough surface and scattered object waves illuminated the image sensor. The object wave at the wavelength λ = 473 nm was extracted from three holograms and the object wave at λ = 640 nm was obtained by the procedures of Eqs. (7)–(15). For comparison, a colored object

Figure 11. Experimental results of 2π-PDM. (a) One of the recorded holograms and (b) its 2D Fourier transformed image. (c) Image reconstructed from the hologram of (a). (d) Whole image reconstructed by 2π-PDM. (c) and (d) are the images digitally focused on 250 mm depth from the image sensor plane. Object images numerically focused on (e) 250 mm and (f) 320 mm depths, which were reconstructed by 2π-PDM.
