The volume phase holographic (VPH) transmission grating recorded in dichromate gelatin (DCG) with a specific spectral
coverage from 420 nm to 760 nm is designed for a novel prism-grating-prism imaging spectrometer. Based on the
Rigorous Coupled-Wave Analysis, its performances are predicted and analyzed. The grating is manufactured and its
properties are measured experimentally. The diffraction efficiency over the spectral range, the bandwidth, and the angular
selectivity of the grating is measured, analyzed and compared with that of the theoretical ones. The results show that by
adjusting and controlling the preparation conditions of DCG plates, the exposure time and the post-processing technique
of the grating, the VPH transmission grating with high diffraction efficiency approximate to the design requirement can
be obtained. The measured peak diffraction efficiency reaches nearly 85% at central wavelength of 590 nm while the
average diffraction efficiency is larger than 75% over the required spectral range from 420 nm to 760 nm.
KEYWORDS: Digital holography, 3D image reconstruction, Holograms, Wavelet transforms, Wavelets, Diffraction, Image quality, Digital filtering, Digital imaging, Linear filtering
The wavelet transform with coefficients selection method and the one of traditional frequency domain filtering
methods named Gaussian high-pass filter method are used for the reconstruction of the digital holograms in this paper.
In order to eliminate the speckle noise near the filtered zero-order diffraction spots more effectively, the high and low
frequency coefficients in the wavelet transform were adjusted. A holographic experimental system based on
Michelson interferometer was set up and digital off-axis holograms were recorded. By adopting the Robert gradient
method in Iconology, the reconstructed results both by the wavelet transform with coefficients selection method and
the traditional Gaussian high-pass filter method were compared. The larger the gradient magnitude value (G) is, the
more information the image contains, and the image is clearer. The G values of the reconstructed images got by
wavelet transform with coefficients selection method and the traditional Gaussian filtering method are 1099891 and
1042562, respectively. The results validated that applying the wavelet transform with coefficients selection method
for digital hologram reconstruction can more effectively eliminate zero-order diffraction image and improve the SNR
and quality of reconstructed image than applying the traditional Gaussian high-pass filter method.
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