Color lensless in-line holographic microscope with sunlight illumination for weakly-scattered amplitude objects

Abstract

We propose a method for acquiring color images with a lensless in-line holographic microscope (LIHM) using sunlight illumination, which is suitable for weakly scattered amplitude objects. In the color LIHM, the sample is illuminated by sunlight, and the in-line hologram is recorded by a color CMOS imaging sensor located behind the sample. For weakly-scattered amplitude object, we show that the hologram can be described as the convolution of object transmission and a point spread function (PSF) that depends on the spectral distribution of the light. The captured color hologram is first separated into the red, green, and blue components, and then the sub-holograms of each color are used to reconstruct the corresponding color components of the sample by a deconvolution process. We proved that the deconvolution process was able to improve the imaging resolution, which was deteriorated because of insufficient temporal coherence of the light. The resolution enhancement capability of our color LIHM was demonstrated by numerical simulations and imaging experiments with the U. S. Air Force target as the sample. We also imaged a stained root of the herb Saposhnikovia divaricata to further demonstrate the capability of our method for color imaging applications. Our proposed color LIHM method provides a way to realize color holographic imaging with white light sources and thus reduces the cost and complexity of the lensless color microscope.