Lensless efficient snapshot hyperspectral imaging using dynamic phase modulation

Spatial light modulator (SLM) is a dynamic optical component capable of real-time modulation of incident light's amplitude, phase, and polarization state under external control. It achieves this by adjusting the refractive index of liquid crystals, thereby controlling the optical path length. By utilizing liquid crystal SLMs, it is possible to simulate diffractive optical elements (DOEs), enabling active diffraction control due to their programmability and flexibility.

Snapshot hyperspectral imaging based on a diffractive optical element (DOE) is increasingly featured in recent progress in deep optics. Despite remarkable advances in spatial and spectral resolutions, the limitations of current photolithography technology have prevented the fabricated DOE from being designed at ideal heights and with high diffraction efficiency, diminishing the effectiveness of coded imaging and reconstruction accuracy in some bands. Here, we propose, to our knowledge, a new lensless efficient snapshot hyperspectral imaging (LESHI) system that utilizes a liquid-crystal-on-silicon spatial light modulator (LCoS-SLM) to replace the traditionally fabricated DOE, resulting in high modulation levels and reconstruction accuracy. Beyond the single-lens imaging model, the system can leverage the switch ability of LCoS-SLM to implement distributed diffractive optics (DDO) imaging and enhance diffraction efficiency across the full visible spectrum. 

Partial Experimental Procedures and Results​

The schematic of the LESHI system is shown in Fig. 1. A light source (CIE standard illuminant D65, Datacolor Tru-Vue light booth) is used to illuminate the object. The reflected light of the sample passes through the polarizer (GCL-050003), is reflected by a beam splitter (GCC-M402103), and impinges on the LCoS-SLM (FSLM-2K39-P02, 8-bit grayscale level of 256 steps, 180-Hz refresh rate) loaded with optimized DOE patterns. Since the liquid crystal layer has different refractive indices for different wavelengths of the spectrum [52,53], it can produce different phase delays for the entire spectrum like DOE, splitting the continuous hyperspectral data cube. Thus, when a light wave passes through the liquid crystal layer of the LCoS-SLM, the modulation of each pixel causes the phase of the light wave to change. Finally, the phase-modulated light reflected from the LCoS-SLM transmits the beam splitter and is recorded by a color CMOS camera (ME2P-1230-23U3C, which contains a Bayer filter).

Fig. 1. Schematic of the lensless efficient snapshot hyperspectral imaging (LESHI) system. LCoS-SLM, liquid crystal on silicon-based spatial light modulator. LESHI comprises hardware-based diffractive imaging and software-based hyperspectral reconstruction algorithms. The diffractive imaging component includes an LCoS-SLM, a polarizer, a beam splitter, and a color CMOS camera. The hyperspectral reconstruction algorithm employs a ResU-net to decode the spectral information.

Fig. 2. Working principle of LESHI. (a) Pipeline of LESHI. (b) Schematic of PSF acquisition process in diffractive optical imaging based on LCoS-SLM with DOE patterns. (c) DDO model design based on LCoS-SLM. DDO fuses the PSFs of inpidual DOEs of the different bands and adds the model of the diffraction efficiency to form a degenerate PSF model. (d) Structure of the ResU-net reconstruction algorithm, which combines the U-shaped architecture of U-net with the residual connections of ResNet.

Fig. 3. Validation of LESHI model. (a) Ground truth from the ICVL dataset. (b) The trained simulated DOE pattern loaded on the LCoS-SLM. (c) RGB image generated by the LESHI model with a single DOE pattern. (d) Reconstructed result of (c). (e) Reconstructed hyperspectral images using LESHI model with a single DOE pattern. (f) Ground truth and reconstructed values of the spectral radiance curves for local area “1” marked in (a). (g) Same as (f) but for local area “2”. (h) Diffraction efficiency as a function of wavelength, using single DOE pattern (LCoS-S) and multiple DOE patterns (LCoS-D) in the LESHI model. The table shows the relative diffraction efficiency gain (RDEG) of LCoS-D compared to LCoS-S at three different bands (400–500 nm, 500–600 nm, 600–700 nm).

Fig. 4. Characterization of the LESHI system performance. (a) Reconstructed image of ISO12233 test chart. (b) Spatial line profiles of two regions on the test chart, highlighted in light orange and teal boxes at the location of label 1 in (a). (c) Spatial line profiles of two regions on the test chart, highlighted in light blue and teal boxes at the location of label 2 in (a). (d) Measurement of the LEHSI system. (e) Reconstruction result of (c) in RGB format. (f) Root mean square error (RMSE) and maximum error of reconstructed image and measurement by the CS-2000 spectrometer at six local regions [marked by white boxes in (c)]. (g) Reconstruction radiance curves of six local regions [marked by white boxes in (c)] as a function of wavelength. Ground truth is obtained by the CS-2000 spectrometer. (h) Seven representative reconstructed spectral channels of (d).

Fig. 5. Application results for focal length modification. (a) Phase modulation patterns loaded onto LCoS-SLM with different focal lengths by end-to-end training. (b) Corresponding captured RGB images of (a). (c) Results of spectral image recovery by applying the LESHI system at different focal lengths. (d) Six representative reconstructed spectral channels corresponding to (c).

Fig. 6. Comparison of spectral reconstruction simulations for different models. (a) Comparing the four reconstruction data results and visual effects, the diffractive optical imaging model based on LCoS-SLM can effectively improve the reconstruction performance and avoid the degradation of the reconstruction results caused by the quantized DOE. (b) Spectral radiance curves for different models. The spectral curves show that the reconstructed spectral curves of LCoS-D are closer to the ground truth values.

The specifications of the phase-only spatial light modulator used in this experiment are as follows:

Final Thoughts

DOE, as a traditional diffractive optical element, has a fixed structure and fixed functionality, but its efficiency is relatively high. In contrast, the liquid crystal spatial light modulator (SLM) modulates the wavefront via electrical control, enabling flexible programming and real-time modulation. However, its efficiency is lower due to losses from pixel gaps and the liquid crystal response. Both have their own advantages and disadvantages, and by using them complementarily, it is possible to optimize optical systems. For example, an SLM can be used to correct aberrations in a DOE, or a DOE can be combined with an SLM to extend the functional boundaries of the SLM.

Article Information: https://doi.org/10.1364/PRJ.543621