Multi-exposure image fusion for phase enhancement in digital holographic microscopy

Phase information in single exposure images is often lost for specimens with non-uniform transmittance. To address this issue, we propose a multi-exposure image fusion phase enhancement technique for holographic images. In this process, exposure time is varied to acquire 11 groups of four-step phase-shifted holographic images from a common sampling area. The resulting images are then decomposed using a wavelet transform. Maximum phase information from the low-frequency regions and regional features from the high-frequency regions were employed to avoid discontinuities in subsequent reconstructions, caused by regional truncation. Holographic images were then obtained after multi-exposure image fusion using a wavelet fusion method. Corresponding phase information was acquired by reconstructing fused holographic images using a four-step phase shifting technique. Experimental results showed that for specimens with non-uniform transmittance, this approach increased information entropy by 4.2%, edge density by 5%, and contrast by 3.8%, in comparison with the single-exposure digital holography phase reconstruction method. This result suggests that clarity and information content are improved, thereby enhancing the reconstructed phase.