Logarithmic dynamic range for improved digital lensless holographic microscopy reconstructions

Under sub-optimal illumination conditions, DLHM often struggles to capture useful holographic information across the full sensor area. This is especially true when using relatively large, spatially extended point sources (≥ 5 µm) or very short working distances (< 10 mm). In such cases, the recorded holograms typically show useful detail only in a small, brightly illuminated central region. Since these conditions are common in practical applications, numerical preprocessing becomes essential. It can substantially improve the quality of the reconstructed wave-fields. This work proposes a hardware-free, single-frame preprocessing method called Logarithmic Dynamic Range (LogDR). It redistributes the recorded irradiance onto a logarithmic scale and then applies local tone mapping. This approach flattens the characteristic Gaussian illumination envelope in DLHM without introducing artifacts. Initial validation with eight illumination-limited holograms of a USAF-1951 target shows that the proposed LogDR method increases the effective field of view by about 13 % and improves the smallest resolvable feature from 3.10 µm to 1.95 µm while reducing background phase noise by four-fold. Compared with other methods, LogDR delivers the best trade-off between field of view, lateral resolution and phase contrast. A biological demonstration on a buccal-mucus smear further reveals peripheral protein aggregates and sub-cellular details that remain hidden in conventional reconstructions. Because LogDR operates without additional exposures, optics or machine-learning models, it offers an immediately deployable enhancement for portable or point-of-care DLHM systems, enabling wider contextual imaging and finer structural analysis under compromised illumination conditions.