Optical Evaluation of Internal Damage to Human Hair Based on Second Near-Infrared Window Polarization Microscopy

This study introduces a novel method using circular polarization microscopy with near-infrared optics in the second NIR window (1000-1600 nm) to evaluate internal hair damage caused by cosmetic treatments. The method, which employs a laser scanning transmission microscope system, effectively improves light transmittance in black hair, allowing for detailed observation of damage. It quantifies changes in the cortex, such as decreased circular polarizability after lithium bromide treatment and its restoration with water. Additionally, it identifies two types of damage and observes black speckles after heat treatment, which are independent of polarizability changes. This noninvasive, cost-effective technique shows promise as a standard in hair care research and medical fields.