Near-infrared imaging of water in human hair

Abstract

Background/purpose: The water content of hair can be evaluated by weighing, the Karl Fischer method, and from electrical properties. However, these methods cannot be used to study the distribution of water in the hair. Imaging techniques are required for this purpose. In this study, a highly sensitive near-infrared (NIR) imaging system was developed for evaluating water in human hair. The results obtained from NIR imaging and conventional methods were compared. Methods: An extended indium-gallium-arsenide NIR camera (detection range: 1100-2200 nm) and diffuse illumination unit developed in our laboratory were used to obtain a NIR image of hair. A water image was obtained using a 1950-nm interference filter and polarization filter. Changes in the hair water content with relative humidity (20-95% RH) and after immersion in a 7% (w/w) sorbitol solution were measured using the NIR camera and an insulation resistance tester. The changes in the water content after treatment with two types of commercially available shampoo were also measured using the NIR camera. Results: As the water content increased with changes in the relative humidity, the brightness of the water image decreased and the insulation resistance decreased. The brightness in the NIR image of hair treated with sorbitol solution was lower than that in the image of hair treated with water. This shows the sorbitol-treated hair contains more water than water-treated hair. The sorbitol-treated hair had a lower resistance after treatment than before, which also shows that sorbitol treatment increases the water content. With this system, we could detect a difference in the moisturizing effect between two commercially available shampoos. Conclusion: The highly sensitive imaging system could be used to study water in human hair. Changes in the water content of hair depended on the relative humidity and treatment with moisturizer. The results obtained using the NIR imaging system were similar to those obtained using a conventional method. Our system could detect differences in the moisturizing effects of two commercially available shampoos. © 2012 John Wiley & Sons A/S.