40, 41 The phase and amplitude maps of a representative hair are shown in Figs. Each hair sample was illuminated with plane waves at 300 different incidence angles, which were systematically controlled by the two-axis GM at a frame rate of 100 Hz.Ĭomplex optical fields of the sample were retrieved from the interferogram recorded by the QPI technique, via a retrieval algorithm based on Fourier transform. The full field of view at the sample plane was 46.18 × 45.76 μ m 2, considering the total magnification of 250 ×. The camera has 1776 × 1760 pixels with a pixel size of 6.5 μ m. For the imaging purpose, a high-NA objective lens (PLAPON, 60 ×, oil immersion, NA = 1.42, Olympus Inc., San Diego, California) was used with an additional telescopic 4 - f system, and the total magnification of the imaging system is 250 ×. For the reconstruction of off-axis interference patterns, the reference and the sample beams are recombined with a tilted angle by another beam splitter, and the resultant interferogams of the samples are recorded by a high-speed CMOS camera (Neo sCMOS, Andor Inc., Northern Ireland, UK).Īn objective lens was used as a condenser lens with the tube lens of a focal length of 200 mm. A two-axis galvanometer mirror (GM, GVS012/M, Thorlabs) varies the angle of the illumination beam impinging onto the hair sample, from which 2-D optical field images of the sample are obtained with various illumination angles. The sample beam passes through a downy hair loaded on the sample stage of an inverted microscope. A beam splitter (BS, BS016, Thorlabs) divides a laser beam into two arms: a reference beam and a sample beam. A diode-pumped solid state laser ( λ = 532 nm, 50 mW, Cobolt Co., Solna, Sweden) is used as a coherent light source. 37 – 39 The schematic of the setup is shown in Fig. 1(b). Optical Setup for Quantitative Phase Imagingįor quantitative measurements of human downy hairs, we employed a Mach–Zehnder interferometric microscope equipped with a two-axis galvanometer mirror. Furthermore, we investigated the effects of hydrogen peroxide ( H 2 O 2), which is one of the widely-used hair bleaching agents, on individual downy hairs using the present method.Ģ.2. The high-resolution two-dimensional (2-D) holographic synthetic aperture images and 3-D RI distribution maps of individual hairs were measured, from which the morphological and biochemical properties were retrieved, including the mean RI, volume, cylinder, and effective radius of individual hairs. 36 The feasibility of noninvasive and quantitative imaging of QPI was demonstrated with individual downy hairs using the QPI techniques. 24, 25 QPI techniques have been applied previously for biological studies of cells and tissues including blood cells, 26 – 33 cell growth monitoring, 34 neurons, 35 and optical imaging of tissue slices. QPI techniques provide quantitative measurements of optical phase delay introduced by intrinsic RI distributions of transparent cells using interferometry. Here, we present quantitative phase imaging (QPI) as an effective imaging tool to study the morphological and optical properties of downy hairs in a noninvasive and quantitative manner. Furthermore, most previous studies have used expensive instrumentation, which prevents these imaging techniques from being utilized in general laboratories in which body hairs are studied. 19 – 23 However, previous approaches have difficulties in providing simultaneous measurements of three-dimensional (3-D) morphologies and the refractive index (RI) of hairs quantitatively and noninvasively. 18 Chemical analyses of hairs for drug detection (e.g., alcohol and cocaine) also have been extensively carried out, particularly in the fields of forensic science. Using an inverse Monte-Carlo method, the melanin contents of human hairs have been quantitatively estimated from digital images. 1, 2 While human body hairs have lost their thermoregulatory roles in maintaining warmth through an evolutionary process, the decorative aspects of hair have gained attention and led many researchers to investigate the microstructures of hairs using diverse microscopic techniques, including atomic force microscopy, 3 – 5 scanning electron microscopy, 6 – 8 transmission electron microscopy, 9, 10 confocal microscopy, 11, 12 infrared spectroscopy, 13 – 15 optical reflectometry, 16 and ellipsometry 17 under various experimental conditions. The characteristics of different hairs have been regarded as one of the evolutionary consequences incurred by human bipedalism. Human body hairs exhibit distinct morphologies at their relative sites in the body, from soft downy hairs on the arms to long stiff hairs on the head.
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