Refractive index measurements of multiple layers using numerical refocusing in FF-OCT.

We propose and demonstrate the novel method of refractive index (RI) measurement for each layer of multilayered samples, which is based on numerical refocusing in full field optical coherence tomography (FF-OCT). The en-face FF-OCT image on an inner layer boundary of a multilayered sample is unintentionally blurred or defocused due to the RI of the sample itself, but can be numerically refocused. The refocusing is performed by numerically shifting the image sensor plane of the system, in general. However, by calculating the corresponding sample shift and then compared it with the actual sample shifting distance, we could extract the average RI of the layer between any two layer boundaries within the multilayered sample. In addition, the thickness of that particular layer could be derived at the same time. For the idea proof, several samples were prepared by stacking, for each sample, two transparent plates with a gap in between. While changing the material of the plate and filling the gap with oil, the RIs of the plate and the oil were measured. For oils of various RIs, from 1.2977 to 1.3857, the measured RIs were well matched with the reported ones within 0.205%. Moreover, even with a stack of various and multiple plates in front of the same oil layer, the oil RI and the physical thickness of the oil layer were extracted with average errors of only 0.065% and 0.990%, respectively.

Qualitative investigation of fresh human scalp hair with full-field optical coherence tomography.

We have investigated depth-resolved cellular structures of unmodified fresh human scalp hairs with ultrahigh-resolution full-field optical coherence tomography (FF-OCT). The Linnik-type white light interference microscope has been home-implemented to observe the micro-internal layers of human hairs in their natural environment. In hair shafts, FF-OCT has qualitatively revealed the cellular hair compartments of cuticle and cortex layers involved in keratin filaments and melanin granules. No significant difference between black and white hair shafts was observed except for absence of only the melanin granules in the white hair, reflecting that the density of the melanin granules directly affects the hair color. Anatomical description of plucked hair bulbs was also obtained with the FF-OCT in three-dimensions. We expect this approach will be useful for evaluating cellular alteration of natural hairs on cosmetic assessment or diagnosis of hair diseases.