Measurement and Analysis of Optical Transmission Characteristics of the Human Skull.

The brain, as a vital part of central nervous system, receives approximately 25% of body's blood supply, making accurate monitoring of cerebral blood flow essential. While fNIRS is widely used for measuring brain physiology, complex tissue structure affects light intensity, spot size, and detection accuracy. Many studies rely on simulations with limited experimental validation. In this study, we used real adult skulls and agar to create a mimic model, building a transmission optical system with 13 wavelength filters and varying agar thicknesses. Peak intensity of transmitted light and size of scattered spot were measured at different wavelengths, and transmittance, total attenuation coefficient, and spot diameter enlargement of cranial mimics at different wavelengths were obtained. Results showed wavelengths below 550 nm struggled to penetrate the skull, while those above 700 nm penetrated deeper and diffused more. This suggests that short wavelengths capture epidermal PPG signals, whereas longer wavelengths detect both epidermal and intracranial signals.