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In medical near-infrared spectroscopy (NIRS), particularly in the study of the brain, light illumination is commonly realised by lasers or laser diodes (LD). However, the rapid development of high-power light-emitting diodes (HPLED), increased optical output power and expansion of wavelengths of choice in particular, have made HPLEDs a feasible alternative, also for in vivo measurements of cerebral blood flow (CBF) and brain metabolism. These applications, however, require using sophisticated modulation techniques that enable distinguishing low-level light signals that are back-scattered from the cerebral cortex. They also rely on separating different wavelengths when the illuminating light is emitted simultaneously by several HPLEDs with different wavelengths. In this paper, we study key properties of commercially available HPLEDs, with a focus on the study of the brain. Of particular interest here are optical output power and available wavelengths. Furthermore, we demonstrate a lock-in amplification technique suitable for use with HPLEDs in brain studies. Based on the presented measurement technique, we conduct experimental measurements on CBF in the cortex and analyse fluctuations in blood oxygen level at different combinations of wavelengths. |
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