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Transillumination, or the passage of light through the body, has been studied since the early 1800s. Attempts to image organs and tissues for the purpose of diagnosis and treatment have developed to the point that light-based monitoring in the form of pulse oximetry is used daily in hospitals and clinics. Transillumination of the head, first described in 1831 by Richard Bright, eventually would be recognized as the first light-based diagnostic technique to identify hydrocephalus. With modification, although still crude, it was used to diagnose intracranial hemorrhage in the neonate at a time before head ultrasonography was available and when computed tomography (CT) was extremely expensive and not widely available.

In early devices, light in the visible part of the spectrum was employed; more recently, light over a narrower band of wavelengths has been used. In 1977, Jobsis showed that tissue absorption...

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