TransilluminationPhotoglottography (PGG) also known as Transillumination comprisesof an optic light source inserted through the nose and positioned above the glottis. Medical supervision is required as accidental contact between the probe and the glottis can send it into uncontrolled spasm causing breathing difficulties. A light sensing probe is held in place externally just above or just below the crycoid cartilage, usually with a neckband. (Note in some systems the light source is external and the sensor in placed internally.)
Also Hoole, P.,"Laryngeal Coarticulation. Section 1: Coarticulatory investigations of the devoicing gesture." In W.H. Hardcastle ad N. Hewlett (eds.), Instrumental Studies of Coarticulation. Cambridge Univesity Press 2000.