Measurement of optical transmittance: A novel method for determination of the glass transition temperature of transparent glasses

A light beam thermal analysis (LBTA) has been applied for determination of the glass transition region (T g ) of Y 2 O 3 -Al 2 O 3 -SiO 2 glasses containing between 0 and 20 mol.% of CaO. The temperature dependence of the optical transmittance of glass plates with parallel surfaces and with controlled surface roughness was determined Healing of defects on the glass surface by viscous flow in the temperature region close to T g was observed, resulting in increased transmission of visible light through the specimen. The T g values measured by LBTA are in a comparatively good agreement with the results obtained by differential thermal analysis (DTA).