Abstract: The melting point of a test substance is determined by placing it in a glass capillary tube within a metal block equipped with an electrical heater and temperature monitoring device. The temperature of the metal block and sample are gradually increased, often to temperatures that would damage electronic components. The output of a light emitting diode located a short distance from the metal block and away from the high temperature is coupled into a hollow tubing having highly reflective internal walls that carries the radiation to the glass capillary tube containing the test substance. A second hollow tube, also having highly reflective internal walls, is connected to a light detector that collects reflected radiation from the capillary tube. At the melting point of a solid, the light reflecting properties of the test substance decrease causing the light signal collected by the second reflective tube to decrease. The melt point is recorded as a change in the output voltage from the detector.