A system for monitoring energy radiant from a material dispensed onto a substrate. As disclosed, an infrared sensor is positioned to receive infrared energy radiated by a substrate and by a heated adhesive applied to the substrate, with the resistance of the sensor varying in dependence upon the amount of infrared energy received. An amplifier has an input capacitively coupled to the sensor, and the voltage at the amplifier input varies in dependence upon the resistance of the sensor. The output voltage of the amplifier varies in dependence upon the amplifier input voltage, and hence upon the amount of infrared energy reaching the sensor. In order to prevent inaccuracies in the amplifier output signal due to, for example, temperature-related resistance variations of the sensor, the amplifier input is grounded during intervals when the sensor receives infrared energy from the substrate and ungrounded shortly before the sensor begins to receive infrared energy from the heated adhesive.