4 Thermocouples simultaneously monitored
a. internal temperature of double glazing gap
b. temperature in gap between external unsealed IR window. IR shield shadows the sensor from the external IR
c. hot temperature plate (internal)
d. warm plate (external) ambient plate temperature is not measured.
Fan blowing ambient air between the double glazing and the separate window. This is to reduce thermal conduction and convection from a different warm/cool source affecting the ambient at the outside of the double glazed window.
Angled BB absorber at back of external source (angled to prevent reflection affecting hot plate.
Lining of polished aluminium foil to prevent IR penetrating the insulation foam.
Voltage input to external warm plate heater is stable but differing ambient and convection/conduction will change its temperature - All this needs to do is provide a temperature less than the hot plate and greater than ambient - so the variation is not important.
The voltage to the hot plate heater is also controlled (by a professional Power supply) the temperature reached is approximately 70°C and the variation with external plate at ambient / warm will be less than a couple of °C. The change in the high quality resistors value will be negligible. So constant voltage equates to constant power..