A testing device for testing thermal detectors includes a hollow sleeve, open on at least one end, configured to receive a thermal detector. The hollow sleeve can have a length greater than a diameter. A heater is positioned is in proximity to the hollow sleeve, and is configured to provide heat for testing the thermal detector. The heater can be a flexible foil heater. A power source is provided to supply power to the heater. A power control module can be provided for variably adjusting a set point temperature or a rate of temperature rise for the heater.