A Gardon gauge or circular-foil gauge is a heat flux sensor primarily intended for the measurement of high intensity radiation. It is a sensor that is designed to measure the radiation flux density (in watts per metre squared) from a field of view of 180 degrees. The most common application of Gardon gauges is in exposure testing of sample materials for their resistance to fire and flames. Explanation While heat flux sensors can be made according to various designs, the sensor of a Gardon gauge consists of a foil connected to the sensor body at its external radius, and connected to a thin wire at the center, named after its originator Robert Gardon. The foil center and side are the hot- and cold joint of a thermocouple respectively. When radiation hits the sensor this generates a signal. It is typically water-cooled and does not require any power to operate. A so-called Schmidt-Boelter Gauge has the same outward appearance as a Gardon Gauge, but employs different sensor technology. The Schmidt-Boelter has a plated constantan wire wrapped around an insulating chip. Both are heat flux sensors. The only difference is practical; Gardon gauges can be manufactured in such a way that they withstand extremely high flux levels. The range for Schmidt-Boelter technology is more limited. On the other hand the Schmidt-Boelter technology can reach higher sensitivities at a lower response time. Please note: Images on this page are of a Schmidt-Boelter gauge. While of similar appearance externally, the internal construction is not that of a Gardon gauge. Construction of both is detailed in the explanation. A high intensity radiation spectrum extends approximately from 300 to 2,800 nm. Gardon gauges usually cover that spectrum with a spectral sensitivity that is as “flat” as possible. For a flux density or irradiance measurement it is required by definition that the response to “beam” radiation varies with the cosine of the angle of incidence; i.e. full response at when the radiation hits the sensor perpendicularly (normal to the surface, 0 degrees angle of incidence), zero response when the radiation is at the horizon (90 degrees angle of incidence, 90 degrees zenith angle), and 0.5 at 60 degrees angle of incidence. It follows from the definition that a Gardon gauge should have a so-called “directional response” or “cosine response” that is close to the ideal cosine characteristic. Design of Gardon gauges In order to attain the proper