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The principle of laser calorimetry

A CO2 laser of a known and stable power has the test optic placed in its beam path, in our case the test optic is nearly always a mirror. The mirror under test is heated by the small amount of laser power that is absorbed, and the resulting temperature rise is measured. Despite the mirror being thermally isolated as well as practicable, there is always a small amount of heat loss, so the rate of cooling is measured as well.

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Calculating absorption

By knowing the laser power, the weight and thermal properties of the test optic, along with the rate of heating/cooling it is possible to calculate the amount of power that the test optic absorbed. For long wavelengths like 10.6um and a metal mirror, any scattering or transmission is ignored so we can say Reflectivity = 1 - Absorption. The rate of heating and cooling of the test mirror is best measured by the slope of the temperature versus time graph of the mirror during testing. This is measured by a thermocouple attached carefully to the mirror, its voltage output recorded by a high speed, high bit rate A-D data logger. A spreadsheet can quickly analyse the thousands of recorded temperature readings and calculate the reflectivity of the mirror. The accuracy of the technique depends on the size and shape of the mirror to be tested as well as the absorption of the mirror. In general, we can measure the reflectivity of a CO2 laser mirror with an accuracy far greater than would ever be needed by customers.

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