Operating Principles

Thermal methods of measuring power and energy are those in which radiant energy is absorbed and converted into heat, thus creating a temperature rise in the absorber. The absorbed energy can then be measured by monitoring a temperature gradient between the hot area (where the laser strikes) and a cool area (where the generated heat is dissipated). This measurement can be done by means of thermocouples arrays (thermopile). The temperature difference will generate a voltage at the end of each single thermocouple, so if the array is duly distributed, the resulting total voltage will be proportional to the incident power or energy. As the generated voltage depends on the temperature difference between the hot and the cold areas it results that there is no influence of ambient temperature on the measurement. Thermal detectors exhibit an intrinsic high degree of linear response at the increase of power levels (linearity); a compensation for the minor drops in linearity occurring at the working temperature extremes are made with the use of thermistors.

Linearity of LaserPoint’s detectors, thanks to their optimised thermal design, is excellent: as an example, for the W-1000 head as measured by PTB (Physikalisch-Technische Bundesanstalt) in the range between 60-1000W, it remains within +/-1 % without compensation.
The generated heat all flows through the thermocouples, whether they deposited on circles (radial thermopiles) or linearly, with the hot and cold areas facing each other (axial thermopiles). Since the total signal is given by the sum of contributions from all thermocouples, the result is the independence from laser beam size and position.

The response time is determined the thermal resistances, the thermal capacities and, mostly, by the geometrical sizes of the sensor disks. The intrinsic response times of detectors is significantly reduced by appropriate acceleration algorithms in LaserPoint’s monitors. A detector like LaserPoint’s A-300, which has an intrinsic response time of 11 sec due to its very large area, displays the final power value only after 3.5 sec. To dissipate the generated heat a thermal sensor must be placed within a housing which, depending on the amount of heat to be driven away, can work by simple convection, have electrical low voltage fans or be water cooled. So as the thermal contact between sensor and housing is very critical since it might generate overheating of the sensor and instabilities in the signals, the final shape and dimensions of the heads must be carefully designed to maintain the sensor temperature within its working limits. An example of excellent thermal dimensioning is LaserPoint’s W-6000 head, which can safely work up to 9KW.