Thermal sensors have a series of bimetallic junctions called a thermopile. Radial or axial heat flow through the sensor creates a voltage proportional to the power absorbed as it flows through the thermopile. The reading is not dependent on the ambient temperature since only the temperature difference is measured and not the absolute temperature. The thermopile elements are so arranged that the reading is almost independent of beam size and position. Generally, Ophir specifies ±2% or better uniformity of reading over the surface.