Correlation of Sheet Resistance to Emissivity
The propagation of electromagnetic waves in matter is described by the MAXWELL equations. The values of transmittance, reflectance and absorptance can be derived from the solutions to these equations. The interaction of the visible light with the layers was very well investigated among industry and science and simplifications for the set of equations have been made and evaluated. MAXWELL equations for the absorptance with consideration of KIRCHHOFF’S Law (emissivity ε = absorptance α) result to εn = αn,IR = 4·R□/zo where R□ is the sheet resistance of the layer and zo = 377 Ω is the wave resistance in vacuum. This results to εn = (4 / 377 Ω) · R□ and is typically written as εn = 0.01061·R□. The industry adds typically some “safety markup” when applying this formula for QA and process control. Often this formula is used e = 0.0108 Rsq.