An LED-driven optical cavity PCR system and method providing fast, accurate and reliable PCR based diagnostics. Thermal cycling for PCR takes place in an optical cavity formed in a microfluidic channel or chamber with thin light absorbing metal films disposed on at least two opposing walls that provide uniform heating between the walls by photothermal light to heat conversion. The opposing metal films may be sized for equal light absorption from a single light source orthogonal to the two opposing films to produce an even temperature elevation between the equally heating films upon a single light exposure. Uniform temperature elevation may also be achieved with films made from two different materials.