Kerma-area-product (KAP) meters are plane parallel ionization chambers used in diagnostic radiology for the assessment of the amount of radiation impinging on the patient during an x-ray examination. Modern medical x-ray units either contain a KAP meter or calculate the kerma-area-product from machine settings. To simplify x-ray beam positioning, most KAP meters are transparent to visible light. This is achieved by using a thin conductive layer containing elements with high atomic numbers. This construction detail leads to severe energy dependence of these ionization chambers, which limits measurement accuracy. Relative errors up to about 20-25% are common and complicate x-ray diagnostic protocol optimization.