In direct radiography (DR), the fill factor refers to what?

In direct radiography (DR), the fill factor is a crucial concept that pertains specifically to the sensitive portion of the detector element. The fill factor represents the ratio of the area of the detector that is sensitive to radiation (i.e., captures the image data) to the total area of the detector element. A higher fill factor indicates that a larger portion of the detector is actively contributing to the image formation, which is desirable for image quality and exposure efficiency.

This is important in understanding how effectively a DR system can convert incoming X-rays into a digital image. While the other options relate to aspects of detector elements, they do not accurately capture the definition or significance of the fill factor in the context of direct radiography.