The portion of the image intensifier device responsible for converting light photons into free electrons is the:

The component responsible for converting light photons into free electrons within the image intensifier is the photocathode. When x-rays strike the input phosphor, which is the first layer of the image intensifier, it emits light photons. These light photons then reach the photocathode, where they interact with the material, typically made of a photosensitive substance like cesium antimonide. This interaction causes the emission of free electrons.

The photocathode is crucial in the functioning of the image intensifier as it effectively translates the light signal from the input phosphor into an electron signal, which can then be focused and intensified further down the line by other components of the image intensifier. Without this conversion process, the entire imaging function would be compromised, as the system relies on the transformation of light into an electron-flooded image that can be more readily processed and displayed.