Although a plain metallic cathode will exhibit photoelectric properties, the specialized coating greatly increases the effect.  A photocathode usually consists of alkali metals with very low work functions.
The coating releases electrons much more readily than the underlying metal, allowing it to detect the low-energy photons in infrared radiation.  The lens transmits the radiation from the object being viewed to a layer of coated glass. The photons strike the metal surface and transfer electrons to its rear side. The freed electrons are then collected to produce the final image.