What is the main function of the photocathode in an image intensifier?

The main function of the photocathode in an image intensifier is to absorb light and emit electrons. This crucial step occurs when the photocathode, which is usually made of materials sensitive to light such as cesium antimonide, is exposed to the light photons generated by the input phosphor. When these light photons strike the photocathode, they cause the emission of electrons through the photoelectric effect.

This process amplifies the light signal, allowing for a brighter image to be produced on the output side of the image intensifier. The emitted electrons are then directed toward the output phosphor, where they are converted back into visible light. Therefore, the role of the photocathode is pivotal in determining the efficiency of the image intensifier in transforming and amplifying the initial light signal from the patient.

The other options describe functions that do not align with the primary role of the photocathode. Converting electrons back to light is the task of the output phosphor, focusing light is the responsibility of other components within the image intensifier system, and filtering unwanted radiation is typically done by beam filters or collimators rather than the photocathode itself. Thus, the function of the photocathode is specifically centered on the absorption