What is the primary cause of peripheral fall-off of image brightness in image intensifiers?

The primary cause of peripheral fall-off of image brightness in image intensifiers is indeed related to the concave, curved shape of the input screen. In image intensifiers, the input screen is designed to capture the incoming light and convert it into an electron image. When this screen is curved, the angles at which photons hit the screen vary depending on their position, particularly for those at the edges.

As a result, light that enters near the periphery of a concave surface does not align as efficiently with the conversion process as light entering near the center. This misalignment leads to a decrease in light intensity—the peripheral areas thus exhibit lower brightness compared to the central region.

Additionally, the design of the output screen can influence other factors in image quality but does not directly contribute to the brightness fall-off observed at the periphery in the way that the curvature of the input screen does. Factors such as calibration and material quality may affect overall system performance but are not the primary causes of this specific phenomenon.