Distortion that occurs with an image intensifier tube showing a low density image at the periphery is called:

Vignetting refers to the phenomenon where the brightness and density of an image decrease toward the edges or periphery compared to the center. This can happen in imaging systems, including those using image intensifier tubes, due to several factors such as the geometry of the system and light absorption characteristics. The result is that images presented at the edges of the field of view can appear dimmer or low in density, which directly corresponds to the conditions described in the question.

In contrast, interlacing relates to the way images are presented in a sequence to reduce flicker, and is not specifically tied to the density of peripheral images. Diminution refers to a general reduction or decrease but does not specifically address the edge effects of image quality. The edge spread function involves how sharp the edge of an image appears and is more related to the resolution and contrast of the edge, rather than the overall density observed within the image periphery. Therefore, vignetting is the term that accurately describes the low density image appearance at the periphery in this context.