Halo value is a metric not often discussed. Halo in a simple sense refers to the amount of “halo” effect seen around a light source viewed through an image intensifier. A large halo will tend to white out or wash out the image in the area of the light source.
Now that we know what halo is in a practical sense let’s talk about why it occurs. Halo is primarily caused by photoelectrons (electrons jettisoned by the photocathode by means of photon intake) hitting the lattice face of the microchannel plate then backscattering.
In terms of imagery; think of it like that carnival game where you throw rings at a bunch of bottles clustered together trying to get it to land on the bottle neck (except in this analogy the game isn’t rigged and most of them land where they’re supposed to). The ring is the photoelectron. The bottle neck is the microchannel plate passage. Now imagine you throw a bunch of rings at once at cluster of bottles. The rings that don’t land on their bottle necks bounce (back scatter of photoelectrons hitting the MCP). This causes “a wider spill” of photoelectrons than their intended trajectory. Meaning there is additional activity over a larger area of the microchannel plate than what is ideal. This is a crude analogy and in actual operation the photoelectron obtains proximity coupling. Meaning it is pulled into the closest mcp passage.
So how do we arrive at the halo value of a tube? The halo value is a multiple of approximately four times the distance between the photocathode and microchannel plate. Thinking back to the analogy this begins to make more sense. The greater the distance the ring is thrown at the bottle neck from the larger potential area it can travel after bouncing. The larger the gap between the photocathode and microchannel plate the greater the distance photoelectrons can scatter.