Answer: The distance between the photocathode and the MCP determines how big the halo will be. As you look to an active light source, you will see a halo around the light source. The closer the MCP is located towards the photocathode, the smaller the size of the halo. Typically 0.8 mm for the XR5™ and 2.0 mm for the GEN III.

Typical GEN III Halo size	Typical PHOTONIS Halo size