The Illuminator-medal can be achieved via capturing Mind Units (MUs) with Control Fields Δ through populated areas.
Mechanics[]
Although there is no concrete equation known on how Mind Units (MUs) are calculated, it is generally agreed on the population density under a Control Field Δ. This means more populous cities like New York City and Tokyo has a potential to yield more MUs per meter squared. Maximizing size of Control Fields Δ in these cities can yield larger figures of MU captured per Control Field Δ.
Strategy[]
Bigger Control Fields Δ tend to grab more MUs, especially over populated land. If one wishes to maximize their MUs, they can layer their Control Fields Δ which allows for MUs to be stacked on top of one other. Although it can be tedious to layer Control Fields Δ (due to the time farming Portal Keys 
and heading from Portal to Portal), it often can end up being worth it due to the AP gained. There is an IITC plugin called FanFields that maximizes the amount of Control Fields Δ, thus MU as well. Planning Portals, Control Fields Δ and a route will give an Agent a quick and time-efficient way to layer Control Fields Δ.