I prefer the idea of making the ability generate its own light source that is the only determining factor of whether you get either boost. Tap and Hold are probably the preference for most people, but the thematic of using light seems to fit with Mirage more. If we can control the radius of the buff area created by the light source with range then it's probably fine.
Perhaps we can have the behavior of the field change depending on the Tap or Hold. If we do one, the field makes light, and standing inside it gives us the full light buff, but leaving it gives us the partial shadow buff. If we do the opposite input, it creates a dark field that gives us the full shadow buff, and leaving gives us a partial light buff.