If somebody asked me if my spot meter was a gun I'd pull my Glock out of its concealed holster and say, "No. It's not a gun. THIS is a gun!"
Okay... No, I really wouldn't do that in real life but that's what I want to say right now.
I actually have been in situations where I have been confronted by police or security guards. My response was always a polite but direct, "We're on public property and, unless there's anything else, we have nothing to discuss," and I keep right on walking.
I know you're going to say that's a good way to get yourself into trouble but I'm not the one who is in the wrong. I have no responsibility to give anything more than a cursory response to such an inquiry. As long as one is reasonably polite and doesn't threaten the other person there is nothing they can do.
If that doesn't work, the phrase, "Title 42, Section 1983" is enough to stop any cop (who is unjustly harassing you) cold in his tracks.
If a person can't tell the difference between a light meter and a gun, they probably couldn't tell the difference between their ass and their elbow, either.