Hi, I've written an iPhone app that can measure light frequency directly called FlickerFinder. Essentially you point the phone towards the light, it will autoexpose to reveal and measure the flicker, then give you a suggested shutter speed. It's in the final stages, I just need some help calibrating the rest of the iPhones. https://flickerfinder.com
Alternatively if you know the frequency you can use a calculator app https://apps.apple.com/app/id1514983816
I hope it helps, I can answer any questions.