This is the lux meter I use.
It works great for matching a new coral to a light zone in your tank so its light isn’t too different from the tank where it came from.
It works great if you’re switching lights for the same reason. Measure the old system and the after the new system is in place, tune it closely to the old system’s output.
There are many free lux meter apps for your smartphone. They use the smartphone camera’s light meter to generate a normal lux reading. “Galactica luxmeter” for IOS is one example.
If you don’t have a lux meter, I’d start with the free download. You can’t beat the price and the results are quite good.
But I’d strongly recommend a handheld meter such as the LX-1010B as they are usually only around $15, delivered. They will deliver better readings and are safer to use around saltwater than your smartphone!