Android and iPhone devices have a light sensor embedded for modifying screen brightness and to do things like help the camera in making some calculations and decisions. I can’t speak much for newer iPhones as I haven’t tested one but Android is definitely improving.
In the past these were very basic sensors with wild jumps between low/med/high light but the quality of these has been improving over the years. Most of the models from 5 years ago weren’t great, but as of late they’ve been getting better.
If you’ve got a Samsung S9 or Pixel 3 or similar newer phone you might have some luck just using the smartphone app (like “Light Meter”,) though understanding the limitations of the sensor is a big part of getting a reading that’s close to reality. Professional equipment will let you take readings from wherever you want to whereas smart phone sensors will generally only give you an accurate reading when it is in certain positions (or in other words, from certain distances from the light and angled “just right”). Once they are outside of the “sweet spot” the readings have a higher error rate; though again this has been improving over the years.
The trick with those apps is to figure out at what distances your phone gives an accurate (enough) reading (i.e. 1ft away, 3ft away, etc) and for which lamp types that distance is (i.e. CFL, MH, LED, the sun, etc) and how to position the phone to get a consistent result from test to test.
Some apps have a calibration feature to help with this but it does take some practice to learn the quirks – initial tests will probably be way off until those sweet spots become better known and understood through some trial and error.
I’d think it’s worth some experimentation at least (for the fun of it) if you’re interested in trying to measure this without purchasing the meter! Can try to test some reference lamps with known LUX values (or published ratings) then see how that compares to readings taken elsewhere.
But I do agree, the lack of a tech spec sheet is annoying.