Short answer, I don't know.
However, you're thinking about it oddly. Light isn't measured in frames per second. Light is measured in shutter speed (seconds) and aperture (f-stop).
So, the first thing to figure out is what shutter speed you're shooting at. In 24fps world, the shutter is open half the time ("180 degrees" out of 360) which translates into 1/48th of a second.
I never heard of a camera that shoots 18fps as a standard. You may want to check the manufacturer and specs. Some cameras vary the framerate and CAN shoot 18fps, and if you want to match the look to the 24fps footage you use a 180 degree shutter, which equals 1/36th of a second.
So, back to your lightmeter, does it have a setting for 1/36th second, and more importantly, is this what your camera is doing?