Thorsten von Overgaard, a Danish writer and photographer in his blogpost Using an external light meter for accurate, failproof metering:
“Despite the fact that light metering is not exactly space technology, lightmeters tend to be too bulky and too complicated. The LUMU lightmeter changes this into a simple to use, accurate and compact - perhaps even trendy - opackage that works with your iPhone. For a price of only $149 is is even less than most other lightmeters.
One of the things that traditional lightmeters do to complicate things, is that they give aperture for example f/2.03. or f/8.04. Not only is it confusing, it is also not applicable on any lens to set the aperture to f/2.03.
And forget about aperture priority or shutter priority when you buy a lightmeter. In the LUMU app on the iPhone you simply lock the ISO and the aperture so as to only allow the lightmeter to figure out an exposure time.
Or you lock the exposure time and the aperture, letting the LUMU only give you the ISO. That’s pretty bright, simple and as it should always have been.
Moreover, it is compact to bring with you, provided you have an iPhone with you as well. Traditional lighmeters has the same size as a camera (at least if you are using a Leica M). The LUMU is the size of the cap of a water bottle.”