Lumu is a digital light meter for photographers that plugs into the iPhone’s headphone jack as a smaller and smarter replacement for traditional analogue light meters. It’s used in conjunction with Lumu’s app.
The basic idea is for a photographer to grab a light reading using Lumu on their iPhone, then input the suggested settings into their camera. Settings are displayed in Lumu’s app, which also allows the user to save data to the cloud so they can retain light-setting and location info, plus add voice records, notes, pictures, photo parameters, and more.
Lumu is not going to help you take better photos on your iPhone — it’s a tool for standalone cameras that have ISO, aperture and shutter speed parameters that can be manually set. The company launched a Kickstarter campaign to fund the product, which is roughly the size of a coin. It’s incredibly elegant and can even be used with a standard iPhone bumper, given that there’s a 2.2mm gap between the headphone jack and the start of the meter.