Digital cameras work out the exposure by measuring the light reflected from your subject towards the camera. This works well enough most of the time, but it falls down when you've got a especially reflective subject, like a white wedding dress, or a non-reflective one like a black cat. That's one reason why all cameras have EV (exposure value) compensation controls, so that you can correct the exposure when it's not quite right.
But old-school photographers and cinematographers know there's another way – you measure the light falling on your subject instead, using a specially-designed translucent dome attachment with a hand-held meter. Taking these 'incident light' readings is more fiddly but measures the thing you really want to know – how much light is falling on your subject, not how much it's bouncing back.
See the light
That's where the Lumu comes in. You can already get light metering apps for the iPhone, but the Lumu takes it further a neat little dome attachment that plugs into the 3.5mm earphone socket so that you can take incident light readings.
You can do this by next by your subject and angling the dome back towards the camera position or, if that's not practical, just make sure you're holding the Lumu in the same light as your subject.
The app measures the light level and gives you shutter speed and lens aperture values which you then set manually on your camera.
The Lumu is not especially cheap at £118/US149$/AU$214, but it's very neat and comes with a case and lanyard, so it does have some style.