The primary function of the app is to be a light meter. It works by using the camera located in the front of your iOS device to approximate illuminance (in LUX).
You can record a sequence of illuminance values which you can save and review at a later point. This feature is useful when working together with a partner in order to record illuminance values at the same time in two different locations. This data could then be used to calculate the Daylight Factor which is useful for architects and interior designers trying to assess the lighting levels inside a building.
If you wish to get better accuracy when measuring illuminance, the app also provides support for connecting an external light sensor produced by Lumu (http://lu.mu/). This sensor simply connects to your iOS device via the audio jack, and provides accuracy comparable to a professional light meter.