ITunes provides different versions of light meter. Taken Lux Light Meter Pro as an example, the app's measurement depends on the front and the back camera. It can measure the instant interior light intensity and the exterior and also record the average and Max value.
The angle of the phone also affects actual readings. When the back camera is used, be sure that you can read the actual reading from the screen.
Overall, the phone apps is also an reasonable choice.
The Decibel X can take instant measurement and recording.
It also can share data and export
into Excel (in Pro version). A graph of value changing will be automatically generated.
So it's not appropriate in extremely quite environment.
However, It's a powerful and portable tool to measure sound level except for quiet environment.