iPhone camera Brightness... How is it physically measured?

Question

I hope someone can answer this question, its been bugging me for a while now and I can't seem to get a solid answer.

How exactly does the iPhone measure scene brightness (or luminance if you prefer) through its camera? Does it measure it off the pixels on the sensor, or does it have dedicated in lens brightness sensor(s)?

Is there a way to read this raw brightness information other than through EXIF data?

Thanks in advance,

Chris

Answer 1

I have determined through experimentation that Luminance is measured off the camera sensor and not from a discrete sensor through the lens. Thanks to everyone who read and/or answered this question!

Kind Regards,

Chris

Answer 2

I don't know of a public API, but this shows a way to get it

http://b2cloud.com.au/tutorial/obtaining-luminosity-from-an-ios-camera

The suggestion at the end is to use this API

https://developer.apple.com/library/mac/#documentation/AVFoundation/Reference/AVCaptureVideoDataOutput_Class/Reference/Reference.html

To capture raw uncompressed image frames and use the luminosity function on them. To get EXIF, you need to bring up a camera and have the user take a picture.

If you do this, you'd be doing it by measuring pixels on the sensor. I have no idea how Apple does it, but I don't see any mention of a brightness sensor anywhere. There is an ambient light one used in the proximity sensor, but that's not the same.