What is the best feature to describe the illumination condition in image?

Questions
1

I am trying to analysis the illumination condition(i.e. exposure) in the image.

How can i get the right camera exposure value from camera live image.

I have already pre-collected image database according to the various exposure setting.

Thanks in advance.

2

Are you adjusting the exposure to get better brightness or contrast? Have you tried adjusting your images with ImageAdjust? Can you give a simple example of the problem you are seeing?

3

I am controlling the camera exposure value dynamically for face recognition.
I got the proper exposure values for face recognition according to the light condition.
So, it’s left to analysis the light condition from image.
Thanks for your caring.