PHP调整2个图像的图像亮度

I have two similar Images, one of it is brighter than the other.
I tried to get a Color or Gamma-Value to handle with -with no results. As endresult i would like to adjust both Image in same brightness/gamma.

<?php
$s1=new Imagick('01.jpg');
$s2=new Imagick('01.jpg');
$gamma=$s1->getImageGamma (); // Output "0" :-(
$s2->setImageGamma($gamma);
?>