To Tone Balance with Average Mode

From the Raster Manager Tools menu, choose Image>Mosaic
The Mosaic toolbox opens
In the Mosaic toolbox, select the Balance Tone tool
From the Tools Settings box, set:
Mode: Average

Source Area: Entire Images

Note: If the images you want to process are already selected (multiple-selection) – proceed to step 6.
Select the first image by pointing at the image and enter a data point.
Repeat step 4 for remaining images to be balanced.
Reset to accept the image selection set.
Enter a data point to process the image selection set.
The images from the selection set are now balanced.