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The standard way of handling this variation is by performing a normalization of the lightning.

• The easiest way is to use cv::equalizeHist / cvEqualizeHist. This will equilize the contrast over the input images.
• Gamma correction can be done using cv::pow / cvPow
• Difference of Gaussian approach can be done with cv::gaussianBlur / cvSmooth to get two different images then use cv::sub / cvSub to get the Difference of Gaussian.

But there are many other ways (including a Contrast Stretch, a Log filter, CLAHE, or even White Color Balancing or Retinex), that can work better or worse depending on your opinion and your project.

Some guidance:

• http://opencv.itseez.com/modules/imgproc/doc/histograms.html#equalizehist
• http://opencv.itseez.com/modules/core/doc/operations_on_arrays.html#pow
• http://movidius.tumblr.com/post/3704283839/opencv-difference-of-gaussians