White circles extraction from a little darker background
Follow the following steps:

Step 1: use an averaging filter to minimise the noise present in the image

Step 2: threshold the image such that pixels with RGB values within [0, 0, 0] and [1, 1, 1] are set to 0 and rest as 1. This step will lead you to generate a binary image.

Step 3: use Hough transform in OpenCV to detect circles. Tune the parameters of the Hough transform to get your desired output.

Check this out for more details of Hough transform for circles: https://docs.opencv.org/2.4/doc/tutorials/imgproc/imgtrans/hough_circle/hough_circle.html