使用opencv如何获得图像的第二亮区域？

I'm currently trying to detect both; the cup and the disc of an image of Glaucoma. They way these two stand out from the rest of the image is they're brighter than the rest of the image. But the cup is brighter than the disc, so I'm only able to detect the cup. What I want to achieve is, to encircle both; the cup and the disc with two different circles. The cup being the brightest of the image, and disc the second brightest. How do I ago about doing it?I'm currently trying to detect both; the cup an