How can I highlight white blobs in a dark image without for loop?

Question

I have a task of highlighting UV marks from a dark image (UV marks light up). I have achieved it using a for loop. But I must do it in a very short time. I have tried colormatrix, but I didn't see a threshold so that I can assign a value black if the pixel value is less than say, 200 and white above it, while using the class.

How do I detect UV marks from a dark image?