Given all the comments, I'm surprised to see no one mention flat field images and flat field correction. This would be the proper way to measure and even correc

Given all the comments, I'm surprised to see no one mention flat field images and flat field correction. This would be the proper way to measure and even correct for blemishes and dust like this. Basically a flat field image is an image of a completely uniform illumination pattern (i.e. given a perfect sensor and lens with no vignetting all pixels would have the same value). This lets you measure all sorts of things, but comparing flat field images of before and after dust/tape were applied would actually highlight the exact effect. Using flat field correction, you can even correct for these issues to a great extent by scaling the pixels as appropriate to basically account for their being light paths between the object in the scene and the image projected onto the sensor.