What are those tiny circles on fan blades of Trent 1000?
These are certainly targets for an optical CMM (coordinate measuring machine) using laser, structured light or stereoscopy. The targets help define common points on the model from one scan to the next.
@mins linked a perfect example of this in his comment below, from which this image is taken. When doing 3D scanning, you want the dots in a random pattern as in the OPs image, rather than the uniform pattern seen below. This helps the identify and match the dots when going from one scan to the next (called &quot;tie-points&quot;). For the vibration analysis process linked by @mins, the requirements are probably different as the camera doesn't move, so it makes sense to place the dots in a grid pattern.