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From Sam Chui's video, there are tiny circles on the surface of three fan blades of next generation Trent 1000 engine. Why aren't they present on remaining fan blades? Below comments suggest they are for optical purposes. What optical information can be inferred from these circle?

My prediction: They are embedded explosive charger to test explosion testing(specifically Blade off testing of an engine.

enter image description here

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  • $\begingroup$ At about 14:11 in the linked video the engineer talks about installing an explosive bolt to test engine containment. He's not referring to the small circles, so whatever they are for, it's not that! $\endgroup$ – CatchAsCatchCan May 20 '20 at 12:01
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    $\begingroup$ Explosive charges would be at the root of the blade, not distributed of the entire Surface. They could be static pressure measurement points or strain gauges. But I suspect they are markers for an optical measurement system. $\endgroup$ – DeltaLima May 20 '20 at 12:01
  • $\begingroup$ my guess is that they're nails to keep the blades in place with the frame, but I'm not sure. $\endgroup$ – Air Canada 001 May 20 '20 at 14:23
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    $\begingroup$ @DeltaLima since they are on both the composite painted surfaces and the stainless leading edges I'm going with stickers for some optical purpose also. $\endgroup$ – John K May 20 '20 at 14:35
  • $\begingroup$ I'm guessing that they are all on a slightly different radius. That would make it easier for a laser to focus on one at a time and so measure defomation/vibration under load at that point. $\endgroup$ – Martin James May 24 '20 at 6:25
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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 "tie-points"). 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. Source https://www.gom.com/metrology-systems/aramis.html

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    $\begingroup$ @eleventyone: I think the poster means "It is 100% certain that these are targets..." $\endgroup$ – TonyK Aug 8 '20 at 11:35
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    $\begingroup$ So do I, but what I think is irrelevant. $\endgroup$ – eleventyone Aug 8 '20 at 11:36
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    $\begingroup$ Likely used with ARAMIS sensors and software for deformation/vibration analysis. $\endgroup$ – mins Aug 8 '20 at 13:30
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    $\begingroup$ Before anyone comments that they seem to be randomly placed: the "randomness" makes it possible to identify each blade from a scan. The blades are also numbered, but this pattern variation is another blade indentification method. $\endgroup$ – Jpe61 Aug 8 '20 at 17:30
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    $\begingroup$ You updated you aswer just as i posted my comment 😂 $\endgroup$ – Jpe61 Aug 9 '20 at 18:28

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