These are microscopic balls of tin, imaged under scanning electron microscope (SEM) at 2500x (about 25x more magnification than the strongest optical microscope can manage). These very cool-looking tin samples are good for looking at when you’re trying to calibrate the SEM to produce the best images possible - since the spheres come in all different sizes, you can start with the large spheres for rough calibration, since the tiny spheres are invisible if the microscope isn’t properly calibrated.

In the UCSD cleanrooms we used carbon tape and specks of dust for calibration, because they’re almost always part of our sample anyways, so we don’t have to change between the calibration sample and the sample we’re interested in - usually our SEM time slots are short, and returning the system to vacuum between samples takes too much time. But I can see how the spheres would be very nice, particularly for detecting astigmatism, an effect that would stretch the spheres into little footballs.

Tin Balls by FEI Company
Via Flickr:
Image of tin balls, which are used to calibrate scanning electron microscopes (SEMs). The image was taken at a tilt of 57 degrees. Courtesy of Mr. Daniel Oldfield , RMIT University Image Details Instrument used: Verios Magnification: 2500X Horizontal Field Width: 170um Voltage: 10kV Spot: 6.3pA Working Distance: 4.0 Detector: SE

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