Keeping particles in the right range is very important, since larger or smaller particles will tend to get captured more easily by the filter, making it appear better, in the test results, than it really is. To verify that the particles remain in the right range throughout the testing process, it is important to measure them somehow.
Measuring the sizes of these tiny particles is another big challenge for laboratories, because 75 nm is such a small size that objects that size cannot be seen, even with a world-class optical microscope.
This is because light waves themselves are closer to 500 nm in size, and do not interact in any usual way with the particles.
To address this issue, engineers have invented a variety of special tools and measurement devices that can pick out particles of different sizes and even count them!
This picking and counting can be a complicated process on its own but, in the end, is the most accurate known way to ensure the efficiency of masks.