meaning of a phrase
As part of a European Union environmental programme, a London council is testing an infra-red spectrometer from the University of Denver in Colorado. It gauges the pollution from a passing vehicle-more useful than the annual stationary test that is the British standard today – by bouncing a beam through the exhaust and measuring what gets blocked.

What does "gets blocked" mean here?
Thank you!

Question

meaning of a phrase
As part of a European Union environmental programme, a London council is testing an infra-red spectrometer from the University of Denver in Colorado. It gauges the pollution from a passing vehicle-more useful than the annual stationary test that is the British standard today – by bouncing a beam through the exhaust and measuring what gets blocked.

What does "gets blocked" mean here?
Thank you!

Answer

gets blocked - is stopped
The beam will show the level of 'pollution' that has been blocked. 
The beam will show what level of 'pollution' is not getting through the exhast.
(this statement is not written too clearly so that is what makes it harder to translate:)

Answer

It just means that if an infrared beam passes though the exhaust fumes it will bounce off any particles it runs into. If there's less pollution then there should be less rays bouncig off particles and hence most will pass through the exhaust fumes.