Small distance measuring

Hi,
I'll build an XY table and on Z axis I'll move an printing head - as a sword of serigrafic machine.
But distance from head nossels to material to be printed should be approx 1mm.
What tehniques could I use in order to measure whith precision such a small distance ?
The only method that I know for measuring distances it is with ultrasonic emiter-receiver but it seems that this is for bigger distances.
I have seen on an laser engraving machine that optical head is moving up and down pretty precisely what method they are using ?
Thanks,
Daniel


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