Such instruments are dual channel, with manganese peroxide used to scrub the ozone from one channel, so providing a reference zero for the other channel. The instrument can be arranged so that it switches the channels periodically too. So yes, it provides the blank you mention. If you do a google scholar  or web of science search on authors M H Proffitt and R J MacLaughlin, you'll find the authoritative reference.

OK, i understood, thanks a lot, I will check the references you suggested.

"Manganese peroxide" is an incorrect scrubber compound in my answer - it should have been manganese dioxide.

Thanks once more