the geometric (area) and optical (average brightness) characteristics of the background increase with the
surface roughness. Note that the luminescent background is hardly notable on the smooth surface (area 1)
even after the minimal time of sample immersion in cleansing liquid. On surfaces with the intermediate
roughness (areas 2 and 3) the luminescent background can be seen, but its intensity drops to a tolerable
value at immersion times of 1–1.5 min.