Derfor er man nødt til at indføre et Bessel-filter, således at filterresponstiden (tF) sammen med den fysiske (tp) og elektriske (te) responstid af det enkelte opacimeter resulterer i den ønskede samlede responstid (tAver).
Therefore, a Bessel filter has to be created in such a way, that the filter response time (tF) together with the physical (tp) and electrical response time (te) of the individual opacimeter must result in the required overall response time (tAver).