The highest contrast ratio of the Acer S191HQLMb is 100000000:1. That's over five times the contrast ratio of a 20000000:1 PC screen, which is the most common contrast ratio.
This PC monitor's contrast ratio (100000000:1) is in fact the best contrast ratio amongst all flatpanel monitors that are a comparable age to the S191HQLMb.
Amongst comparable types of PC monitor the worst contrast ratio is Mega Contrast.
Other PC displays that have a 100000000:1 dynamic contrast ratio currently available that you might want to consider include cheaper PC displays such as the 192HQL (£59.99), the K192HQ, or the 193HQLHb all from Acer.