problem from the Ash thread of differentiating between dense ash and light Ash
Exactly. 'Light' in your categories means 'not visible' and unfortunately probably not visualised by the IR system discussed. But where do the current limits (2000 and 4000 microgrammes per cu metre) fit in? It's my understanding that even at the 4000 level, the cloud would still neither be easily visible nor detectable by IR. Presumably under optimum viewing conditions (eg. daylight, sun behind aircraft and shining towards VA cloud, good visibility, perhaps even contrasting cloud beyond the VA, plus a fairly thick layer of VA and/or viewing from slightly above / below) a cloud at 4000 microgrammes
might be visible as a darker line or area. But once conditions become less optimal, a low-density cloud will not be visible by eye. Whether the Easyjet IR system can get useful data under poor viewing conditions is another matter. As far as has been publicly revealed so far, it's never been tested using a horizontal view of a VA cloud at low densities - only relatively close to erupting volcanoes where the ash density would be order(s) of magnitude greater.