PMTs do get dirty








These are nowhere near the dirtiest I’ve seen, but this is to demonstrate that even in a clean shop, over time, a haze forms on most of the optical components, especially those that face upward, like the photo-multipliers, the color filters, and the neutral density filters.  It’s important to get your scanner serviced by a Fachmann (someone who knows what they are doing)!  In this photo, one PMT has been cleaned.