Photoplot film density is calibrated to achieve optimal optical density in dark areas while keeping clear zones
https://www.newsbreak.com/dmr-news-321522575/4524631068041-artnet-pro-sets-the-standard-in-high-resolution-pcb-photoplotting-for-aerospace-and-advanced-electronics
Photoplot film density is calibrated to achieve optimal optical density in dark areas while keeping clear zones transparent, maximizing contrast during UV exposure of dry film or liquid photoresist.