Выражение встречается в следующем контексте:
письмо. что-то о телескопах :)

Заранее спасибо

а может это?
Light Pollution (LPR) Filters are designed to suppress the common emission lines generated by artificial lighting, yet allow the important nebula emission lines to pass, thus enhancing the contrast of astronomical objects, particularly emission nebulae. For both visual and photographic use, Light Pollution (LPR) Filters are most useful under light-polluted skies, from where it reveals faint objects like star clusters, galaxies, nebulae and comets by darkening the background of space.

поройтесь здесь
http://www.universetoday.com/guide-to-space/telescopes/telescope-filters/