The Cokin Clearsky Light Pollution filter targets and blocks a specific yellow/orange area of color that common street lamps emit. The sodium and mercury vapor lamp spectrums are bright and travel great distances which makes them great for lighting up your streets at night but a nightmare for astro and cityscape photographers.  That yellow/orange glow can be seen many miles from the source and becomes even more apparent in long-exposure astro-photographs. Unfortunately, a cameras white-balance cannot target this orange glow without affecting other colors.

The Clearsky filter blocks that small area of the color-spectrum before it ever reaches your camera. This allows you to capture more accurate colors in camera and avoid wasting time trying to color correct your images in post-processing.