Such filters are used to eliminate wavelengths that are originated from sources of light pollution, mostly various kinds of urban light. This means, that they are produced so that wavelengths of those malicious sources are cut down to a minimum, whereas other wavelengths of light are allowed to pass the filter. The result is a darker sky background, that permits faint details of nebulae to be observed. Unfortunately as no pain no gain, the filter also eliminates some of the starlight, therefore the limiting magnitude decreases slightly, therefore such filter will not help in observing faint galaxies under heavy light pollution skies.