CLS filters are broadband filters, which means they allow wide incident light wavelength ranges to pass through. They block light only in selected sections of the spectrum. These blocked sections are chosen in such a way that they coincide with the emissions from artificial light sources. In addition to artificial sources, there are also natural sky brightness phenomena, which are also blocked by CLS filters. Ultimately, the sky background appears darker with a CLS filter. The object being observed stands out more clearly against the background.
The figure below illustrates the principle using the example of the NGS1 filter from IDAS. The black transmission curve shows which wavelength ranges the filter allows to pass through. 100% would mean no filter effect at all. The transmission curve also shows which light wavelength ranges are blocked by the filter. These ranges are associated with the emissions from sodium-vapour lamps and other disruptive light sources. The CLS filter filters out this unwanted light.
The CLS filter allows broad ranges to pass through and is therefore also suitable for small optics.