The ZWO Duo-Band Narrowband Filter (1.25") is a two-in-one filter, as it passes some wavelengths of light through two narrow bands while blocking other wavelengths.
This filter was designed specifically for ZWO's colour cameras and is 5 mm thick. It allows astrophotography of emission nebulae in light-polluted urban conditions. The filter effectively blocks moonlight and light pollution, while providing 80% and 90% light transmission at Hα (656.3 nm) and OIII (500.7 nm) wavelengths.
The excellent choice of materials and machining precision that ZWO is known for are, of course, also a basic requirement for this product.