Baader's UHC-L filter (1.25")is an effective response to the changing composition of light pollution.
Deep-light filters make it easier to see faint, diffuse fog spots and their details. The principle of their operation is quite simple: they only pass specific wavelength(s) of incoming light, precisely those that a particular object is emitting. This makes the object appear more contrasty, almost jutting out from the sky.
Baader's UHC-L filter has a relatively wide passband among UHC filters, so it does little to "hide" stars in the field of view. It is recommended for both visual and photographic use (optimized for CMOS). It is also effective in minimising light pollution from the now common LED lights and car headlights, while remaining effective against the harmful effects of conventional lights. It is also well suited for L-RGB photography, for L channel preparation, as it prevents both UV and IR wavelengths from passing through, so there is no need to use a separate UV/IR block filter.
The edges of the glass surfaces that make up the filter are sealed to Baader's standard and painted matte black, making the anti-reflective layers more protective.
The OIII filters transmit light in an even narrower band, which means they cut off even more of the light from background stars, making it easier to see details of diffuse nebulae.
The UHC version is recommended for telescopes under 20 cm diameter or as a first low-light filter.