Optolong L-Quad Enhance Filter
A broadband light pollution filter for one-shot-colour cameras, intended for galaxies, star clusters and reflection nebulae, the targets where a dual narrowband filter is no help at all. Optolong themselves do not recommend it above Bortle 7, which makes it an interesting thing to test from a Bortle 8 balcony.
The filter for everything narrowband cannot help
A dual narrowband filter is transformative on emission nebulae and useless on everything else. Galaxies, globular clusters and reflection nebulae emit broadband starlight, so a filter that passes only two narrow windows throws away most of what you came for.
The L-Quad Enhance is the other tool. It is a broadband light pollution filter, designed to suppress the parts of the spectrum that street lighting occupies while leaving the continuum starlight largely intact.
Why the results are location-dependent
This is the part most reviews skip. A light pollution filter works by blocking the wavelengths your local lighting emits. Low-pressure sodium produces a narrow spike that is straightforward to reject. Broad-spectrum white LED does not, and much of its output sits exactly where you want starlight to pass.
That means a filter of this kind can be genuinely effective in one city and close to useless in another, depending entirely on what the local authority installed. A glowing review written under sodium lighting tells you very little about a sky lit by LED.
Realistic expectations
Broadband filters give smaller gains than narrowband filters do, because there is much less they can safely block. Expect a modest improvement in contrast and easier colour calibration, not a transformation. From a very bright sky, no filter substitutes for integration time.
They can also introduce a colour cast that needs correcting in processing, and on fast optics any interference filter's passband shifts slightly, which is worth knowing if you image at f/4 or faster.
Who it suits
Broadband imagers under moderately light-polluted skies, and anyone who already owns a dual narrowband filter and has found it does nothing for galaxies. Under dark skies, the honest advice is to skip it and spend the money on integration time or a better mount.