The common name derives from the leaves which are very thin, only a single cell thick, and translucent, giving the appearance of a wet film.

In contrast to the similar H. wilsonii the fronds are more divided, flattened, appressed to the substrate and tend to have a bluish tint.

Up to 5–10 purse-shaped sori are produced per frond, each covered by two strongly convex, flattened indusial valves.

In common with all ferns, H. tunbrigense exhibits a gametophyte stage in its life cycle (alternation of generations) and develops a haploid reproductive prothallus as an independent plant.

Gametophytes of the related Killarney fern (Trichomanes speciosum) have been found outside the geographical range of the sporophyte[6] and this may prove to be true of Hymenophyllum tunbrigense.

[9] Apparently worldwide, but discontinuous, in distribution but most frequent in western European oceanic regions like the British Isles.

In continental Europe it occurs much further east than H. wilsonii but these sites are very disjunct and may represent relict populations from a different climatic period.

The specific name is derived from Royal Tunbridge Wells in Kent, England which was one of its disjunct eastern localities in Britain.

Comparative Ecophysiological Measurements on the Light Responses, Water Relations and Desiccation Tolerance of the Filmy Ferns Hymenophyllum wilsonii Hook.