"Manifestation" of glazing has been in the building regulations since 1991 or earlier. It was in Part N, but is now in
Part K: protection from falling, collision and impact. This requirement does not apply to dwellings. Here is section 7:
Section 7: Manifestation of glazing
Critical locations
7.1 Critical locations (see paragraph 5.1) include large uninterrupted areas of transparent glazing which form, or are part of, the internal or external walls and doors of shops, showrooms, offices, factories, public or other non-domestic buildings.
7.2 The risk of collision is greatest when two parts of the building, or the building and its immediate surroundings, are at the same level but separated by transparent glazing and people may think they can walk from one part to the other.
Permanent methods to indicate glazing, and alternative methods
7.3 People moving in or around a building might not see glazing in critical locations and can collide with it. To avoid this one of the following should be adopted.
a. Use permanent manifestation to make glazing apparent (see paragraph 7.4).
b. Use alternative indications of glazing, such as mullions, transoms, door framing or large pull or push handles (see Diagram 7.1).
7.4 Provide glass doors and glazed screens (including glazed screens alongside a corridor) with all of the following.
a. Manifestation at two levels, as shown in Diagram 7.2.
b. Manifestation that will contrast visually with the background seen through the glass, both from inside and outside, in all lighting conditions.
c. Manifestation in the form of a logo or sign, a minimum of 150mm high (repeated if on a glazed
screen), or a decorative feature such as broken lines or continuous bands, a minimum of 50mm high.
d. Where glazed doors are beside or part of a glazed screen, they are clearly marked with a high-contrast strip at the top and on both sides.
e. Where glass doors may be held open, they are protected with guarding to prevent people colliding with the leading edge.
Part of that can be read as saying "put something visible across the width of the window, at suitable heght(s), but avoid narrow lines". There are pictures to help explain this.
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