An oculus is a circular opening or round window in a wall, facade or dome. The word is Latin for “eye”. An oculus brings daylight into places where an ordinary window would be structurally awkward or visually intrusive.
The classic case: the dome
The most famous oculus sits at the crown of the Pantheon dome in Rome. It is not glazed: rain genuinely falls through it, and the floor below has drains. Structurally it is a clever move — the crown of a dome is where the masonry goes into tension anyway, and the opening saves considerable weight at the most critical point.
Where you find an oculus
- In a dome — as a light opening at the crown, with or without a lantern above it
- In a facade or gable — a round window above a door or beneath a pediment
- In a church front — as a forerunner of, or a simpler alternative to, the rose window
- In a dormer or parapet — as a small round light
- In a vault bay — as a light opening between two spaces
Oculus and related windows
| Term | Form |
|---|---|
| Oculus | Round window or circular opening |
| Oeil-de-boeuf | Small round or oval window, often in a roof or dormer |
| Rose window | Large circular Gothic window with stone tracery |
| Lantern | Glazed structure raised over a dome |
Points to watch in new build and renovation
- Water shedding — a round frame has no natural sill; provide a proper drainage profile or a drip under the frame
- Detailing around it — in masonry a round frame requires a curved brick arch or a precast concrete surround; this is specialist work and priced accordingly
- Glazing — round insulating glass is a bespoke item and considerably dearer than rectangular glass of the same area
- Heat loss — in a small round window the edge length is large relative to the glass area, so the linear thermal bridge at the edge seal weighs heavily
Related terms
- Dome
- Rose window
- Round arch
- Roof window
- Rooflight
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