Design Nomenclature / Light & Optics
Antumbra
The region beyond the umbra's apex, where the occluder sits entirely inside the source's disc and is ringed by light on every side. It exists only where the blocker's angular size is smaller than the source's — the geometry that makes an annular eclipse rather than a total one.
Not to be confused with
- UmbraThe part of a shadow from which the source is completely hidden — no direct light arrives from any point on the emitter. With a finite source it is the fully dark core; with an ideal point source it is the entire shadow, which is why point lights give shadows a knife edge.
- PenumbraThe graded margin of a shadow from which part of the source is visible and part is blocked. Its width grows with the source's angular size and with the distance from occluder to receiver — the whole of what is called a soft shadow is simply this region being wide.
Origin
Latin ante (before) and umbra. The newest of the three, coined in the twentieth century to name what lies past the cone's tip.