The umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.
These names are most often used to refer to the shadows cast by celestial bodies.
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The umbra (Latin for "shadow") is the darkest part of the shadow, where the light source is completely blocked by the occluding body. An observer in the umbra experiences a total eclipse.
The penumbra (from the Latin paene "almost, nearly" and umbra "shadow") is the region in which only a portion of the light source is obscured by the occluding body. An observer in the penumbra experiences a partial eclipse. An alternative definition is that the penumbra is the region where some or all of the light source is obscured (i.e., the umbra is a subset of the penumbra). For example, NASA's Navigation and Ancillary Information Facility defines that a body in the umbra is also within the penumbra.[1]
In legal terminology, a penumbra is a grey area where a piece of legislation has no clearly defined meaning.
In radiation oncology, the penumbra is the space in the periphery of the main target of radiation therapy, and has been defined as the volume receiving between 80% and 20% of isodose.[2]
In medical neurology, an ischemic penumbra is a transitional perimeter that surrounds the core area of the brain where an ischemic stroke has occurred.[3] The cells in the penumbra are still viable but metabolically lethargic,[3] resulting in non-functional neurons with potential to be salvaged.[4]
The antumbra is the region from which the occluding body appears entirely contained within the disc of the light source. If an observer in the antumbra moves closer to the light source, the apparent size of the occluding body increases until it causes a full umbra. An observer in this region experiences an annular eclipse, in which a bright ring is visible around the eclipsing body.