umbra

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Related to umbras: umbral

umbra

 [um´brah]
the part of a shadow in which there is no light from any light source.
a sharp appearance to the edges of a structure on a radiograph.

um·bra

(ŭm'bră)
radiology An image with sharply defined margins.

umbra

(ŭm′bră) [L., shade, shadow]
The edge of the radiographic image proper.

penumbra 

1. Region of very low illumination on a dark background.
2. Zone in which the brightness varies from some illumination to zero (umbra) in the shadow cast by an opaque object intercepting light from an extensive light source. See shadow.
References in periodicals archive ?
Instead, it occurs because light that would otherwise occupy the innermost penumbra has been refracted by the atmosphere into the umbra.
If there were no ozone the outer umbra would appear lighter overall, the change from penumbra to umbra would be more gradual, and the maximum in the brightness gradient would shift outward.
Could Horace have meant with this word to recall the adjective grauis that Vergil uses twice of umbra in the lines here considered (75, 76)?
It's been a pleasure working with Remedy on Alan Wake," said Marko Nurro, Umbra Software's CEO.