umbra

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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.