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.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

um·bra

(ŭm'bră)
radiology An image with sharply defined margins.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

umbra

(ŭm′bră) [L., shade, shadow]
The edge of the radiographic image proper.
Medical Dictionary, © 2009 Farlex and Partners

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.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann