penumbra
[pĕ-num´brah] 1. the part of a shadow in which there is a small amount of illumination from a light source.
2. blurring at 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.
pe·num·bra
(pe-nŭm'bră), The region of partial illumination or radiation caused by light or x-rays not originating from a point source; also called geometric unsharpness.
[Mod. L., fr. L. paene, almost, + umbra, shadow]
Farlex Partner Medical Dictionary © Farlex 2012
pe·num·bra
(pĕ-nŭm'bră) 1. radiology The blurred margin of an image.
2. radiation physics The region at the edges of a radiation beam over which a rapid change in dosage rate occurs.
Synonym(s):
geometric unsharpness.
[Mod. L., fr. L. paene, almost, + umbra, shadow]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012
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
pe·num·bra
(pĕ-nŭm'bră) 1. radiology the blurred margin of an image.
2. radiation physics the region at the edges of a radiation beam over which a rapid change in dosage rate occurs.
Synonym(s):
geometric unsharpness.
[Mod. L., fr. L. paene, almost, + umbra, shadow]
Medical Dictionary for the Dental Professions © Farlex 2012