penumbra

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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.

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]

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]

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.

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]

penumbra

a blurred edge to an image, a halo effect, in an x-ray film caused usually by an overlarge focal spot exacerbated by a long object-to-film distance.