speckle پَکال pakâl Fr.: tavelure Optics:
An image defect, one of a large number of bright and dark spots, that
appears when an object is illuminated by monochromatic, highly
→ coherent light.
This phenomenon results from the → interference
of a number of randomly phased complex contributions of electromagnetic
→ wavefronts scattered from an object with
rough structure, such as a piece of paper, a display screen, or a metallic surface.
In particular, whenever the object is rough on the scale of an optical
wavelength, the image has a grainy appearance.
Also called speckle noise.
Astro.: The pattern produced by a short-exposure image of a
→ point source, such as a star,
when the → wavefront is torn apart
under the effect of the → atmospheric turbulence.
Speckles change very rapidly with time as a function of the atmospheric turbulence.
→ speckle lifetime.
Long exposure images of these changing speckle patterns result in a blurred image of
the star, called a → seeing disk.
→ Fried parameter.
Etymology (EN): Speckle “a speck or small spot, as a natural dot of color on skin, plumage, or
foliage,”
from M.E.speck (from O.E. specca “small spot, stain,”
of unknown origin; probably related to Du. speckel “speck, speckle”) +
-le a noun suffix having originally a diminutive meaning. Etymology (PE): Pakâl, from pak “spot” (Lâri, Gerâši), pašy
“mingled, confused” (Tâleši),
probably related to
pisé “dappled, variegated,” pis, pisi “leprosy,”
neveštan “to write,” pišé “profession,”
→ professional astronomer;
Mid.Pers. parš “speckled, spotted,” pēsīdan “to color, adorn,”
pēsit “adorned;”
O.Pers. pais- “to adorn, cut, engrave;”
Av. paēs- “to paint, adorn,”
paēsa- “adornment;”
cf. Skt. peś- “to adorn, hew out, decorate,”
piśáti “adorns; cuts;” Gk. poikilos “multicolored;”
L. pingit “embroiders, paints;” O.C.S. pisati “to write;”
O.H.G. fēh “multicolored;” Lith. piēšti “to draw, adorn;”
PIE base *peik- “colored, speckled.” |
