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.
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.
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."
Fr.: interférométrie des tavelures
A technique for generating a clear composite image of a celestial object blurred by → atmospheric turbulence in which a large number of short-exposure photographs are mathematically correlated by a computer. By comparing the behavior of the → speckles in a series of images it is possible to approach the theoretical resolution of the telescope.
→ speckle; → interferometry.
Fr.: durée de vie de tavelures
The time scale on which a stellar image changes significantly due to → atmospheric turbulence. It is proportional to the ratio r0/Δv, where r0 is the → Fried parameter and Δv the standard deviation of the distribution of wind velocities weighted by the turbulence structure coefficient. Typical lifetimes in the visible range from about 3 to 30 milliseconds.
Omr "life-time;" from Ar. 'umr; pakâl, → speckle.
Fr.: bruit de tavelures
An image defect associated with the → speckle phenomenon.