From The Collaborative International Dictionary of English v.0.48 [gcide]:
Spectrum \Spec"trum\, n.; pl. {Spectra}. [L. See {Specter}.]
1. An apparition; a specter. [Obs.]
[1913 Webster]
2. (Opt.)
(a) The several colored and other rays of which light is
composed, separated by the refraction of a prism or
other means, and observed or studied either as spread
out on a screen, by direct vision, by photography, or
otherwise. See Illust. of {Light}, and {Spectroscope}.
(b) A luminous appearance, or an image seen after the eye
has been exposed to an intense light or a strongly
illuminated object. When the object is colored, the
image appears of the complementary color, as a green
image seen after viewing a red wafer lying on white
paper. Called also {ocular spectrum}.
[1913 Webster]
{Absorption spectrum}, the spectrum of light which has passed
through a medium capable of absorbing a portion of the
rays. It is characterized by dark spaces, bands, or lines.
{Chemical spectrum}, a spectrum of rays considered solely
with reference to their chemical effects, as in
photography. These, in the usual photogrophic methods,
have their maximum influence at and beyond the violet
rays, but are not limited to this region.
{Chromatic spectrum}, the visible colored rays of the solar
spectrum, exhibiting the seven principal colors in their
order, and covering the central and larger portion of the
space of the whole spectrum.
{Continous spectrum}, a spectrum not broken by bands or
lines, but having the colors shaded into each other
continously, as that from an incandescent solid or liquid,
or a gas under high pressure.
{Diffraction spectrum}, a spectrum produced by diffraction,
as by a grating.
{Gaseous spectrum}, the spectrum of an incandesoent gas or
vapor, under moderate, or especially under very low,
pressure. It is characterized by bright bands or lines.
{Normal spectrum}, a representation of a spectrum arranged
upon conventional plan adopted as standard, especially a
spectrum in which the colors are spaced proportionally to
their wave lengths, as when formed by a diffraction
grating.
{Ocular spectrum}. See {Spectrum}, 2
(b), above.
{Prismatic spectrum}, a spectrum produced by means of a
prism.
{Solar spectrum}, the spectrum of solar light, especially as
thrown upon a screen in a darkened room. It is
characterized by numerous dark lines called Fraunhofer
lines.
{Spectrum analysis}, chemical analysis effected by comparison
of the different relative positions and qualities of the
fixed lines of spectra produced by flames in which
different substances are burned or evaporated, each
substance having its own characteristic system of lines.
{Thermal spectrum}, a spectrum of rays considered solely with
reference to their heating effect, especially of those
rays which produce no luminous phenomena.
[1913 Webster]
From WordNet (r) 3.0 (2006) [wn]:
spectrum
n 1: an ordered array of the components of an emission or wave
2: a broad range of related objects or values or qualities or
ideas or activities
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