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Colour of objects
The upper disk and the lower disk have exactly the same objective colour, and are
in identical gray surrounds; based on context differences, humans perceive the
squares as having different reflectances, and may interpret the colours as
different colour categories; see same colour illusion.
The colour of an object depends on both the physics of the object in its
environment and the characteristics of the perceiving eye and brain. Physically,
objects can be said to have the colour of the light leaving their surfaces, which
normally depends on the spectrum of the incident illumination and the
reflectance properties of the surface, as well as potentially on the angles of
illumination and viewing. Some objects not only reflect light, but also transmit
light or emit light themselves (see below), which contribute to the colour also.
And a viewer's perception of the object's colour depends not only on the spectrum
of the light leaving its surface, but also on a host of contextual cues, so that
the colour tends to be perceived as relatively constant: that is, relatively
independent of the lighting spectrum, viewing angle, etc. This effect is known
as colour constancy.
Some generalizations of the physics can be drawn, neglecting perceptual effects
for now:
* Light arriving at an opaque surface is either reflected "specularly" (that is,
in the manner of a mirror), scattered (that is, reflected with diffuse
scattering), or absorbed – or some combination of these.
* Opaque objects that do not reflect specularly (which tend to have rough
surfaces) have their colour determined by which wavelengths of light they scatter
more and which they scatter less (with the light that is not scattered being
absorbed). If objects scatter all wavelengths, they appear white. If they absorb
all wavelengths, they appear black.
* Opaque objects that specularly reflect light of different wavelengths with
different efficiencies look like mirrors tinted with colours determined by those
differences. An object that reflects some fraction of impinging light and
absorbs the rest may look black but also be faintly reflective; examples are
black objects coated with layers of enamel or lacquer.
* Objects that transmit light are either translucent (scattering the transmitted
light) or transparent (not scattering the transmitted light). If they also
absorb (or reflect) light of varying wavelengths differentially, they appear
tinted with a colour determined by the nature of that absorption (or that
reflectance).
* Objects may emit light that they generate themselves, rather than merely
reflecting or transmitting light. They may do so because of their elevated
temperature (they are then said to be incandescent), as a result of certain
chemical reactions (a phenomenon called chemoluminescence), or for other reasons
(see the articles Phosphorescence and List of light sources).
* Objects may absorb light and then as a consequence emit light that has
different properties. They are then called fluorescent (if light is emitted only
while light is absorbed) or phosphorescent (if light is emitted even after light
ceases to be absorbed; this term is also sometimes loosely applied to light
emitted due to chemical reactions).
For further treatment of the colour of objects, see structural colour, below.
To summarize, the colour of an object is a complex result of its surface
properties, its transmission properties, and its emission properties, all of
which factors contribute to the mix of wavelengths in the light leaving the
surface of the object. The perceived colour is then further conditioned by the
nature of the ambient illumination, and by the colour properties of other objects
nearby, via the effect known as colour constancy and via other characteristics of
the perceiving eye and brain.
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A |
C |
K |
G |
R |
O |
U |
N |
D |
|
C |
O |
L |
O |
U |
R |
RUN CURSOR OVER
COLOUR TO CHANGE PAGE BACKGROUN COLOUR |
Colour of objects
The upper disk and the lower disk have exactly the same objective colour, and are
in identical gray surrounds; based on context differences, humans perceive the
squares as having different reflectances, and may interpret the colours as
different colour categories; see same colour illusion.
The colour of an object depends on both the physics of the object in its
environment and the characteristics of the perceiving eye and brain. Physically,
objects can be said to have the colour of the light leaving their surfaces, which
normally depends on the spectrum of the incident illumination and the
reflectance properties of the surface, as well as potentially on the angles of
illumination and viewing. Some objects not only reflect light, but also transmit
light or emit light themselves (see below), which contribute to the colour also.
And a viewer's perception of the object's colour depends not only on the spectrum
of the light leaving its surface, but also on a host of contextual cues, so that
the colour tends to be perceived as relatively constant: that is, relatively
independent of the lighting spectrum, viewing angle, etc. This effect is known
as colour constancy.
Some generalizations of the physics can be drawn, neglecting perceptual effects
for now:
* Light arriving at an opaque surface is either reflected "specularly" (that is,
in the manner of a mirror), scattered (that is, reflected with diffuse
scattering), or absorbed – or some combination of these.
* Opaque objects that do not reflect specularly (which tend to have rough
surfaces) have their colour determined by which wavelengths of light they scatter
more and which they scatter less (with the light that is not scattered being
absorbed). If objects scatter all wavelengths, they appear white. If they absorb
all wavelengths, they appear black.
* Opaque objects that specularly reflect light of different wavelengths with
different efficiencies look like mirrors tinted with colours determined by those
differences. An object that reflects some fraction of impinging light and
absorbs the rest may look black but also be faintly reflective; examples are
black objects coated with layers of enamel or lacquer.
* Objects that transmit light are either translucent (scattering the transmitted
light) or transparent (not scattering the transmitted light). If they also
absorb (or reflect) light of varying wavelengths differentially, they appear
tinted with a colour determined by the nature of that absorption (or that
reflectance).
* Objects may emit light that they generate themselves, rather than merely
reflecting or transmitting light. They may do so because of their elevated
temperature (they are then said to be incandescent), as a result of certain
chemical reactions (a phenomenon called chemoluminescence), or for other reasons
(see the articles Phosphorescence and List of light sources).
* Objects may absorb light and then as a consequence emit light that has
different properties. They are then called fluorescent (if light is emitted only
while light is absorbed) or phosphorescent (if light is emitted even after light
ceases to be absorbed; this term is also sometimes loosely applied to light
emitted due to chemical reactions).
For further treatment of the colour of objects, see structural colour, below.
To summarize, the colour of an object is a complex result of its surface
properties, its transmission properties, and its emission properties, all of
which factors contribute to the mix of wavelengths in the light leaving the
surface of the object. The perceived colour is then further conditioned by the
nature of the ambient illumination, and by the colour properties of other objects
nearby, via the effect known as colour constancy and via other characteristics of
the perceiving eye and brain.
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