Colour is a perceptual experience, not a
directly measurable physical phenomena.
Of course there are underlying physical phenomena creating our
but we can experience the same physical phenomenon differently under
circumstances. Similarly we may experience two different physical
as appearing the same. In general, there is no one-to-one mapping
the spectrum of the light reflected into our eyes and the colour as we
A fundamental assumption is that humans
exhibit colour vision to give
us information about the surface properties of objects. The stability
reliability of colour information is important to our understanding of
the world. The problem of colour stability arises because the light
our eyes from an object is the product of the object's surface
and the spectrum of the light illuminating the object.
We do not have direct access to the
properties of the incident light,
so somehow we must estimate them from the light we observe from the
To make matters worse, our eyes only measure the spectrum at extremely
low resolution. Light energy is a continuous phenomenon that the human
visual system samples over the wavelength range of approximately 300 -
700 nanometres using three sensors. Thus, the eye's output signal is a
discrete triplet of colour values.
All commonly used imaging systems, from
digital cameras, to scanners
and CRTs, use a similar 3 band colour encoding scheme since their
is to provide input to the human visual system. One consequence of this
colour encoding scheme is that there are many spectra that can lead to
the same triplet of responses. Such spectra are called metamers,
and reflect the fact that the relationship between spectra and sensor
is many-to-one. We cannot perfectly reconstruct the original spectrum
its reduced counterpart, because information has been lost that cannot
be recovered. There is no hope of recovering the precise surface
properties of surfaces in a scene from a 3 band image of that scene.
is there any hope of recovering the exact spectrum of the light
the scene. The best that we can expect is a 3 parameter specification
the reflectance and illumination properties.
Another problem is that a change in the
illumination can lead to a very
different sensor triplet response, even though the surface properties
the illuminated object remain the same. This makes identification of
using just sensor triplet information unreliable. Yet the human visual
system, using a 3 band encoding of colour, exhibits a remarkable
to changes in incident illumination.