Page 17 - Computational Colour Science Using MATLAB

4 INTRODUCTION
Programmers who wish to use languages other than MATLAB may wish to
create their own version of the backslash operator in order to easily translate the
code within this book. All the MATLAB code contained within this book can be
downloaded from http://www.colourware.co.uk/compute/ and from
http://www.mathworks.com/matlabcentral/.

1.3 A brief review of the CIE system of colorimetry

Light is a term that we use to describe that range of wavelengths (approximately
380–780 nm) of electromagnetic radiation to which the human visual system is
sensitive. When we observe the light reflected from surfaces in a scene, or when
we look directly at the light emitted by light sources, we experience the sensation
of colour. Colour is just one attribute of a complex and not fully understood set
of properties that define the appearance of the world. Surfaces interact with light
in a complex and varied way that includes the processes of absorption,
scattering, refraction and diffraction, but it is the light that is reflected by the
surfaces in a scene that we use to identify those surfaces by their colour. The
reflectance properties of surfaces can be defined by the spectral reflectance
factors that are normally measured at regular intervals in the visible spectrum of
radiation. Typical reflectance spectrophotometers are able to measure the
reflectance factors at intervals of 10 nm in the range 400–700 nm (though some
instruments extend their measurement to shorter or longer wavelengths).
Reflectance factors are normally in the range 0–1 and represent the proportional
amount of light reflected in each wavelength interval. The light that we see when
we look at a point in a scene clearly depends upon the spectral power distribution
of the illuminating source and the reflectance properties of the surface at that
point. Our visual systems detect the reflected light using the light-sensitive sense
organs or retinas that form the inner lining of the back of the eyeball. Light
enters the eye through the pupil and is focused onto the retina by the lens. The
human retina consists of a mosaic of specialized cells called rods and cones that
contain pigments that respond to light. The chemical changes that take place
when the visual pigments in the rods and cones absorb light initiate electrical
impulses that are subsequently processed by a neural network of brain cells and
which eventually lead to the excitation of cells in various specialized areas of the
outer region of the brain known as the cortex. It is still unknown where in the
brain colour perception actually occurs, if indeed it occurs in any localized area,
but activity in the visual cortex at the back of the brain is strongly implicated.
The properties of the visual system have been reviewed elsewhere (e.g. Roberts,
2002) and only a minimal summary of the retinal processes is now presented
before methods for the measurement of colour are outlined.
The rods are responsible for our vision at low levels of illumination, referred to
as scotopic or night vision. At higher or photopic levels of illumination colour