An insight into the use of colour in personal computing, the
processes involved and the limitations they have
Historical perspective
Prior to the 17th century, very little was known about the nature
of light and colour because university teachings were derived from
the Greek philosophers, who were good at inventing theories but
sadly lacking when it came to testing them by practical experiment.
It was generally accepted that light was inherently white and the
objects that it interacted with added colour to it. During the late
19th and early 20th centuries, commercial colour printing became
well established and various processes for colour photography were
developed. By the 1940s, most of the basic systems used in colour
photography had been developed, although the processes continue to
be improved even today. The next major development was colour
television in the 1950s, and in the 1980s and 1990s computer
printers with colour capability brought personal colour printing
within everyone's reach. The next major step in the recording and
reproduction of colour images will undoubtedly be the
popularisation of digital photography, when electronic cameras and
colour printers become good enough, and affordable enough, to
replace conventional film.
The observer as part of the system
Most colour systems contain three basic elements: A source of
illumination (light source) whose spectral content can affect how
colour is perceived. Apart from street lighting, which has a gross
effect upon colour, the main everyday sources of illumination are
"white" light such as daylight, incandescent lamps and fluorescent
lamps. Even these have widely differing spectral content An object
(or objects) upon which the light falls, which reflects light and
usually modifies the spectral contentAn observer, whose eyes
receive light reflected from the object(s). Each person's
perception of colour is different and standards for colour can only
be derived by averaging the results from a large number of
observersThe majority of everyday objects are viewed by reflected
light, but in some cases the object may be self-luminous, for
example a television screen or a firework displayWhen assessing
colours, all the above factors must be taken into account.
Attributes of light and colour Colours have three main
perceptual attributes: Qualitatively, hue is whether a colour is
red, orange, yellow or green. Although we may think of a pastel
colour, such as pink, as being different from red, in fact the two
have the same hue, it is just that the red in pink is diluted with
white. Hue can be defined as the dominant wavelength of a light
source or the light reflected from an object. To get around this,
the spectrum can be made into a colour wheel in which the purple
colours join the ends of the spectrum.The hue is then defined as a
hue angle, with the spectral colours ranging from 0-degrees (red)
to 267-degrees (violet), and the purple colours occupying angles
between 268- and 359-degrees. This allows all hues to be defined
quantitatively, and it also ties in neatly with colour television,
where hue is encoded as the phase angle of the colour subcarrier.
ColourfulnessThis is the degree to which the hue is
apparent. The pure colours of the spectrum are very colourful, but
pastel colours are less colourful because the predominant colour is
diluted with white. Light sources will also appear less colourful
the less bright they are, and reflecting objects will appear less
colourful if they contain black or the illumination is less
bright.
BrightnessThis is the extent to which an area appears
to emit light. The sun is very bright, as are some man-made light
sources. Light reflected from a white area may be bright. Reflected
brightness from greys and browns may be medium, and black very low.
The objective term corresponding to brightness is luminance. The
brightness of objects reflecting light varies with the
illumination, so a piece of grey paper viewed outside on a sunny
day may appear brighter than a piece of white paper viewed inside
on a dull day, even though we know that the white paper is
"lighter" than the grey paper. In order to compare the relative
brightness of colours under differing illumination, the term
lightness is used. This is the relative brightness of a colour
compared to a white area under similar illumination.
The eye
A basic understanding of how the human eye works is essential to
the understanding of colour perception and colour reproduction.
Light entering the eye first passes through the conjunctiva, a thin
transparent membrane that is not part of the eyeball. Next it
passes through the cornea, which is a transparent window in the
otherwise opaque outer layer of the eyeball, the sclera or white.
It then passes through the pupil, the circular hole in the iris,
the coloured part of the eye. Light passing through the pupil is
focussed by the lens, which is held in place by a ring of ligaments
connected to the ciliary or focussing muscle. When the ciliary
muscle is relaxed, the lens assumes a fatter shape with a shorter
focal length. When the ciliary muscle contracts, the suspensory
ligaments pull the lens into a flatter shape with a longer focal
length. In this way the images of objects at any distance can be
brought into focus on the retina at the back of the eye. The image
formed on the retina is inverted.
Colour response of the eye
It is impossible to measure the response of the eye directly, since
one cannot get at the outputs. All that we can do is average the
subjective responses of many observers under experimental
conditions or carry out post-mortem experiments on the pigments in
the retinal cells to determine their spectral response. The problem
is made more difficult by the way in which the eye adapts to
different light levels. Firstly, the iris can vary the amount of
light falling on the retina over about an 8 to 1 range. Secondly,
it is generally accepted that the rod cells depend for their
operation on a compound called visual purple, which is broken down
by light. Thirdly, the sensitivity of the cone cells is reduced by
a brown pigment that builds up at high light levels. This is why it
takes several minutes to become adapted to bright sunlight after
being in the shade (much longer than it takes the iris to close
down).With the increase in the use of digital images, consumers can
now expect better colour recording and reproduction. The
software-supporting, top of the range digital camera's and printers
has become increasingly user-friendly and affordable, thus allowing
a better standard all-round. Although not all aspects of colour is
covered in this article, the key topics discussed will help develop
a better understanding of the concept of colour and the
developments in colour technology.
Compiled by Paolo Formenti