The Nature of Colour As Light
Light, the Eye, and Colour
For most of us, the perception of colour occurs
so easily and naturally that it seems to suggest that colour resides
fully and directly in the objects that we see. In fact, colour perception
is the creative act of our uniquely configured visual system.
We will begin our exploration of colour perception
with a discussion of colour's underlying physical basis - the electromagnetic
energy. This will be followed by a discussion of spectral sensitivity,
and then an explanation of colour mixing.
from a very small portion of the electromagnetic (EM) spectrum,
a spectrum that also includes x-rays, gamma-rays, radio waves. Our
visual system is able to convert electromagnetic wavelengths between
380 nm and 700 nm into light (1.0 nm = one-billionth of a metre);
we are blind to all the rest of the EM spectrum. In short, without
an eye, there is no light, only EM energy of different wavelengths.
shown in the accompanying figure, wavelength refers
to the distance between corresponding points of the wave cycle. Within
the visible portion of the electromagnetic spectrum, wavelength is
measured in billionths of a metre (nm). Alternatively, you
can refer to a light's frequency (measured in Hz), which is
simply the reciprocal of its wavelength.
The primate eye's overall daylight
(i.e., photopic) spectral sensitivity function is based on the more
limited sensitivities of short (S), medium (M), and long (L) wavelength
cones (see figure). Although, as shown in the figure, each cone type
is sensitive over a range of wavelengths if provided with sufficient
electromagnetic energy, each has a peak sensitivity (i.e., requires
the least amount of energy) at a specific wavelength. S cones are
maximally sensitive to wavelengths of 419 nm, M cones to 531 nm, and
L cones to 558 nm. Because there are relatively few S cones, however,
overall photopic sensitivity is due largely to the summed responses
of the M and L cones, accounting for the overall sensitivity peak
at 555 nm.
molecules do not respond to these wavelengths, humans are relatively
insensitive to ultraviolet and infrared light.
The Experience of Colour
Colour perception can be
described in terms of three dimensions: hue, brightness, and saturation.
These three dimensions can be represented in simple terms as a spindle-shaped
hue of a colour refers the colour name (e.g., red or blue),
and is a function of the wavelength(s) reaching the eye.
Brightness refers to the intensity
of a colour, and is roughly proportional to the amplitude of the incident
Saturation refers to the "depth"
or "purity" of a colour, and is related inversely to the number of
different incident wavelengths. At the brightness extremes (i.e.,
black and white), the spindle comes to an infinitely small point,
indicating a lack of apparent hue and thus complete desaturation.
Mixing Colours: Lights vs. Filters
colour that we see is not the "colour" of a surface per
se, but rather the mixture of wavelengths that are reflected back
to the eye. For example, an orange appears orange because its skin
absorbs very short and medium wavelengths, while reflects medium (yellow)
and long (red) ones.
In short, colour perception is a function
of both lights that are added up (i.e., additive mixing) and
those that are absorbed (i.e., subtractive mixing).
Additive Colour Mixing
If two or more light sources
of different hues are added by being directed to the same white surface,
the colour experienced would be different from that elicited by either
hue alone. The hue that results from the mixture of red,
green, and blue additive primaries is shown in the accompanying diagram.
For example, if red and green spotlights were shone on a white dress,
it would look yellow. If red, green, and blue lights of equal intensity
illuminated the same dress, however, it would still look white. In
other words, its brightness would be greater but its "colour" would
Additive mixing of this type is the
basis for colour on t.v. and computer screens. You can test this yourself
by examining a yellow patch on your computer monitor with a powerful
magnifying glass; you'll see that it's composed of yellow and green
dots. Additive mixing by the placement of small colour patches next
to one another is referred to as partitive mixing.
In addition to t.v. and monitors,
partitive mixing can occur with fabrics as well as mosaics. Certain
fabrics (e.g., tweeds) are woven with threads of different colour,
but from a distance, the eye perceives a global additive colour.
For example, some wedding dresses are woven with very desaturated
primary colours (red, green, blue usually) to give a shimmering white
appearance (much like a pearl). In clothing design this fabric is
Depicted in the image at left is another
type of fabric, called "shot", which is composed
of interwoven threads of two (or more) different colours. In the figure,
the purple and yellow fibres do not actually appear as you see them
in the image, but rather the eye combines these to produce a brown
hue (a camera cannot accurately capture the appearance of shot).
Partitive mixing can also be used
as a technique in art. In this technique called pointillism,
small dabs of paint are placed adjacent to one another to produce
a brighter, more vibrant work. This form of art and was influential
in France, Holland, and Italy in the 1890s and 1900s. The image below
is a pointillist rendering of a summer scene at a beach. When viewed
from a distance so that the dabs of paint cannot be discriminated,
a somewhat different global perception of colour results. Even when
the colour dabs can be seen, pointillist paintings tend to be more
lively and luminous than those based completely on subtractive mixture.
mixing is based on the selective removal of wavelengths from light
to produce a different colour. Common examples of this include paint,
dyes, inks, and colour filters. For example, red paint is a paint
that reflects red while absorbing all other wavelengths. The accompanying
figure illustrates subtractive mixing.
As you can see, a green "filter" is one that transmits wavelengths
in and around green; likewise, a blue "filter" transmits the shorter
wavelengths associated with blue. The only wavelengths that would
be transmitted by the two filters placed in sequence are those not
absorbed by either filter. Thus, when green and blue filters are used
together a dimmer, a dimmer colour intermediate between them results,
in this case teal.
Paints work similarly. Colour particles
are suspended in a clear medium (e.g. linseed oil). As light passes
through that clear medium, it is transmitted selectively by the colour
particles. The wavelengths that are reflected back to the eye (i.e.,
not absorbed) determine the colour seen.
Thus, mixing paints of different colour
is completely analogous to placing several filters of different colour
in a sequence.