The Context - Artificial Human Sensors: Science and Applications - page 40

ties and performances of different approaches of perceptual sensing system. The
human perception is a natural process of interaction including the brain. More
detailed information and facts regarding the human perception process is avail-
able in the literature, e.g., Wenzel (1973).

2.2 LIGHT AND COLOURS

The human eye is sensitive to a narrow band of electromagnetic radiation that is
available in the wavelength range between 400 and 700 nanometers. This band-
width is commonly known as the visible light spectrum. The visible spectrum is
located between the infrared and ultraviolet light, as can be seen in Fig. 2.5, which
is the only source of detectable colours of the human vision. When combined, all
of the wavelengths presented in the visible light, is about one third of the total
spectral distribution that successfully passes through the Earth's atmosphere. The
light forms colourless white light that can be refracted and dispersed into its com-
ponent colours by means of a prism. The visible colours and subdivided into seven
basic colours — red, orange, yellow, green, blue, indigo, and violet. The colours
red, green, and blue are classically considered primary colours, because they are
fundamental to human vision. Humans perceive light as white when all three cone
cell types in the eye are simultaneously stimulated by equal amounts of red, green,
and blue light.
A majority of the common natural and artificial light sources emit a broad
range of wavelengths that cover the entire visible light spectrum, with some
extending into the ultraviolet and infrared regions as well. For simple lighting
applications, such as interior room lights, flashlights, spot and automobile head-
lights, and a host of other consumer business, and technical applications, the wide
wavelength spectrum is acceptable and is quite useful. However, in many cases it
is desirable to narrow the wavelength range of light for specific applications that
require a selected region of colour or frequency. This task can easily be accom-
plished through the use of specialised filters that transmit some wavelengths and
selectively absorb, reflect, refract, or diffract unwanted wavelengths.
Colours may be defined in objective physics, and accurately specified as
specific electromagnetic frequencies in the visible-light range. The objective com-
ponents of colour was first proposed in the late 1600's by the Dutch physicist,
Christian Huygens, as a source of radiant energy that travels in a medium and hits
an object that reflects and absorbs different portions of the light spectrum. Colours
may also be described in subjective terms, as sensed by a vision system, perceived
and experienced by an individual or an artificial camera device. Nowadays, it
is also an easy task to process the colours and even manipulate the environment
through devices, like sun glasses, filters or even by modifying colours in digital
photographic pictures by computer programs.
The subjective components of colours are related to the visual perception, i.e.,
the cones and rods in the retina of the eye, that are connected to the brain that