Perception of what human beings describe as reality is intimately entwined not only with the physical disposition of the world but also with the visual competence of the observer. At a basic physical level, each human being perceives only those characteristics of the environment that are detectable by his or her particular pair of eyes. The sensitivity of the typical human eye to electromagnetic wavelengths extends from about 390 to approximately 740 nm - a frequency range of roughly 769 to 405 terahertz. This range is described as the visible spectrum for obvious reasons, and ranges from red to blue, but it represents only a tiny part of the radiation to which the Earth is subjected. Some potentially harmful elements of cosmic radiation are filtered out by the planet's magnetic field or atmosphere, but the human eye still detects only a small part of the energy striking the Earth's surface. Human beings are so accustomed to this somewhat blinkered view of their environment that we tend regard what we perceive as reality. In fact it is nothing more than a small slice of physical reality, but our eyes still allow us to see about 10 million different colours.

Other creatures with which we share our world have quite different perception of the same environment. Some, including mammals such as cats and dogs, have monochromatic or near-monochromatic vision and consequently know little concept of colour. They see the world in various shades of grey, or at least in muted tones comparable with human red-green colour blindness. Others have dichromatic vision which relies upon two sets of colour receptors and a limited range of colours. Such notions may be relatively easy to accept - after all, we are the dominant species on the planet and not particularly surprised by the more limited capabilities of other creatures.

A more disturbing realization is that human beings not only perceive their environment in a restricted fashion, through the window of the visible spectrum and the inherent limitations of a their own three-dimensional (trichromatic) vision, but are also blind to some aspects of their environment clearly visible to other "lesser" creatures. Snakes have superb night vision based upon the detection of infrared, enabling them to "see" what humans regard as heat. Some insects and birds are capable of detecting ultraviolet wavelengths to which the human eye is completely insensitive. Since colour vision depends upon the outputs from various different sets of receptors, this means not only that these creatures can see ultraviolet colours but also that their perception of the what human beings see as the colours of the visible spectrum is significantly different. Many species of birds have four sets of colour receptors and hence tetrachromatic vision. They are consequently able to see many colours that trichromats, such as human beings, can never perceive. There is some evidence that pigeons may have pentachromatic vision which incorporates five sets of receptors and an even wider range of colours. The most sophisticated colour vision system known is found in stomatopods, a type of shrimp, and features twelve different sets of colour receptors.

The human eye also has characteristics which significantly affect the accuracy of our trichromatic observations. Even within what we call the visible spectrum, the eye has varying efficiency. It is most sensitive to yellow wavelengths which lie broadly in the middle of the visible spectrum. Wavelengths at the red and blue ends of the spectrum with the same energy are much less efficiently detected. This is the underlying reason why yellow sodium street lights are used to enhance the night vision of car drivers.The varied efficiency of the human eye as a function of the wavelength of light has such profound significance that two separate units of measurement are used for physical observations and human perceptions. Objective physical measurements use radiometric units whereas measurements of the human perception of light use photometric units. So which measurements represent "reality"? An objective scientific mind would surely accept the physical measurements as closer to the truth, and accept that human beings perceive the world through eyes with particular characteristics that not only filter out part of what is before us but also distort in particular ways what we are capable of detecting.