| Natural
light
The primary light source that man has relied on for thousands of years is the sun. It is only relatively recently in evolutionary terms that we have developed light sources based on fire, such as candles and gaslights, and more recently still sources run by electricity.
Therefore, it is reasonable to suppose that our eyes have developed with the expectation that vision will be provided by natural light, which contains all wavelengths in roughly the same proportion. There is speculation, but little evidence, to say that our bodies need the full complement of wavelengths in order to function optimally, and that by working or living under artificial lights we are somehow causing ourselves damage. Probably more important than the spectral composition of the light is the variation in light levels over the
24 hours of the day, for two reasons. First, our bodies have natural rhythms, and these are thought to be
'tuned' to, or controlled by, the day-night cycle. Second, we appear to have a biological need for
light - in Scandinavia (where the winter days are very short and the winter nights very long) the depression rate is much higher in the winter than in the summer.
Artificial
light
There are two main types of lamp used to provide interior lighting. The first of these is the
incandescent light - the traditional 'light-bulb' - which consists of a thin piece of wire (a
filament) sealed into a glass case along with a special gas. When electricity passes through the wire it glows, giving off light and
infrared radiation (which we feel as heat). The second type is a fluorescent tube, which is again a glass tube filled with gas. This tube has two electrodes, one at either end, and as electrons fly from one end to the other they excite the gas which then gives off electromagnetic energy. Some of this energy is in the form of visible light, some of it is invisible. The name of the tube comes about because they have a fluorescent coating on the inside which absorbs this invisible radiation, and then emits radiation in the form of light.
Colour
rendering properties
There are two main areas where there are differences between the light produced by these three light sources, the sun, the incandescent lamp and the fluorescent tube. The first is in the relative amounts of radiation that each emits. The sun emits roughly equally across the visible spectrum, the lamp emits more radiation the higher the wavelength, and the fluorescent tube emits unevenly across the spectrum. It is worth mentioning one further lamp here, the sodium
street-lamp, because these are often monochromatic,
that is they only emit light at a single wavelength, sodium yellow.
Because of the different spectral compositions of the sources, colours
of things that we look at under each source can appear different.
This is worth remembering when performing any colour-critical task, such as choosing
clothes or doing inspection work. Have you noticed that under
sodium street-lamps everything looks a dim shade of yellow and
no other colours are visible?
The following two photographs
were taken of the same bathroom interior walls and ceiling. In
the first picture the room is lit only by artificial halogen lamps, and there is little difference seen
in the colour of the walls and the ceiling. In the second, the room is lit by daylight, which has a broader spectrum and provides better colour rendering, and the colour difference between the wall and ceiling is quite apparent.
 
Time
properties
When a light is flashed at a slow rate,
10 or 20 times per second, it appears to flicker. As the flash rate increases the flickering appearance
decreases, until eventually the light appears to be steady. Both the sun and an incandescent lamp
produce light which is essentially uniform, whereas the amount of light produced by a fluorescent tube (like that produced by a television or a VDU) varies rapidly. Usually the frequency of this flicker is so fast that
the eye can't follow it, and the light appears to be steady. When a tube gets old, however, it may produce some slower flicker and this can often be seen by eye.
As well as affecting the eye, a flickering lamp can alter the appearance of an object that the eye is viewing. A similar effect can be seen in television or films where a wheel
(for example, a wagon wheel in a western) appears to stop moving, or even go backwards. This happens because the camera does not make a continuous record of the scene, but takes a number of pictures in rapid succession. Because each is similar to the previous picture, and they are shown in quick succession, the eye sees the film as continuous movement.
The following
pictures show two successive frames of a cinema
film. The wheel has rotated clockwise between frames, and the
red and blue spokes have moved round ('forward'). In the second
frame, the original position of the red spoke is shown as a dashed line, and
you can see that the blue spoke is closer to this than the solid
red spoke. Your eye sees this (blue) spoke as if it were the original
(red) spoke. Therefore, it appears that the wheel has moved
anti-clockwise (backwards) rather than clockwise (forwards).
This will only happen if the wheel moves round at less than one
spoke-distance between successive frames.
 
|
Although
light, microwaves and radio waves are the same
type of radiation, they cover different parts of the electromagnetic spectrum
- BBC Radio 5 Live in the UK broadcasts on the wavelengths 693 and 909
metres, for example, in the range of radio wavelengths. Radio waves are generally not detected by your
body, and actually pass straight through it without causing any damage. The visible spectrum,
light, is between about 380 nanometres and 700 nanometres (one nanometre is 1/1,000,000,000
metre).
Light is absorbed by our skin and also by special cells within the eye called photoreceptors. This absorption starts a chain of events, chemical changes and electrical signals, which ends up as the sensation of light.
Our eyes see different wavelengths as different colours. A rainbow is formed by sunlight being refracted
(bent) by droplets of rain into individual wavelengths, and short wavelengths are seen as blue, middle wavelengths are seen as green or yellow, and longer wavelengths are seen as red. The rainbow will also contain
ultraviolet (shorter than blue) and infrared (longer than red) radiation, but as our eyes are not sensitive to these wavelengths we
can't see them.
The eye is similar in
some ways to a digital camera. In each case there is an optical system, generally of variable
focus.
There is a variable aperture (pupil) to regulate the amount of
light passing through the optical system, and there is a
light-sensitive layer (which may be of variable sensitivity) at
the back, which is composed of an array of photocells that absorb the radiation focussed onto it. The analogy
can't be taken further, however, because in photography the picture is held in memory in the form of a digital signal whereas with vision there is considerable further processing in the brain which leads to the perception of colour, depth and motion as well as the perception of detail.
