(part 2 of 6)
For the normally sighted person, vision is by far the most powerful sense.
Visual receptors respond to light waves. The term 'light' is used to describe electromagnetic energy in the visible range of wavelengths, those approximately from 400 to 700 nm. Shorter wavelength electromagnetic energy is know as ultraviolet, X-rays and gamma rays. Longer wavelength energy is know as infrared to radio waves.
As discussed above, it is due to the physics of the eye and the chemistry of the photoreceptor pigments in the eye that we only see wavelengths between about 400 and 700 nm. We do sense infrared energy as heat but the other wavelengths are imperceptible without the aid of devices such as the radio. Ultraviolet energy is destructive to living tissue, and thus the yellow pigment in the lens of the eye is adaptive in filtering out ultraviolet energy. People who have had their lenses removed because of cataracts are actually able to see ultraviolet energy as light. Their experience of this is not as a new colour sensation though; they see ultraviolet as the same colour that people with normal vision see violet.
Light can also be thought of as consisting of individual packets of energy known as photons or quanta of light. Such quantity measures help to consider the energy relationships concerning light; for example, the minimum number of quanta necessary for vision.
The eye is like a camera is some ways, having a lens and a surface on which the image is registered (the retina).
However, as with many metaphors and analogies, the use of the camera metaphor is problematic, and is often called 'naive realism'. It is an inadequate theory of perception.
Some critical differences between the eye and visual perception and photography mean that there is not a simple one to one correspondence between the context of a scene and what we perceive. Two important points are:
A quick description of the eye (OHP from Pheasant), showing cross-over to brain. The important things to note physiologically are the lens, the retina, the rods and cones (the sensory receptors in the eye which respond to different light waves), the fovea, and the optic nerve.
The fovea constitutes a small part of the retina. The receptor systems that permit visual acuity are concentrated and properly arranged only in the "fovea" of the retina, so it is necessary for us to stare directly at an object in order to have a clear image of it.
The image of an object is formed by the cornea and lens and is imaged on the retina. Muscles attached to the sides of the lens contract in order to thicken the lens and achieve a clear focus when the stimulus is near and relax when it is far away, a process called 'accommodation'. As a person ages, the lens stiffens and its curvature cannot be changed as much. Thus, one needs glasses to supplement the accommodative ability of the eye.
The pupil of the eye reacts to the amount of light falling on the retina by expanding and contracting in such a way as to keep the amount of light approximately constant. This feedback mechanism operates within a certain fairly narrow range of illumination (abut 16 to 1), but the enormously greater variation in retinal illumination (about a billion to 1) demands an intermediate mechanism to allow the eye to function over the whole range of illumination.
This mechanism is called adaptation. Adaptation is one of the most profound and pervasive sensory phenomena. You may have had the experience of entering a theatre from the bright outdoors and stumbling in the darkness to your seat over the feet of people already seated. In a few moments you will have adapted to the darkness, that is, you have become more sensitive to the light. This mechanism works for smell too: when walking into a kitchen where someone is cooking you will notice the smells as very strong, but after a few minutes the smells become less noticeable. These examples illustrate that after exposure to a stimulus the sensitivity to a stimulus is decreased and after removal of the stimulus, the sensitivity returns. Adaptation may be defined as the various outcomes of the operation of repeated or steady presentation of s stimulus to an organism. There are many texts describing these adaptation processes. Suffice it to say, in designs we may need to consider these processes.
As an example of using this information in design, consider the role of red light in photo development rooms. Rods are relatively less sensitive to light in the red end of the spectrum than the blue. Therefore red is the best colour to use if one wishes to dark adapt the eye without totally depriving it of light. The rods begin dark adapting because they are relatively insensitive to the longer wavelength, dim red light. (Rods and cones are equally sensitive to red light). The psychophysiological principle behind this is the "Purkinje Shift". This will be discussed below when considering rods and cones.
The retina: composed of several types of cells in layers. Light passes through the ganglion cells and bipolar layer cell layers as well as interconnecting cells such as amacrine and horizontal cells before reaching the rods and cones that are the receptors proper. In this way the retina is "inverted", as the neural structures overlie the receptors. Excitation begins in the rods and cones and travels through the bipolar cells to the ganglion cells, in the opposite direction the light passed.
According to the "duplicity" theory the rods and cones differ in a number of ways, actually yielding two different receptor systems in the eye. The cones, found mostly in the fovea, function in daylight conditions and are completely responsible for colour vision. The rods, distributed throughout the retina outside the fovea are much more sensitive to light than the cones and therefore are active during night vision. There are approximately 120 million rods and 6 million cones in the retina.
You can demonstrate this to yourself by staring at a dot in the centre of a page. Only the area immediately surrounding it will be clear. The result of this foveated vision is that the eyes must move constantly in order to perceive an object of any size. These semi-conscious movements are called "saccades" and take approximately 0.5 of a second each, just about the interval of persistence of vision. Notably there are movements that are even faster and smaller than saccades (micro-saccades) that prevent our eyes from 'habituating'. Habituation describes how the receptor response dies away after prolonged stimulation, because receptors are basically change detectors.
The periphery of the retina is the area most sensitive to movement. This is the way in which we perceive items we may want to track or focus on.
The retina at the back of the eyeball converts light energy into electrical energy and begins the conversion from sensation to perception. The retina consists of rods and cones that serve different functions.
Rods perceive in poor light but are not located in the central (foveal) vision - thus in dim light we perceive the world better off centre. Conversely, the cones being chiefly in the fovea mean that we can only perceive colour well in good light and in central vision. Also, we can only make fine visual judgments in foveal vision.
These differences are important mainly in the large control room environment we have talked about, where there is a lot of activity occurring in peripheral vision.
The Purkinje Shift: if you look at two flowers one blue and one red, first in daylight and then at dusk, you will notice that under low illumination both appear faded, but the blue seems brighter than the red. Likewise, a piece of green paper and a piece of red paper, matched for brightness in good light, will not be matched in dim light. This is called the Purkinje Shift, after the man who discovered that long wavelength colours such as red appear duller under low illumination than shorter wavelengths. The basis of this phenomenon is the shift from high illumination vision (cones) to low illumination vision (rods). The rods are relatively more sensitive to light in the blue region than are the cones, thus the apparently greater brightness of the green in dim light. These relationships can be seen in spectral sensitivity curves which illustrate that maximum sensitivity goes from red to blue green (i.e. to shorter wavelengths) when we shift from bright to dim light and from the cones to the rods.
Blindspot There are no receptors (rods or cones) present at the point where the optic nerve leaves the retina. Therefore when the image of an object falls on this spot nothing is seen. This is called the "blind spot".
A basic distinction is the measurement of incident light (light falling on an object) and reflected light (light reflected from an object). Incident light is referred to as illuminance whereas reflected light is termed luminance. White surfaces typically have reflectances of 80% and black surfaces around 10%. Some related terms are discussed below.
Luminance is the light reflected from the surface of an object and can be measured in candelas per square metre. As the luminance of an object becomes greater, the eye's visual acuity or ability to discern small detail also increases. The pupil diameter decreases and therefore increases the depth of focus in the same way as a standard camera lens when the aperture is adjusted. An increase in luminance of an object or display will also make the eye more sensitive to flicker.
Contrast describes the relationship between light emitted from an object and light emitted from the background surrounding the object. Contrast is defined as the difference between the luminance of the object and its background divided by the luminance of the background. This will produce a positive number if the object is emitting more light than the background or a negative number if the background is emitting more light than the object. Objects can therefore be described as having positive or negative contrast.
Brightness is a subjective response to light. There is no real means of measuring absolute levels of brightness as there is of measuring luminance or contrast, but in general a high luminance from an object implies a high brightness. It is possible to experience odd effects around areas of high-to-low brightness boundaries. e.g. Hermann grids. Designers should be wary of creating effects like the Hermann grid.
Visual angle and visual acuity Visual angle is defined as the angle subtended by an object at the eye. Visual acuity defines the minimum visual angle that can be resolved. Because these angles are fairly small they are usually measured in minutes or seconds of arc.
To assist the user, the designer of a visual display should note that in good viewing conditions a minimal perceptible visual angle of about 15 minutes of arc should be maintained and in poor viewing conditions this should be increased to 21 mins. These correspond to a 4.3 mm object and a 6.1 mm objects respectively viewed from 1 m.
Visual field is the area discernible to the average person. This field of view obviously varies depending on whether the head and eyes are stationary, whether the eyes are allowed to move while the head remains stationary or whether both the head and eyes are allowed to move.
The recommended maximum field of view depends on these points.
The visual field is very important factor in defining the size of a particular display screen or the layout of displays and control equipment.
We have already explained some of the basic concepts in colour vision. Colour is increasingly important in the work environment. It can provide information, warnings, and change the mood of a room or display.
Some useful terms in considering colours:
Brightness is a measure on the black-white dimension, Saturation refers to the purity of the sensation as opposed to grayness and Hue is primarily dependent on wavelength; it is closest to the everyday term "colour". There are many theories of how we perceive colour.
As discussed, visible light is a small part of the electromagnetic spectrum. It occupies the 400-700 nm wavelength region that extends from ultraviolet to infrared. If the wavelength of visible light is varied between 400 and 700 nm with constant luminance and saturation (amount of white light added), a person with normal colour vision is able to distinguish approximately 1228 distinct differences in colour. If luminance and saturation are varied in addition to the wavelength of the light, approximately 8000 distinct differences in colour can be detected. Although up to 8000 different colours can be distinguished comparatively, only 8 to 10 different colours can be identified accurately without training when viewed in isolation by a person with normal colour vision.
People's sensitivity to colour is not uniform across their field of view. The eye is not sensitive to colour at the periphery of vision. Accurate colour discrimination of colour is only possible to around 60 degrees of the straight ahead position (with the head and the eyes stationary) and the limit of colour awareness (as opposed to discrimination) is approximately 90 degrees of the straight ahead position. The eye is the least sensitive to red, green and yellow light at the periphery of colour vision and most sensitive to blue light.
The retina is not equally sensitive to all colours - it is best adapted to the perception of yellow-green light, and colour is only well perceived in foveal (central) vision.
Colour perceptions are affected by any prolonged exposure to other colours - this is because different cones are responsive to different dimensions of colour (e.g. red-green or yellow-blue). Looking at red light causes the red cones to adapt (go to sleep!). This could be important if working at a terminal (VDU) of some constant colour. This is often seen in colour after effects, or afterimages.
Colour constancy means that we perceive colours the same where they are not the same. Our clothes do not actually change their colour when we go indoors, but the wavelengths hitting the retina have changed. Vegetables and meat in supermarkets are most compelling example of constancy not working - they tend to be lit so that they give off particular wavelengths - when you get home with different lighting the colours are substantially duller.
Colour constancy is a classic example of seeing what we expect to see - there are no good theories of how we achieve constancy.
Colour blindness may be the most important feature of the colour system for human factors experts. 6 percent of men and 0.5 percent of women are deficient in their colour discrimination. Most of us can discriminate all 3 primary colours, (red yellow and blue) and are known as 'trichromats'. Some people cannot distinguish between two of these - usually being unable to tell red from green, and they are know as 'bichromats'. 'Monochromats' are unable to distinguish any colours having only one set of cones (or just rods). They perceive the world in monochrome.
Colour constancy and colour adaptation mean that colour blindness is not a complete failure to discriminate - some people use adaptation and constancy cues to help distinguish colours.
Use of colour: colour can be used to code items in the interface. Also program comprehension and ease of debugging may be improved by using colour coding and indentation in a programming language.
We are sensitive to flickering lights - above a certain rate we do not perceive flicker (e.g. movies, the "flicks", in order for us to perceive constant movement we need to present an image more than 'critical flicker fusion': films: beam is broken 50 times per second. Critical flicker fusion is responsible for apparent movement effects). Note however, that flickering lights can trigger epileptic fits in some people (7-10 flickers per second). There are regulations and guidelines related to this for VDUs.
As mentioned, vision is an ability that declines with age. A lot of this can be dealt with. With age we need more light to see by (the lens lets less light through), and the process of focusing gets harder (the muscles get less effective).