While ambient illumination strongly influences the overall impres- sion of a space, it is obvious that illumination also influences how well we are able to see. When we need to read small print, or we want to see the fine detail of an object, we switch on a task light or carry the object across to a window. We are particularly likely to choose the window option when we want to be sure of the colour of an object, such as an article of clothing that we are think- ing of buying.
There is no simple measure of how well lighting enables us to see. A lot of factors can become involved, such as what it is that we are trying to see and how good our eyesight is, and even then, there is no objective basis for comparing how well two different people have seen the same thing.
Initially, studies of human vision examined peoples’ ability to discriminate small differences, in particular, small differences of detail and of colour. These studies have provided the basis for the concepts of visual performance and colour rendering.
Visual performance
Everyone has seen a chart like Figure 2.7. Optometrists use them to test eyesight. The letters are black on a white background to maximize contrast, and the chart is viewed at a distance of 6 m.
The lines of letters are designated by a series of numbers, 6/4, 6/6, 6/8 and so on, from the bottom of the chart. If a person can read down to the 6/8 line but no lower, this fraction may be used to classify their eyesight, as it indicates that at a distance of 6 m, this person can only discriminate detail that would be visible to a person with normal sight at 8 m. In this way, normal eyesight for a healthy young person is 6/6 vision, although this is still often referred to as 20/20 vision because the chart’s inventor, Dr Snellen, stated that the chart should be viewed at 20 feet. A person with 6/4 vision has better than average eyesight.
While Snellen’s system satisfies optometrists, scientists prefer to measure the ability to discriminate small detail in terms of visual acuity. The smallest detail that has to be discriminated in order to identify an object is termed the critical detail, and for a letter on the Snellen chart, this might be the gap that distinguishes a ‘C’ from an ‘O’. When an observer is just able to discriminate the criti- cal detail, this is described as a threshold condition. The size of the critical detail may be measured in terms of the angle that it subtends at the eye, as shown in Figure 2.8, and for a threshold condition, visual acuity is calculated from the expression VA = 1/␣, where ␣ is the angular size of the critical detail in minutes of arc. For example, if the critical detail at threshold subtends an angle of 2.5 minutes, visual acuity is 0.4.
It can be seen that the Snellen chart is a simple device for measuring a person’s ability to discriminate detail, and it can be
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P U R W C Z L K H F D A Y N K G T W N X D J S P H F M V L N 2.7.applied quite readily for the needs of optometrists or visual scien- tists who are seeking to measure a person’s visual ability. The crucial factors are the contrast of the detail against its background and the angular size of the detail, and when the chart is viewed at the correct distance, both of these are controlled. So, what is the role of lighting in all this? The answer is that providing that the ambient illumination is ‘acceptably bright’ (note the discussion in the previous section) it makes no practical difference. If the optometrist’s room appears to be generally well lit, the test condi- tions are satisfied if the chart is fixed onto a wall and viewed from 6 m, using a mirror if necessary. This is not to deny that scores would be affected by providing noticeably high or low light levels, but nonetheless, consistent scores can be expected over the broad range of illuminances that is commonly encountered in well-lit rooms.
This might seem to be a rather strange conclusion, as it implies that illuminance makes little difference to how well we can see. The answer to that is that there is more to seeing than visual acuity, and it is for this reason that vision scientists have devised the concept of Visual Performance.
Human performance in carrying out various types of work tasks can be measured in several ways. The most obvious measures are how long it takes to complete the task or how many times the task can be completed in a set time. After that, various measures that have been devised for quality control may be applied, and for any work task, there will be a range of factors that influence perfor- mance. One of these is likely to be the visual conditions, but tasks differ greatly in the extent to which they are vision dependent, ranging from tasks that require 6/6 vision to ones that can be performed ‘with one’s eyes shut’.
Scientists have devised visual tasks that are highly dependent upon vision, an example being the Numerical Verification Task (NVT) for which the experimental subject scans two similar columns of numbers and has to identify any differences. The
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Eye Visual task α 2.8.The angular size of the critical detail at threshold determines visual acuity
researcher measures both the time taken and the number of mistakes made in completing a NVT, and this test is repeated under different visual conditions. The data are processed to give a scale of visual performance that takes account of both speed (the inverse of time taken) and accuracy (the inverse of mistakes made). This enables the experimenter to measure the effects of alternative visual conditions in terms of visual performance. As has been noted in the previous paragraph, overall performance may be influenced by many other factors, so that application of visual performance data to actual work performance requires assessment of both the visual task difficulty and the extent to which overall work performance is vision dependent.
The Relative Visual Performance (RVP) model is due to Rea (1986) and Rea and Ouellette (1991). A RVP value of 0 represents a ‘readability threshold’, which means that the visual conditions are just sufficient to enable a normally sighted person to read slowly, and a value of 1 corresponds to an experimentally determined level of performance that is ‘unlikely to be exceeded in practice’. It is worth taking a careful look at this model as it reveals the underly- ing factors that govern how visual performance is influenced by the visual conditions.
Trolands Trolands
Trolands Trolands
Contrast Contrast
Contrast Contrast
Relative visual performance Relative visual performance
Relative visual performance Relative visual performance
1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 1000 100 10 1000 100 10 1000 100 10 1000 100 10 1.0 0.8 0.6 0.4 0.2 0.0 0.1 1 10 1.9 Microsteradians 4.8 Microsteradians 15 Microsteradians 130 Microsteradians 2.9.
The Relative Visual Performance model. RVP depends on three variables: task size
(microsteradians), retinal illuminance (Trolands), and task luminance contrast. A 1.9 microsteradian task is a very small task, and high task luminance and particularly high contrast are required for a high level of RVP. As task size increases, lower values of luminance and contrast can provide for high RVP. The model shows an enlarging ‘plateau’ over which performance will not be inhibited by visual difficulties, but at the edge of the plateau is the ‘scarp’, where the
combination of task luminance and contrast start to become inadequate. In this vicinity RVP falls away sharply (IESNA 2000)
Figure 2.9 illustrates four examples of how RVP varies with visual conditions, where the visual conditions are represented by just three factors:
• Target size, being the angular size of the detail to be seen, measured in microsteradians (µsr), and represented by the four different diagrams ranging from 1.9 µsr (very small detail) to 130 µsr (large detail).
• Luminance contrast of the detail against its background, which, for a small target seen against a brighter background, may have a value somewhere between 0 (no contrast) and 1 (hypotheti- cal perfect black on perfect white). A contrast value greater than one indicates that the detail is brighter than its background. • Retinal illuminance for the observer, measured in trolands,
which provides an indication of the level of the stimulus to the retinal photoreceptors.
The terminology may seem formidable, but do not be discouraged. While scientists need measures that ensure controlled conditions, these can be related to practical, everyday concepts. In every case shown in Figure 2.9, the vertical scale is relative visual perfor- mance, and the first thing to note is that for even the smallest detail, RVP approaches the maximum value providing both contrast and illuminance are high. The important differences between the four examples concern what happens when either or both of these factors is less than optimum.
Boyce and Rea (1987) have described the RVP model as a ‘plateau and escarpment’ landscape, and this topography can be seen in Figure 2.9. As noted, high contrast and high retinal illumi- nance correspond to high RVP, and this will always be so unless the detail is so small that the resolution capability of human vision is challenged. The principal difference between the four cases of task size is the plateau area. Providing that illuminance is sufficient to provide for a high level of RVP for a given visual task, increas- ing the illuminance does not enable that task to be performed better, but rather enables better performance of more difficult visual tasks. If there are no visual tasks of smaller detail or lower contrast present, there is no performance advantage to be gained from increased illuminance.
For large-detail visual tasks there is a correspondingly large, flat zone within which neither contrast nor light level have to be high to provide for a high level of RVP. This is the big plateau: the RVP high country where the seeing conditions are good. Providing contrasts and light levels are maintained at reasonable levels, visual tasks are easily performed. As visual tasks diminish in size, the level of the plateau drops only very slightly; but far more
threatening is the diminishing area of the plateau. The brow of the escarpment draws close enough to be a source of concern, and we become conscious that if we have to cope with either small task size or low task contrast, or worse still a combination of them, there could be a real problem in providing light levels suffi- cient to keep a footing on the high ground.
The security of being on the plateau comes from the fact that when you are there, RVP is not a problem. More to the point, providing you know that you are there, you do not need to know the value of RVP precisely. Even so, in order to have any notion of where we are, we have to cope with those microsteradians and trolands. So let’s look at those, and we will start with target size: