ILLUMINANCE ON THE TASK

Under a very low level of lighting, almost any visual task is difficult, and increasing its illuminance enhances performance. But the improvement does not continue indefinitely as more and more light is added. This is shown in Figure 9.1, a graph of task illuminance against visual performance (measured, perhaps, in the rate of working or freedom from error): each curve eventually flattens out, and continuing to increase task illuminance has little effect.

78 The design of lighting

Figure 9.1 Visual performance and task illuminance.

The illuminance at which the graph becomes almost horizontal depends on the visual difficulty.

A demanding task—with small detail to be perceived and low contrast —requires a high illuminance before the final level of performance is reached; much more light is needed to read tiny print on grey paper than to read newspaper headlines. Furthermore, the ultimate perfor-mance that can be achieved with a visually difficult task cannot be as good as with an easy one.

Most countries have standards that specify the minimum illuminance to be achieved on tasks—at least in situations such as schools and workplaces. During the last 50 years, as the real cost of lighting has decreased, the minimum illuminances required have been gradually raised. This is not a coincidence. Derivation of standards from graphs such as Figure 9.1 is a matter of judgement. Where exactly does each curve flatten off? Is it adequate to take a fraction, say 90%, of optimum performance as a minimum standard — especially as the graph gives only averages for large numbers of people, and there are considerable differences between individuals, even when measured under laboratory conditions? Lighting standards are always set within a context of economic and social values.

Typical recommended illuminances for different types of task are given by Table 9.1.

We can see that the range of these illuminances is great—the highest is 40 times the lowest—and the scale moves in large steps. A small variation of illuminance, 10%

perhaps, makes no noticeable difference to task performance. Most codes of practice, such as the CIBSE Code for Interior Lighting in the UK or the Lighting Handbook of the IES of North America, give extended schedules defining the level to be achieved for particular activities and particular types of building.

But a figure from a general schedule or from Table 9.1 is just a starting point. Various factors can modify the basic recommended illuminance:

• Is the task detail significantly lower in contrast or smaller in size than the normal for this activity?

• Is the consequence of error unusually serious?

• Are the tasks carried out for longer periods than normal?

• Are the users significantly older than normal, or do they have poorer eyesight?

For each question with an answer ‘yes’, the required illuminance should be increased by half a step on the scale. Conversely, if the visual task is notably easier or done for signif-icantly shorter periods than normal, the adopted value can be reduced proportionally. Fast-moving tasks, such as activities with machinery or rapid identification of objects, may require higher illuminances.

Table 9.1 Typical recommended task illuminances

Task requirements Lux Examples

General awareness of space; perception of detail is unimportant

50 Access routes to service areas Movement of people; recognition of detail for

short periods; background lighting

100 Corridors, store rooms for large items, auditoria, bedrooms Recognition of detail for short periods in areas

where errors may be serious

150 Plant rooms, domestic bathrooms Areas without difficult visual tasks but occupied

for long periods; short-period tasks with moderate contrast or size of detail

200 General lighting in control booths, foyers, factory areas with

automated processes Tasks such as reading normal print (moderate

contrast and size of detail) over long periods

300 Workshops for large items, general library areas, school classrooms, domestic kitchens

Tasks with some details of low contrast and moderate size

500 General offices, laboratories Tasks with low contrast and small size

Very small visual and low-contrast tasks Tasks with extremely small detail and low contrast

Tasks with exceptionally small detail and very low contrast

An electric lighting system does not give a constant output throughout its lifetime.

Lamp flux declines with age, and dirt collecting on luminaire surfaces absorbs light.

There may be a 25% drop between the illuminance produced by a brand-new installation and that from the system after 2 years of operation in a commercial environment.

Recommended levels in codes are normally taken to be minimum levels in practice; the effects of ageing and dirt must be taken into account. In the CIBSE Code the recommended values are specified as maintained illuminance: that is, the lowest value that a system will produce in practice, taking into account the anticipated programme of cleaning and lamp replacement. The initial illuminance is that produced by a new installation (with discharge lamps this is taken to be after 100 hours operation). The subject of maintenance is covered in Chapter 14.

For rooms with overall lighting for visual tasks (such as general offices), there is often a requirement in standards for uniformity of illuminance across the actual task

700 Drawing offices

1000 Electronic assembly, tool rooms 1500 Fine work and inspection 2000 Assembly of minute mechanisms

80 The design of lighting

area—usually the extent of each desk or, if task positions are not defined, the horizontal working plane as a whole. Typically the required uniformity is that the lowest illuminance in the area should not be less than 0.8 of the average illuminance, excluding an area around the perimeter of the room (usually taken to be 0.5 m wide), which is often at a lower illuminance. With ceiling-mounted luminaires this is achieved by ensuring that the ratio of luminaire spacing to their height above the working plane does not exceed a given value, which depends on the type of luminaire. An example of this is given in the lumen method calculation in Chapter 16. However, where there is a variety of small task areas, or if a combination of general lighting and local task lighting is used, considerable illuminance diversity may occur across the working plane as a whole. It is typically recommended that the ratio of the highest to the lowest illuminance across the plane should not exceed 5:1.

There can be benefit in providing people with the means of adjusting the level of their task lighting. There is also some evidence that performance improves if a task is lit preferentially. This means that individuals can set the illuminance they feel necessary, and alter it for different activities. Not only do many workplaces involve several different tasks carried out at different times by different people, but the sense of being able to control his or her own working environment is important to an individual’s job satisfaction. It is also essential to good energy management.