Task Illumination

In planning lighting for tasks, the first issue is to understand the nature of the tasks and to some extent the visual ability of the people concerned.  For example, if the task is critical in terms of accuracy and perhaps the detail is small, such as when applying stitches to an open wound, then lighting will need to be at a higher level, or illuminance, than for a less visually critical task such as moving equipment around the ward.  Also if lighting is to enable patients to read and the patients are elderly with perhaps poor sight, then a higher level of illuminance will usually be necessary than if the patients are teenagers.

The following table indicates typical values. A range of values is shown to indicate that the particular situation needs to be considered, which will affect the actual value selected. Also to achieve the higher values, local task lights might be a good solution, rather than a high value of uniform illuminance.

Task description Illuminance (lux)
Circulation areas (corridors night – day) 50–150
Reading (casual – critical)    200–500
Examination/treatment (minor – critical)   500–750

It must be stressed that recommended illuminance values apply only to the task area and not necessarily to the whole room unless the task is carried out anywhere in the room. There can be visual benefits to highlighting the task as it helps the person to focus on the work and improves visibility.   Also there will be energy-saving benefits by not lighting the whole room to the same level unless absolutely necessary.  But the designer must decide on what is required. 

Colour representation is another important issue in task lighting.  In virtually all hospital situations, it is essential to be able to see surface and object colours accurately to avoid errors. To enable this, the lamps used must be able to render colours accurately; and where reflected light contributes to the task illumination, strong saturated colour surfaces must be avoided. The colour-rendering quality of a lamp is described by its CIE general colour rendering, usually described by the term Ra.  The Ra scale has a maximum of 100 and for most hospital tasks a minimum value of 80 should be used, but where the task is particularly colour-critical, a minimum of 90 will be required.  In theory, in areas where colour rendition is not critical, then lower values could be used, but experience has shown that where different coloured lamps are used indifferent situations, it is likely that they will be mixed up during maintenance. 

Disability and discomfort glare are potential problems and need to be avoided to ensure good performance. Disability glare is where there is a source of light, seen either directly or by reflection, which is so bright that it makes seeing the task impossible. Usually this can be avoided by screening all bare lamps from normal directions of view and ensuring that any bright reflected images are also outside the normal direction of view. Discomfort glare is similar although the offending source is at a lower brightness but still enough to cause annoyance and a reduced performance. Reflected glare can be a particular problem for computer or other screen-based users.  However, modern screens usually have a diffusing surface that helps to minimise the problem.  Problems of glare are not just the problem of electric lighting, but also daylight and particularly sunlight.

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