Different tasks call for different lighting levels
The lighting level (viz. the amount of light incident on a surface) can be expressed in terms of illuminance, in lux. Years of research and experience have yielded a large variety of lighting recommendations based on illuminance. These recommendations, or tables of illuminance often provide a convenient answer to the question of what lighting level to adopt for a given situation.
In many applications illuminance is the most practical lighting parameter
Although a large number of investigations based on luminance also have been carried out, for interior and sports lighting practice, it is preferable to use illuminance as the principal lighting parameter. Luminance measurements involve the use of complicated measuring equipment and are time-consuming. Moreover, experience shows that under normal circumstances in these fields of application, the recommended illuminances offer a reliable guarantee of good lighting.
Road lighting quality is defined in terms of luminance
It is only in relatively straightforward applications (viz. well-defined visual environments) that luminance distribution can usefully serve as a quality criterion. The advantage of the luminance method is that it fairly well describes or defines the visual scene. Take the case of road lighting. The environment is basically the road ahead and its immediate surroundings. Road-reflectance properties can easily be defined in terms of standardized categories, which means that luminance (including luminance uniformities), can be easily calculated. In road lighting, lighting levels are expressed in terms of luminance (cd/m2).(Fig 1)
Fig 1 .Under the same lighting conditions, luminance and uniformity can vary widely as a result of different reflective properties of the road surface.
Apart from national recommendations which are very often strongly influenced by the economic situation of a country, there are International recommendations produced by the ‘Commission Internationale de l’Eclairage’ (CIE). The CIE has published a Code in which recommended indoor lighting levels are listed for almost every conceivable visual task. These are minimum values, and it will be self-evident, therefore, that opting of a lower level in a particular situation may well be detrimental to human comfort, safety or productivity. On the other hand, higher levels will improve the quality of the lighting, increase worker efficiency and uplift morale, but these may not always be economically viable.
One should bear in mind that the eye, under daylight conditions, is equipped to function with values of about 100 000 lux in summer under a cloudless sky. In interiors, however, the eye is relatively strongly stressed because it has to observe smaller objects for a longer period of lime at much lower levels in comparison with observations in the open.
Not only the illuminance, but also the uniformity of illuminance is of important in majority of lighting applications. This is particularly true in the case of installations providing general lighting, both indoor and outdoor. The recommended illuminance uniformity ratio (minimum¬ to-average value) for general lighting then ranges from 0,8 for interiors to 0,4 for roads.