Theoretically, lighting must meet the following requirements
- Lighting has to ensure the accurate and speedy vision that is necessary for the given task.
- For accurate vision, the details of the object as well as the colour and spatial location of the details have to be seen as clearly as the task requires.
- Visual discomfort caused by lighting has to be limited to an acceptable degree.
- While meeting the requirements of the visual task, lighting has to be cost effective.
The above theoretical requirements can be met in practice if the requirements of lighting are quantified. The requirement of accurate vision can be met with definable values of the following characteristics of lighting
- Illuminance on the reference plane,
- Colour rendering, and
- Shadow effect.
Visual discomfort can be limited with definable values of the following characteristics of lighting
- The colour of light,
- Glare
- The ratio of luminances.
Lighting serves the visual task effectively: If the installation and operation of the artificial lighting system is cost effective, and if the process of visual perception is efficient. Requirements vary from case to case, i.e. the requirements of a given activity is one of a great number of possible combinations.
Illuminance on the Reference Plane
Every visual task has a reference plane. As most tasks have to do with work, the reference plane is sometimes called the working plane, too. Unless otherwise specified, the reference plane is generally a horizontal plane 0.85 m above the floor in working areas, or the floor in circulation areas.
Every visual task requires a certain degree of visual accuracy and contrast sensitivity, i.e. a certain degree of visual ability. As visual ability depends on the average luminance of the field of view, and the luminances of the elements of the field of view depend on the ρ quality of the surface and on E illuminance of the surface according to the L = ρ * E equation, the average luminance of the field of view can be changed by changing the illuminance if the surfaces of the interior (ρ ) are constant.
In this way, the requirement of the average luminance of the field of view can be defined as a requirement for illuminance. The usual values of reflectance of the bounding surfaces of the interior are
- ceiling 60 – 80 %
- walls 40 – 60 %
- floor 20 – 30 %
and it is recommended to keep the values within in the above range. The reflection factor of the reference plane may vary greatly in practice, still it can be defined rather accurately for a given activity.
Thus, we can define the illuminance by which the average illuminance of the probable field of
view satisfies the requirements of the visual ability necessary for the task if we know the characteristics of the visual task as well as the probable shape of the room (i.e. the reflectance of its surfaces).
Luminance greater than necessary results in a greater average luminance of the field of view (higher level of adaptation) and, consequently, in greater visual ability. While greater illuminance improves achievement and decreases fatigue, it also raises both the initial and the running costs of lighting systems.
In certain cases, we can define the optimum illuminance for visual processing. It is usually much higher than the minimum value necessary for the task. In present practice, illuminance for an activity is greater than the minimum value necessary for the task, but smaller than the optimum. For example, we are able to read by less than a hundred lux of illuminance, still the standardized nominal illuminance for reading is several hundred lux, the optimum illuminance, however, is about a thousand lux.
Depending on their financial possibilities, countries specify different values of illuminance for various activities. Optimum illuminance – usually several thousand lux – is only permitted for a narrow range of tasks, even in the most well-to-do countries.
The visual task that lighting has to serve follows from the activity in the room. So relevant standards specify the requirements of illuminance for rooms for certain activities. Requirements vary country by country, depending mainly on their financial possibilities.
emand on illuminance is given as En nominal illuminance. This recommended illuminance is the average value for the reference plane. The values of nominal illuminance are generally standardized, the usual values being 20, 30, 50, 75, 100, 150, 200, 300, 500, 750, 1000, 1500, 2000, 3000 and 5000 lx.
The nominal illuminance for a given activity can be chosen from the above values. The required average value of illuminance on the working plane has to be provided with standardized uniformity. This is to guarantee that there is enough illuminance on the worst illuminated part of the reference plane.
