Theoretically, lighting must meet the following requirements :
1. Lighting has to ensure the accurate and speedy vision that is necessary for the given task.
2. 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.
3. Visual discomfort caused by lighting has to be limited to an acceptable degree.
4. 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.
For accurate vision it is necessary to perceive the details of the task to an appropriate degree. The exact perception of a detail of the environment means being able to differentiate between the
details, their luminances and colours to the required degree as well as perceiving their spatial location.
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, and
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.
L E
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.
Demand 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.
The spatial uniformity of illumination can be characterized by the inequality of illuminance on the reference plane as shown in the following equation and figure.
ε is the coefficient of spatial uniformity.
The required value of spatial uniformity is usually between 1/3 and 1/10. If the logarithmical character of visual perception is taken into account, that means that the perceived difference of luminances of a homogeneous surface illuminated with 1/3 -1/10 uniformity is between 70 and 130
%.
Spatial uniformity has to be larger than 1/3 in working places used for activity needing uniform lighting.
In rooms that are not used for work, in rooms used for relaxation, for communication or for waiting, minimum spatial uniformity is 1/10.
The required ratio of average illuminances of parts of a room used for different activities or of rooms opening into each other must be greater than 1/5.
Another problem connected to lighting is its change in time. A lighting system provides its greatest illuminance when it is installed. Then illuminance is gradually reduced by the aging of the lighting system and by the dirt gathering both on the lighting system and on the surfaces of the room. Consequently, the efficiency of lighting gets worse and worse.
This reduction of output due partly to the deterioration of the conditions of vision and partly of the efficiency of the lighting system is acceptable only to a certain degree.
80 % of the nominal illuminance is generally regarded as the lowest acceptable limit of average illuminance. If illuminance is reduced to this lowest limit, the lighting system has to be at least partly renewed, i.e. first of all the lamps should be replaced, the luminaires should be cleaned, and at less frequent intervals the surfaces of the room should be cleaned.
In designing lighting, aging is usually taken into account through the so-called coefficient of aging ν ( its usual values are 1.25 - 1.65) by designing lighting systems for a value of
Ei = ν * En
A smaller coefficient is used in cleaner environments, and a bigger one in dirty environments.
So illuminance at the time of installing the lighting system exceeds nominal illuminance, and the period between maintenances may be longer, too.
How to meet the requirement of illuminance?
The required Ei (lx) average illuminance on the A(m2) working plane can be ensured by building in Φo luminous flux generated by light sources. The amount of this luminous flux depends on the manner of lighting and the characteristics of the room. Only part of the Φo built in luminous flux will reach the reference plane either directly, or indirectly, reflected from other surfaces. The efficiency of lighting is the ratio of luminous flux illuminating the reference plane (Ei* A) and of the built in (Φo) luminous flux
The efficiency of illumination indicates what portion of the luminous flux generated by lamps reaches the reference plane.
The efficiency of illumination depends on:
- the manner of lighting (direct, semi-direct, general diffuse, semi-indirect, indirect),
- the efficiency of the luminaire,
- the geometrical shape of the room and the reflection of the bounding surfaces.
The more direct lighting is, the better the efficiency of the luminaire, the more symmetrical the shape of the room, and the larger the reflection factor of the surfaces, the greater the efficiency of illumination will be.
Various methods are used to calculate the efficiency of illumination, and the resulting values are given in tables. These values vary between 0.1 and 0.6 in practice. The efficiency of illumination is an important element of the cost-efficiency of a lighting system.
A given requirement of Ei average illuminance in a given interior can be met by various values of Φo built in luminous flux, depending on the different ways of lighting. One has to choose the way that can satisfy the other requirements, as well.
How to meet the requirement of uniformity in space ?
We can meet this requirement by taking into consideration the following circumstances affecting uniformity:
- the light distribution of the luminaire (the way of lighting)
Spatial uniformity improves as we move from indirect to direct lighting.
-luminous flux per one luminaire , ΦL
The smaller the luminous flux per one luminaire is, the greater number of luminaires are needed to ensure uniform illuminance.
- the reflection of the bounding surfaces
The greater the reflection of the ceiling, the walls and the floor, the better the spatial uniformity will be.
The role of certain surfaces depends on their illuminance, in other words on the way of lighting.
Thus, in direct lighting, it is the reflection of the walls, while in indirect lighting, it is the reflection of the ceiling that is the most important. In intermediate ways of lighting, the reflection of both the walls and the ceiling may play a role.
In practice, the spatial uniformity of illuminance is considered adequate if the location of the luminaires representing different ways of lighting meet the following ratios.
Most computer programmes used for designing artificial lighting calculate the values of spatial uniformity, too.