4.5.3. Illuminating Engineering
Besides the demand on the illumination intensity specified in
Section 4.5.1., a few further principles have to be taken into account for the
installation of illumination systems. The difference in brightness
between working field and surround is of great importance. The
best visual efficiency is ensured when working field and surround have the same
brightness (see Fig. 4.20.). On no account should the surround be brighter than
the working field. This can be explained best when one tries to investigate the
texture of a black strip of fabric on a black or on a white ground.
Fig. 4.20. Visual power in dependence
on the brightness difference between working field and surround
brightness of working field/brightness of surround =
smallest perceptible contrast
It is quite difficult to avoid dazzling. Direct dazzling or
glare occurs when the source of light is in the line of vision. It can be
avoided by an appropriate sheathing of the light source and arrangement of the
latter outside of the angle of view. Fig. 4.21. shows that within an angle of
20° with respect to the line of view no light source should be arranged.
Indirect glare occurs when the ray of light emitted from the source of light is
reflected by an object of work into the eye. Fig. 4.22. shows how indirect glare
is brough about. Remedy can be provided by an appropriate arrangement of the
lighting fitting, whenever possible, objects of work should be mat (diffusedly
reflecting) and not bright on the surface in order to avoid high lights. In the
case of objects of work having an intensely directed reflection, mainly diffuse
light should be used for work.
Fig. 4.21. Avoiding direct glare
1 - Eye
2 - Direction of sight
3 - Source of
Fig. 4.22. Origin of an indirect
1 - Source of light
2 - Reflecting object of
3 - Eye
Light which is incident on the object of work when being emitted
in a directed manner from the source of light is called direct light.
When, however, the light of the source is directed to large large diffusing
screens or to the ceiling of the room and then to the working place, we speak of
indirect or diffuse light (see Fig. 4.25.). For most of the problems of vision,
a correct mixture of direct and indirect light should be provided. Spatial
sensation is dependent on the formation of shadow and, thus, on a portion of
direct light. Cast shadows which impair the perceptibility of objects are due to
intense direct light.
Fig. 4.23. Origin of primarily
a) direct light,
1 - Lighting fitting with screen impervious to
2 - Directed light
b) indirect light
3 - Opaque fillet
4 - Source of
The angle of light incidence is also of importance to an
avoidance of fatigue in work. One should take care that daylight and artificial
light have the same angle of incidence to avoid double shadows. The illumination
intensity should take the same course inside a room; this can be ensured by
suspending the lighting fittings asymmetrically and closer by the windows.
Well-being and efficiency of man largely depend on
type and intensity of light. Good illumination leads to an increase in labour
productivity, reduction of rejects and of the number of accidents at work. When
daylight is missing or insufficient, illumination with artificial light is
necessary. The light sources used differ with respect to luminous efficiency,
spectral composition of the light, size and service life. There are types of
light sources which have to be operated by means of special ballast and to be
ignited by means of ignition devices. High-pressure discharge lamps reach their
full brightness only after a few minutes and, after an interruption of voltage,
require a few minutes rest until repeated ignition will be
All light sources and their properties are largely
dependent on the mains voltage. For indoor lighting, the fluorescent lamp is
widely used today, for high rooms and outdoor facilities the high-pressure
discharge lamps which, as point light sources, require only small lighting
fitting dimensions though the luminous flux is high. The light colours should be
adapted to the illumination intensity involved and to the desired colour
The lighting installation must be designed in such a way that
glare is avoided, the problem of vision, can be solved without fatigue,
sufficient shade without cast shadow is available and an appropriate light
distribution in the space under consideration is attained.
1. Which factors of production are influenced by
2. What are the factors on which the different colour rendition
in case of different light sources is dependent?
3. Why is the luminous efficiency an important factor in the
evaluation of light sources?
4. What are the properties and fields of application of the
various light sources?
5. Why is the fluorescent lamp particularly suitable for rooms
of small height?
6. For which tasks of illumination are fluorescent lamps of
different light colours used?
7. By means of which measures can direct and indirect glare be
8. Why should the working field be not darker than the