|Introduction to Electrical Engineering - Basic vocational knowledge (Institut für Berufliche Entwicklung, 213 p.)|
|4. Electrical Energy|
|4.5. Conversion of Electrical Energy into Light|
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 light
Fig. 4.22. Origin of an indirect glare
1 - Source of light
2 - Reflecting object of work
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 light
2 - Directed light
b) indirect light
3 - Opaque fillet
4 - Source of light
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 possible.
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 perceptibility.
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 illumination?
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 avoided?
8. Why should the working field be not darker than the surround?