 |  | Lighting Installation - Basic vocational knowledge (Institut für Berufliche Entwicklung, 164 p.) | ||||
 |  | 4. Measurements at Lighting Installations | ||||
|  | 4.1. General Remarks | ||||
| | 4.2. Illuminance Measuring | ||||
| | 4.3. Luminous Density Measuring | ||||
|
| |||||||||||||||||||||||||||||||||||
According to the measuring principle, similar instruments are applied to the measurement of illuminance and luminous density.
These are always physical measurements.
The photoelectric receivers are photoelements, photoresistors, photodiodes, photomultipliers and others. Their spectral sensitiveness is various and not congruent with the photopic eye response curve. For the correct measuring of illuminance and luminous density values, however, it is necessary that the photoelectric receivers evaluate the light in the same way as the human eye. Therefore, the sensitiveness of the physical receivers must be adapted.
Mostly, this is achieved by filters, but it can also be realized by diaphragm mechanisms. Of course, with supplementary filters and diaphragms, a reduction of the irradiated light energy is connected; the better the adaptation is the greater becomes the mentioned reduction. Generally, it is compensated by electronic intensifiers.
Since the required measuring accuracy for lighting installations is not very great - it is between 1 and 5 % - only approximate adaptations are practised. For this reason, correction factors are indicated on some measuring instruments.
When working with physical receivers, attention must also be paid to the ambient temperature. Sensitiveness alters with higher or lower ambient temperature. Therefore, the receiver must either be thermo-stabilized or this dependence on temperature must be counteracted by correction curves. An especially great importance for the correct assessment of the incident luminous flux is the direction from which the light hits the physical receiver.
An exact measurement is possible only with vertically incident light.
From the general requirements on physical measuring procedures it derives that the following requirements must be met by illuminance measurements:
- Adaptation of the physical receiver to the photopic eye response curve.- An adaptor for the receiver for the cosine-true measurement of the incident light.
- As far as possible, linear correlations between illuminance values and indication value of the measuring instrument or correction curves for the individual measuring ranges.
- No influence of the measured value by fatigue of the receiver or by its ambient temperature.
When carrying out measurements, make sure that the measuring instrument and/or its photoelectric receiver is not shaded partially or completely by people or things.
The selection of the measuring points is determined by the measuring task. If the value of illuminance shall be measured in certain check points, make sure that the measuring conditions are the same as with previous measurements. If, for example with a project to be handed over, the average illuminance values shall be measured for comparison with the values demanded by the plan, a great number of measuring points has to be fixed the measuring height of which shall generally be 850 mm above the floor and with which the receiver element must be placed horizontally. The size of the measuring errors has to be chosen in such way that it can be supposed that the illuminance value is constant in each measuring field. This leads to the number of measuring points for the space or part of space to be measured.
However, there shall be at least 9 points.
With the measurements the mains voltage has to be checked during the measuring process and indicated in the measuring record.
For measuring the luminous density (surface brightness) also measuring instruments for illuminance may be used if simple additional instruments are applied. It goes without saying that the general requirements in connection with illuminance measuring instruments also apply to luminous density measuring. In this context, attention has to be paid that the receiving area is illuminated as evenly as possible. For the image formation of the surfaces to be measured a luminous density attachment with or without optical system is used.
These kinds of luminance meters are especially suited for measuring floodlighted or transilluminated areas and not for measuring the luminous density at lamps and lighting fittings.
For measurements in interior spaces devices of a wide angular aperture are preferred, because - as is known - the medium luminous density in the field of view determines the adaptation of the human eye. Decisive for the adaptation conditions are the surrounding surfaces which are covered by an angle of view of 30 to 50 degrees. If from a number of areas of different luminous density the average value of the luminous density of all the areas shall be calculated, the areas have to be measured individually.
Questions for repetition and knowledge test
1. On what basis do the luminance meters work?
2. What are
the measuring methods for measuring illumination
installations?
 |
 |