Calculations and Measurements - Road Lighting: Fundamentals, Technology and Application - page 207

Fig. 14.5 Example of
observer positions in relation
to the field of calculation
according to the American
Standard, for a three-lane
dual carriageway

obs obs

obs

14.2 Measurements

The measurements carried out in connection with road lighting fall into four cate-
gories, namely those to determine the lamp properties, the luminaire properties, the
reflection properties of road surfaces, and the installation properties. The first three
categories mostly concern laboratory measurements. Sometimes, however, road-
lighting engineers want to check the spectral lamp properties, especially the S/P
ratio of the light source, in the field. The same holds true for the reflection properties
of road surfaces where both laboratory measurements from samples and in-situ mea-
surements with reflectometers can be carried out. Field measurements made on road
lighting installations are carried out on new installations to check whether they fulfill
the quality specifications and on installations already in use, to reveal whether there
is a need for maintenance, modification, or perhaps replacement. The principles of
both laboratory and field measurements will be outlined here.

14.2.1 Light Detectors

14.2.1.1 Types

For all light measurements photoelectric cells are used that convert the light incident
upon them into electrical energy, the amount of which is measured. The principle of
these cells is basically the same as that of solar cells of the photovoltaic type: they
convert incident light directly into electric power without the need for an external
voltage source. However, photovoltaic cells used for accurate and low lighting-level
measurements usually employ external batteries to amplify the output current of
the cell. Photovoltaic cells consist of two layers of semiconductor material: n and
p types (as with LED light sources). Again as with LED light sources, they are
in fact semiconductor diodes and are therefore often referred to as photodiodes.
Formerly, selenium, with a barrier layer in between the n and p layers, was used as