Road lighting has a particular impact on improving road safety, and it is the luminance measurement that determines the level of “brightness” of the road surface observed by the driver. In this article we will discuss how to use an Imaging Luminance Measuring Device for road lighting verification in accordance with current regulations.
Why are luminance measurements recommended?
Road lighting has a particular impact on improving road safety, and it is the luminance measurement that determines the level of “brightness” of the road surface observed by the driver. Since there are other traffic participants and various obstacles may appear in the driver’s field of vision, road lighting improves the response time of the driver. In order to ensure proper lighting conditions, both the design and selection of lighting devices and their installation should comply with current standards and technical conditions.
The investment process associated with the construction or replacement of a lighting system is multi-stage, long-term and very complex. It requires the involvement of many people and institutions, and therefore it seems that the only objective method of verifying lighting quality and compliance with standards is on site measurements upon installation. In this way, the final effect of improving visibility conditions on the road can be clearly assessed. Until now, in practice, such measurements have been carried out sporadically or not at all. This was probably for a number of reasons, but one of them was the limited availability and high prices of suitable measuring instruments and the time-consuming and complicated measurement process.
The new standard EN 13201:2016 Road lighting consists of five parts and forms the basis for the design and evaluation of road lighting parameters. The general assumptions concerning both the selection procedures and the quantitative lighting requirements included in the new standard are based on the 2010 publication of the International Commission on Illumination “Lighting of roads for motor and pedestrian traffic”.
The basic requirements for the illumination of roads intended mainly for high-speed and medium-speed traffic are based on criteria relating to the level and uniformity of the luminance of the road itself, the illumination of its immediate surroundings and to the limitation of glare. Properly designed road lighting contributes to the safety of drivers and pedestrians involved in road traffic. The luminance method used to assess the quality of road lighting applies to M lighting classes. M classes are intended for use on motorised roads where medium and high speeds are permitted. Table 1 shows the minimum luminance requirements for lighting class M.
Table 1: Lighting classes with defined minimum luminance levels and overall uniformity Uo and longitudinal uniformity Ul
New European imaging luminance meter project.
In 2020, work on the creation of a new European measuring instrument was completed as part of the cooperation between GL Optic from Poland, a company specialising in the production of lighting measurement devices, and the Department of Lighting Technology of the Poznań University of Technology. The result of the development works and industrial research is the GL Opticam 3.0 4K TEC, an innovative system for imaging luminance measurement on the roads. This project was carried out as part of the “Fast Track” programme co-financed by the National Centre for Research and Development.
The team of the University of Technology carried out many luminance measurements on expressways and in cities and supported the project with their experience and expertise. In the opinion of Professor Krzysztof Wandachowicz from the Poznań University of Technology, an extremely well thought-out system was created, fully adapted to measurements in accordance with the road lighting standard EN 13201:2016. Apart from a modern measurement system, the meter is connected to a dedicated computer and appropriate software. The device has a hermetic housing and is equipped with an independent power supply system, which is essential for measurements often made on closed road sections without access to the power network. The set is also equipped with all necessary additional elements such as cables, adapters and even screwdrivers and reflective vests.
Pic. 1 GL Opticam 3.0 4K TEC during field measurements
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Conditions necessary to carry out measurements
Measurements of the luminance distribution on the road must be taken after dusk, when the luminance of the road is no longer “additionally illuminated” by a skylight and in favourable weather conditions. It is recommended that the weather conditions do not significantly affect the measurements.
High or low temperatures can affect the light output of lamps or the accuracy of measuring instruments. High-speed winds can cause luminaires to swing and lower the temperature of lamps and change their luminous flux. Moisture in the road surface can have a significant impact on road luminance. The degree of transparency of the atmosphere influences light dispersion and, in the case of luminance measurements, affects the level of the signal reflected from the measured surface recorded by the meter’s sensor.
In addition, attention should be paid to issues relating to lighting installation and the environment. Discharge lamps and LED lamps and modules require some time to stabilise the light output. Luminance measurements should be taken after the start-up process has been completed and after making sure that stabilisation has been achieved.
In urban or built-up areas or in the vicinity of other facilities, direct light and light reflected from the external installation should be eliminated. Foreign light may come from shop windows, advertising boards, and traffic lights. These lights should be switched off or masked, or corrections can be made with the traffic lights off on the basis of separate measurements.
Determination of the measurement field
According to the standard, the measurement of luminance distribution should include the section of road between two consecutive lighting posts. Figure 1 gives general guidance on the method of determining the measurement field. Depending on the lighting class adopted in the design and the type of luminaires chosen and the height of the post, the measured distance between the lighting columns may vary from several metres to several dozen metres. Therefore, the size of the measurement field must be specified in the design and verified on site, as the actual distribution of the columns may differ from the planned one, which affects the measurement results.
Figure 1: Luminance calculation field
The measurements are made according to the direction of traffic and the whole width of the road including all lanes is analysed. The width of the measurement field is therefore dependent on the width and number of designed lanes. According to the Regulation, the minimum lane width depends on the road category and ranges from 2.50 m (access roads) to 3.75 m (motorways outside built-up areas). The maximum lane width in a built-up area is from 3.5 m (motorway) through 3.0 m (collecting road) to 2.5 m (two-lane access road)[2].
For example, the width of the measurement field for a two-way road in a city may be 5-6 m. In the case of a 3-lane section of a motorway, on the other hand, the width may be 10.5 m or more.
Therefore, a road section with the smallest possible curvature should be selected. When the measurement results are analyzed, it must be verified that the measurement points do not overlap the horizontal road markings, e.g. on lane dividing lines. In addition, the road behind the measurement field should be lit at a distance of at least twelve times the height of the lighting columns.
In order to ensure efficient measurement, the corners of the measurement field should be marked visibly and practically. As part of the design of the new meter, special “active markers” have been developed which can be easily positioned on the road and can be easily detected by software during the analysis of the recorded image. This is unique on a global scale and the markers are protected by a patent; a relevant application to the patent office for the protection of the project results has been filed.
Pic. 2 Active marker, protected by a patent
Meter setting and measurement geometry
The meter should be placed in the centre of the lane at a height of 1.5 m above the road surface and 60 m in front of the measurement field, and the optical system used should be capable of measuring luminance over the entire width of the road being measured. In the case of a road with several lanes, the section to be measured must be measured sequentially by moving the meter to the centre of each lane. The angle between the optical axis of the meter and the road surface only changes between 1.5° and 0.5°, i.e. slightly below the horizon line to cover the entire measurement field. In this way, the field of vision of the driver driving the vehicle and observing the section of road ahead of him between 60 m and 160 m is reproduced.
Figure 2. Ideal positioning of the meter with the appropriate measuring geometry.
Measurement and analysis of results
If a spot luminance meter is used, the measuring axis must be accurately aimed at the individual measuring points. Due to the very large number of points and their density related to the observer’s perspective, such measurements are very difficult and time consuming. Therefore, spot luminance meters only serve for evaluation of luminance levels. Detailed analysis and calculation of average and longitudinal luminance is in some cases impossible.
When an Imaging Luminance Meter is used, the analysis of luminance levels and the calculation of results for compliance with standards are based on image analysis. Once the measurement has been taken, i.e. a photograph of the marked road section is taken, the relevant software functions allow the measurement grid to be superimposed on the image and the calculation and comparison with the requirements of the standard to be carried out immediately. The software contains a library of requirements for different lighting classes and automatically analyses the data received from the measurements.
The complete GL Opticam 3.0 4K TEC measuring set includes a calibrated luminance meter, battery power module, professional tripod, measuring wheel, set of active field markers, reflective vests, necessary wiring and even a set of screwdrivers. In addition, the system is equipped with special transportation cases which form a stable measuring stand with a laptop on which analytical software is installed. Additionally, it is possible to buy a car converter and GPS module. GL Optic offers Opticam 3.0 with professional training in the system operation, which takes place in real field conditions. Thanks to such a solution, within a dozen minutes, it is possible to set up the measuring equipment, mark and measure the measurement field, measure the luminance levels and distribution together with determining whether the installation meets the requirements of the standard and technical conditions.
Conclusion
Verification of the design assumptions by evaluating the lighting parameter on the road is possible thanks to the use of high-class meters adapted for these purposes. The choice of the meter is particularly important in the case of luminance measurements, as these are difficult, time consuming and require stopping traffic to prepare and measure. In this situation, the best solution is to use a measuring system consisting of a Imaging Luminance Meter with appropriately selected optics and friendly software for evaluating road parameters in accordance with the requirements of road lighting standards.
The compact form of the measurement system allows for quick and efficient measurements of luminance distribution on the road even by one person and minimizes the nuisance connected with the limitation of vehicle traffic during measurements if the measurements are made on roads in use.
Literature
[1] Road Traffic Law (Journal of Laws of 2020, item 110)
[2] § 15 of the Regulation of the Minister of Transport and Maritime Economy of 2 March 1999 on technical conditions to be met by public roads and their location (Journal of Laws of 2016, item 124).
[3] Journal of Laws of 2006, item 625 – Energy Law Act of 10 April 1997.
[4] CEN/TR 13201-1:2016 – Road lighting. Part 1: Guidelines for selecting lighting classes.
[5] EN 13201-2: 2016 – Road lighting. Part 2: Performance requirements.
[6] EN 13201-3: 2016 – Road lighting. Part 3: Calculation of performance.
[7] EN 13201-4: 2016 – Road lighting. Part 4: Methods for measuring lighting performance.
[8] EN 13201-5: 2016 – Road lighting. Part 5: Energy performance indicators.
[9] M. Zalesińska: Projektowanie oświetlenia drogowego [Design of road lighting]. Przewodnik projektanta No 3/2019 (July-September), WPIIB, 2019.
